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L IG HT SC IENCE a FOR

LEI SURE H OU R S .

5 mm! Suits.

FAMILIAR ESSAYS ON

SCIENTIFIC SUBJECTS, NATURAL PHENOMENA , &c.

WITH A

SKETCH of flu: LIFE of MARY SOLVERVILLE.

RICHARD A . PROCTOR ,B .A. CAMB.

,

HONORARY SECRETARY OF THE ROYAL ASTRONOMICAL SOCIETY ;AUTHOR OF

mmSUN ’ ‘O I

‘

HER WORLDS ’ ‘SATURN ’ ‘

Basns ON ASTRONOMY ’ ‘THE ORBS AROUND U S

’

ETC .

‘ T ruths of Sc ience waning to be caught .

’

LONDON

L ON G M AN S, G R E EN, AND 0 0 .

1873 .

A l l r i g hts r e s e r ved .

P R E F A C E .

THE First Series of Light Science Es says met with asucces s so far beyond my expectations , that I should

have found in that circumstance alone a reason for

adding the present volume to the series. But I havealso felt aWish to publish these essays because they con

tainfacts collected at the cost ofmuch labour and care

fully discussed, —useful, therefore , I trust, to others as

well as to myself, whenthus gathered into avolume .Those Who have read my former series of essays, viz . ,

Light Science , Series ‘The Orbs around Us ,’ and‘ Essays onAstronomy,

’ will perceive that evenwhen

I treat here of subjects already dealt with by me else

where,I have been careful to avoid the repetitionof

any statements, except those few Without which a sub

ject would be incomplete . Fo r instance, it will not

be easy to find inmy two papers on comets in ‘ The

Orbs around Us,’ statements or reasoning repeated in

the two papers oncomets inthe present volume .

However, for the most part, the paper s inthis series

v i PREFA CE .

are distinct insubj ect as well as intreatment from any

ofmy essays which have formerly appeared .

I invite special attentionto the second essay ontheTransit of Venus . The time is drawing near whenit

will be too late to take action to extend and render

complete and satisfactory our preparations for this 1m

portant phenomenon—the most important, I ventureto assert, of all the astronomical phenomena of the

present century. Without imputing blame to any pers on, I must dwell strongly ou f the

,fact

'

that the share

proposed to 'be taken: by Great Britain inthe observations

“

of the transit, i s unworthyfof her positioninthe

scientific world , and '

as a nation. ~ There'

i s great risk

that,for :want

"

ofanadequate number ‘

of southernsta

tions, the whole s er i es of obs ervati ons by all countr i es

engaged in, the work wi ll result in fai lure ; and it

appears to 'me—nay, more pos itively—it certainly i s

a deplorable that ' while Russia and

America are providing for ‘more than thirty northern

stations, whereof sixteen’

are' Halleyan, Great Britain

will supply but ‘

three southernstations, of which only

one chances to be Halleyanas well as Deli slean, while

even as respects this one station, Mr . Groschenhas told

the country, speaking in his place inParliament, that

either Halley’s method will not be applied at all,

or at least very little reliance will be placed upon it.’

Yet at sixteennortherns tations, some of them most

PREFA CE . v ii

difficult of occupation, Russia and America will apply

this very method ; while even the criterion devi sed to

minimi se the value of the method , leaves it superior ,whenapplied at the stations I have indicated, to Delisle’s, as applied at s elected stations .

I appeal to all who have influence inthese matters toexamine the evidence for themselves (whether as pre

sented here , o r with charts inmy Essays inAstronomy,o r in recent numbers of the M onthly Notices of the

Astronomical Society) , and to form their ownjudgment

as to the pos itionof affairs . That is all I ask , sinceI am satisfied that that will be altogether sufficient tosuggest the promptest and most energetic action?“

RICHD. A. PROCTOR.

LONDON : May 1873 .

Since this was wr ittenI have received letters fromthe g reates tmas terofmathematical as tronomy thi s country has p roduced s ince Newton

’

s day ,

s trong ly confirming my views as to the extreme imp ortance of p rovidingmany s outherns tations for app lying Halley

’

s method in1874 , and urg ingme, moreover , to app eal to Amer ica to take part inthi s sp ecial work , forwhich she i s p eculiar ly fitted, because of the bravery and enterpr i se of her

s eamen, the s k i ll and ing enuity of her as tronomers and phys icis ts , and

her s ingular liberality as anationinall s cientific matter s .

CONTENT S .

LIFE OF MRS . SOMERVILLE .

THE COMING TRANSIT OF VENUS , AND BRITISH PREPARATIONSFOR OBSERVING IT

THE EVER-WIDENING WORLD OF STARSMOVEMENTS IN THE STAR-DEPTHSTHE G REAT NEBULA IN ORIONTHE SUN ’S TRUE ATMOSPHERESOMETHING WRONG WITH THE SUN

NEWS FROM HERSCHEL ’

S PLANETTHE Two COMETs OF THE YEAR 1868

PART I.—BRORSEN’

S COMETPART II.

-WINNECKE’

S COMETCOMETS OF SHORT PER IODTHE G ULF STR EAMOCEANIC CIRCULATIONADDENDUM IN REPLY To DR . CAR PENTERTHE CLIMATE OF G REAT BR ITAINTHE Low BAROMETER OF THE ANTARCTIC TEMPERATE Z ONE

LIST OF ILLUSTRATIONS.

CHART OF THE NORTH ATLANT IC ON AN EQUAL-SURFACE PROJECT ION , SHOWING THE G ULF STREAM , &c .

CHART OF THE NORTHERN HEMISPHERE , SHOWING THE CURVESOF EQUAL MEAN ANNUAL TEMPERATURE AND EQUAL M ID

WINTER TEMPERATURE FOR LONDONTHE SAME , SHOWING TH E CURVES OF EQUAL MEAN ANNUALTEMPERATURE AND EQUAL M ID-SUMMER TEMPERATURE FOR

LONDONDIAGRAM SHOWING THE ANNUAL VARIATION OF MEAN DIURNALTEMPERATURE AT G REENWICH

DIAG RAM SHOWING THE BAROMETR IC PRESSURE OVER SOUTHERNHEMISPHERE

D IAGRAM SHOWING THE BAROMETRIC PRESSURE OVER NORTHERNHEMISPHERE

FIG URE ILLUSTRATING A THEORY IN E! PLANATION OF THE Low

BAROMETER O F THE ANTARCTIC Z ONE

PAGE

LIG HT SCIENCE

FOR LEISURE H OU RS .

SECOND SERIES.

MRS. SQMER VILLE .

MARY SOMERVILLE (nee FAIRFAx) was bornat JedburghonDecember 26 , 1 780, and died onNovember 30, 1872 ,at Naples, aged nearly ninety-two years . Inconsidering her education, we have not to mentionimportantseminaries , where skilled teachers make it their chiefbusiness to impart to others the knowledge for whichthey are themselves eminent, but to speak only of

studies pursued in the calm of a quiet home . This,rightly understood , is perhaps the most remarkablefeature of her career . There are few mathematiciansso eminent as she deservedly was , inWhose fame greatpublic schools and universities do not insome degreepartake. But we owe almost to accident the discoveryof the powers ofMary Fairfax’ s mind, while the gradualdevelopment of those powers proceeded under theguidance of tutors unknown to fame, and wi th accessonly to such assistance as could be givenby the friendsofher ownfamily.3 ,

2 LIGH T SCIENCE F OR LEISURE H OURS.

Mrs . Somerville has herself described hOw it chancedthat the peculiar powers of her mind came first to berecognised . She was in the habit of working at herneedle in the window-seat, while her brother took hi slessons in geometry and arithmetic . Fortunately (inher case) the work which is regarded as most suitableto the capacity of womenleaves the mind unoccupiedand consequently there was nothing to prevent MaryFairfax from attending to the lessons intended for herbrother . She gradually became interested inthe subjectof these lessons, and took care no t only to be presentregularly, but to study her brother

’s books inher ownroom . It happened that, onone occasion, young Fairfax failed to answer a question addressed to him, and

his sister involuntarily prompted him . The tutor wasnaturally surprised that the quiet Mary Fairfax shouldhave any ideas beyond the needlework which had ap

parently engaged her attention; but, being a sensibleman, ,he was at the pains to ascertain the degree andsoundness of her knowledge, and, finding that she hadreally grasped the first principles of mathematics

,he

‘ took care that she should have liberty to go on in

her ownway .

’If

'

a boy had shown similar fitness formathematical research , anxious attention would havebeen devoted to the choice of books and teachers,school and university ; but the case of a girl showingsuch tastes seemed to be adequately met by accordingto her the privilege of following her owndevices . Weshall never know certainly, though it may be thathereafter we shall be able to guess

,what science lost

through the all but utter neglect of the unusual powers

MRS. SOMER VILLE . 3

ofMary Fairfax’smind . Wemay rej oice that, throughan accident, she was permitted to reach the positionshe actually attained ; but there i s scarcely a line ofher writings which does not, while Showing what shewas , suggest thoughts ofwhat she might have been.

While studying mathematics ‘ in her ownway,’ she

found a difficulty which for a time threatened to interfere with her progress . She was unable to read theP r incip ia, because she could not understand Latin.

In this strait, she applied, ‘ after much hesitation,’ toProf. Playfair. She asked if a woman might

,without

impropriety, learnLatin. After ascertaining the purpose which the young lady had in view— possibly indoubt lest she might follow inthe steps ofAnne Daci er—Prof. Playfair told . her that it would not, in hi sOpinion, do her any harm to learnLatin in order toread the P r incip ia. It i s noteworthy

,as having pro

bably a bearing on the course which Mrs . Somerville’sreading subsequently took, that P layfair was one of

the few in this country who at that time appreciatedthe methods of

'the hi gher mathematical analysis,and

had formed a just Opini on of their power— ‘a power ,

however,’as Sir JohnHerschel well remarks

,‘ which

he was content to admire and applaud rather thanready to wield .’ His excellent review of the Méeani queCeles te probably gave (as Herschel suggests) a strongerimpulse to the public mind in the direction'

Of thehigher analysis than he could have communicated byany researches of his own.

It was not, :however , as a mathematician that M rs .

B 2

4 LIGH T SCIENCE F OR LEIS URE'

H O URS.

Somerville first became knownto the world. A subjectof research, exceedingly difficult and only to be pursuedsuccessfully under very favourable conditions , was

undertakenby her during the life ofher first husband ,CaptainGreig, sonofHigh-Admiral Greig of the RussianNavy. She sought to determine by experimentthe magnetising influence of the violet rays of thesolar spectrum.

‘ It is not surprising,’ says Si r JohnHerschel on this subject, ‘ that the feeble thoughunequivocal indi cations of magnetism which sheundoubtedly Obtained should have been regarded bymany as insufficient to decide the question at i ssue .’

Nevertheless it was justly regarded as a noteworthyachievement that, ina climate so unsuitable as ours

,

any success should have been attained ina research ofsuch extreme difficulty. That she achieved

, and, whati s more, deserved success, will be inferred from thewords inwhich Si r JohnHerschel indicates his ownOpinion of the value of her results : To us

,

’ he says,their evidence appears entitled to considerable weight ;but it is more to our immediate purpose to notice thesimple and rational manner inwhich her experimentswere conducted, the absence of needless complicationand refinement in their plan

, and of unnecessary or

costly apparatus in their execution, and the perfect

freedom from all pretensionor affected embarrassmentintheir statement.’

In 1832 Mrs . Somerville published the work on

which , inour Opinion, her fame infuture years will beheld mainly to depend. The Mechani smof theHeavens


was originally intended to form one of the workspublished by the Society for the Diffusion of UsefulKnowledge , though it soon outgrew the dimensionssuited for such a purpose . Indeed, it i s remarkablethat either Mrs . Somerville herself or Lord Brougham

,

at whose suggestion the work was undertaken, shouldsuppose it possible to epitomise Laplace’s magnumopus , or so to popularise it as to bring it withinthescope of the Society’s publications .It will be well, inweighing the value of the book ,to consider it first with reference to the purpose of itsauthor, though a judgment based onthat considerationalone would not be a fair one . These, then, are thewords inwhich Mrs . Somerville presents the scope andpurpose ofher work

‘ A complete acquaintance with physical astronomycan only be attained by those who are well versed inthe highest branches of mathematical and mechanicalscience such alone canappreciate the extreme beautyof the results, and the means by which these resultsare Obtained . Nevertheless, a sufficient skill in ana

lysis to follow the general outline, to see the mutualdependence of the several parts of the system, and tocomprehend by what means some of the most extraordinary conclusions have been arrived at, i s withinthereach ofmany who shrink from the task

,appalled by

difficulties which perhaps are not more formidable thanthose incident to the study of the elements of everybranch of knowledge

,and possibly overrating them by

not making a sufficient distinctionbetween the degree

6 LIGH T SCIENCE FOR LEISURE H 0 URS .

of mathematical acquirement necessary for makingdiscoveries and that which is requisite for understand

ing what others have done . That the study of mathematies , and their applicationto astronomy, are full ofinterest, will be allowed by all who have devoted theirtime and attention to these pursuits ; and they only

canestimate the delight of arriving at truth, whetherit be the discovery of a world or of anew property of

numbers .’

It cannot be doubted that Mrs . Somerville here indi

cates her belief in the possibility of presenting hersubject in a form suited to the capacities of a largenumber of readers, and to some extent advocates thisas her object. Whether she succeeded or failed inthispurpose must therefore be the first questionto engageour attention. Sir JohnHerschel considers that shesucceeded, for all those parts of’ her subj ect, at least,which the work professes to embrace, that is to say,the general exposition of the mechanical principlesemployed, the planetary and lunar theories , and thoseofJup i ter

’

s satellites, with the incidental points naturally arising out of them.

’ With the utmost respect for

popularise , I venture to express a different op inion.

I find it impossible to come to any other conclusionthan that, as respects the main purpose of her work ,Mrs . Somerville failed entirely ; though I hasten toqualify this statement by the remark that

,in my

opinion, success was altogether impossible . I believe,

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8 LIGH T SCIENCE F OR LEISURE H O URS.

Heavens— the boldest attempt ever made, perhaps, in

this direction—not only is precision of expression not

anotable feature, but, onthe contrary, the most strik

ing fault inthe work is the inexactness of the language .

EvenSi r JohnHerschel, whose perfect familiarity with

the subject of the work would tend to render the faultless Obvious to him, was nevertheless sstruck by itThe most considerable fault we have to find,

’ hewrote

,with the work before us consists inanhabitual

laxity of language, evidently originating inso completea familiarity with the quanti ti es concerned as to inducea disregard of the words by which they are designated,but which

,to any one less intimately conversant with

the actual analytical operations than its author , mustinfallibly become a source of serious errors , and which ,at all events, renders it necessary for the reader to beconstantly onhi s guard.’

These words form the penultimate sentence of SirJohn Herschel’s critique . I have preferred to speakfirst of the subject touched on, so as to pass withoutreservation to a more pleasing topic—the real andunquestionable value of Mrs . Somerville’s chief work .

And, after all, the good qualities of the work are intrins ic , while its main fault relates to a purpose which thework never could have fulfilled

, no matter how carefully the fault had beenavoided.It is inthis sense—regarding the work apart fromits special purpose, and judging of it only as a contribution to advanced scientific literature— that we mayfairly say, with Sir John Herschel, that the work is


one Ofwhich any geometer might be proud . There is ,indeed, ample evidence of the disadvantage underwhich Mrs . Somerville laboured, in the want of thorough mathematical training ; but so much the morewonderful i s it that she should have completely mastered her subj ect. Every page indicates her appreciationof the methods employed by Laplace and Lagrange .Where she does not strictly follow the Me

’

cani gue

Celes te, she evidences a clear recognition of the purposes to be subserved by adopting a different course .I would not be understood as commending all thedepartures thus made -ou the contrary, there are caseswhere i t

'

appears to me that onthe whole it would havebeen preferable to have followed the processes of theMéeani gue Celes te more closely, while there are otherswhere certain more modern processes might perhapswith advantage have been introduced . But even insuch instances we recognise in the course pursued byMrs . Somerville the decis ion of one perfectly familiarwith the subject in hand . ’

And many of the changesmust undoubtedly be regarded either as improvements

,

o r else as altogether desirable when the scale of M rs .

Somerville ’ s treatise is takeninto account . Amongstinstances of the former kind must be classed the methodemployed inthe investigation of the equations of continui ty of a flui d ; amongst instances of the latter , Iwould specially cite the treatment of the theory of

elliptic motion, inthe opening chapters of the secondbook .If however I were asked to point out the feature of

IO LIGH T SCIENCE F OR LEISURE H OURS.

this work which , inmy opinion, most strikingly indicated the powers of Mrs Somerville’s mind, I shouldunhesitatingly select the preliminary dissertation. In

this we have anabstract of the Newtonian philosophysuch as none but amaster-mind could have produced .Apart from its scientific value—and it has great s ci entific value—it is awork of great literary merit. If iti s not in plan and purpose altogether original, inasmuch as i t must be regarded as to some degree anabstract of Laplace’s Sys teme du Monde, it is nevertheles s , as Herschel has well remarked, anabstract sovivid and judicious as to have all the merit of originality, and such as could have been produced only byone accustomed to large and general views

,as well as

perfectly familiar with the particulars of the subj ect .’

Three years after the appearance of the Mechani sm

of the Heavens , M rs . Somerville published the work bywhich she is probably best known to general readers .The Connexi onof the Phys ical Sci ences was , I believe ,written at the suggestion of Lord Brougham, as an

expansionof the admirable introductionto the Celes tialMechani sm. It is a work full of interest

,not only to

the student of advanced science , but to the generalreader. Insaying this we indicate its chief “merit andits most marked defect. It is impossible to conceivethat any reader, no matter how advanced o r how limitedhis knowledge, could fail to find many most instructivepages in this work ; but it is equally impossible toconceive that any one reader could find the who le work ,o r evenany considerable portion

,instructive or useful .

MRS. SOMER VILLE . I I

The fact was that Mrs . Somerville recognised, or whichis practically the same thing, wrote as if she recognised ,no distinction between the recondite and the simple .She makes no more attempt at explanation whenspeaking of the perturbations of the planets or discussing the most profound problems of molecular physics ,thanwhenshe is merely running over a series of statements respecting geographical or climatic relations .It would almost seem as though her mind was so constituted that the difficulties which ordinary minds ex

peri ence inconsidering complex mathematical problemshad no exi stence for her. A writer , to whom we oweone of the best

“

obituary notices of M rs . Somervillewhich hitherto have appeared, tells us that the sort ofpressure M rs . Somerville underwent from her publisheras the earlier editions of the Connexi onofthe Phys i cal

Sci ences passed through the press convinced her ofherownunfitne s s for popularising science . Whenthere wasalready no time to lose inregard to her proof sheets, shehad hint uponhint from Mr .Murray that this and thatand the other paragraph requi red to be made plainer topopular comprehension. She declared that she tried veryhard to pleaseMr. Murray and others who made the samecomplaint, but that every departure from scientificterms and formulas 'appeared to her a departure fromclearness and simplicity ; so that, by the time she hadexplained and described to the extent required, herstatements seemed to her cumbrous and confused . In

other words , this was not her proper work .’

Respecting her two other works , I shall merelv

1 2 LIGH T SCIENCE F OR LEISURE H OURS .

remark that the Phys i cal Geograp hy appeared in 1 848 ,and the Molecular and Mi cros cop i c Sci ence in 1 8 69,

whenshe had reached the advanced age of eighty-eightyears .I may be excused for regardingMary Somerville’s lifewith reference rather to herastronomical andmathemati

cal researches thanto her proficiency inother branchesof science . Inthis aspect of her career it is difficult,great as was the reputationshe deservedly Obtained, notto contemplate with regret those circumstances, theeffects of unfortunate prejudices, whereby she was prevented from applying the full powers of her mind tothe advancement of science . It i s certain that nodepartment of mathematical research was beyond herpowers, and that inany she could have done originalwork. Inmere mental grasp few men have probablysurpassed her but the thorough training

,the scholarly

discipline, which canalone give to the mind the powerof advancing beyond the point up to which it hasfollowed the guidance of others , had unfortunate ly beendenied to her . Accordingly

,while her writings show

her power and her thorough mastery of the instrumentsofmathematical research

,they are remarkable less for

their actual value, though their value is great, thanasindicating what, under happier auspices , she mighthave accomplished.I have mentioned that Mrs . Somerville was twicemarried. By her first marriage she had one son, Mr .

Woronzow Greig, since deceased. A few years afterCaptain Greig’s death she married her cousin, Dr.

MRS . SOMER VILLE . I 3

Somerville, by which marriage she had three daughters ,two ofwhom survive her. The latter years ofher life

(twenty-three years, we believe) were passed inItaly.It has beensaid by one who was well acquainted withthe circumstances that the long exile which occupiedthe latter portion of her life was a weary trial to her.She carried athoroughly Scotch heart inher breast and

the true mountaineer’s longing for her native countrysickened many anhour of many a tedious year. Sheliked Londonlife, too , and the equal intercourses whichstudents like herself canthere enj oy ; whereas, inItaly,she was out of place . She seldom met any one withwhom she could converse on the subj ects which interested her most ; and if she studied, it could be for nofurther end thanher owngratification. It was felt byher friends to be a truly pathetic incident that, of allpeople inthe world , Mrs . Somerville should be debarredthe sight of the singular comet of 1 843 ; and the circumstance was symbolical of the whole case of herexile . The only Italianobservatory which afl’orded thenecessary implements was in a Jesuit establishment,where no womanwas allowed to pass the threshold. Atth e same hour her heart yearned towards her nativeScotland, and her intellect hungered for the congenialintercourse of London; and she looked up at the skywith the mortifying knowledge of what was to be seenthere but for the impediment which barred her accessto the great telescope at hand. With all her gentlenessof temper and her lifelong habit of acquiescence , she

I4 LI GH T SCIENCE FOR LEISURE H OURS .

suffered deeply, while many of her friends were indignant at the sacrifice.’

I shall venture to quote , inconclusion, some remarksby Sir Henry Holland on features of Mrs . Somerville’scharacter and life which have beenhiddenfrom generalknowledge She was a womannot of science only,

’

he tells us,but of refined and cultivated tastes . Her

paintings and musical talents might well have wonadmiration, evenhad there beennothing else beyondthem. Her classical attainments were considerable,derived probably from that early part of life whenthegentle Mary Fairfax— gentle she must ever have been—was enriching her mind by quiet study inher Scotchhome . A few words more onthe moral partof Mrs . Somerville’s character ; and here , too , I speakfrom Intimate knowledge . She was the gentlest andkindest of humanbeings—qualities well attested evenby her features and conversation

,but expressed still

more inall the habits of her domestic and social life .Her modesty and humility were as remarkable as thosetalents which they concealed from commonobservation.

Scotland ,’ he justly adds

, i s proud ofhavmg produced a Crichton. She may be proud, also,inhaving givenbirthplace to Mary Somerville .’

(FromM onthly Notices of the R oyal A stronomical Societyfor February, 187


the determinationofthe distances and the magnitude of

the celestial bodies ."6

I propose here , after inquiring briefly into thegeneral question of the determination of the sun

’sdistance, to describe the nature of the opportunitieswhich will be afforded during the transit of 1874 , andto discuss the preparations which are being made bythis country to take her part inthe work of observation.

It will be seen, as I proceed, that this discussionof the

I venture to quote here the appeal made by Halley (whenAs tro

nomer Royal)forty-five years before the trans it of 176 1 , the earlier of

the pair of trans its thenlooked forward to . It wi ll show that, indealingwith a trans it 2 1 months before the date of its occurrence, I amnot

looking forward s o inordinately as might be suppos ed by thos e unfami

liar with the nature ofthes e inquiries . I should remark, however, thats ince Halley

’

s day other methods for determining the sun’

s di s tance have

beendevi s ed and employed. Six methods are des cribed inmy treatis eon the Sun

,

’and a s eventh has , wi thin the las t few months

,been

suggested by the great French as tronomer Leverrier . Thus , then, wrote

Halley in1716 I could wi sh, indeed, that obs ervations of the trans itshould be undertakenby many pers ons indifferent places firs t, becaus eof the greater confidence which could be placed inwell

-according Obs er

vations ; and, s econdly, lest a s ingle Obs erver should, by the interventionof clouds , be depr ived of that spectacle which, s o far as I know,

will not be Vi s ible againto the men of thi s and the next century , andonwhich depends the certainand suflficient s olutionofamost noble andotherwi s e intractable problem. I therefore againand againurge uponthos e inquir ing obs ervers of the celes tial bodies , who , whenI have departed this life, w ill be res erved to obs erve thes e thing s , that, mindfulofmy couns el

, they should devote thems elves s trenuous ly and wi th allthei r energies to conduct the obs ervation I des ire and pray that theymay be favoured in every way, and especially that they may not bedeprived ofthatmost des irable spectacle by the inopportune darknes sofaclouded sky ; and that, finally, the magnitudes of the celestial

bodi es , forced into narrower limits (of exactnes s ), may, as i t were, makesubmi s s ion—to the glory and eternal fame of thos e obs ervers .

’

Thes e home were not fulfilled, s o far as the trans it of 176 1 was concerned but the trans it of 1769 was Obs erved with great care at no les sthans eventy-four s tations , fifty ofwhich

,however , were inEurope .

THE COIVIING TRANSI T OF VENUS. I 7

subj ect does not labour under the fault of being premature . Onthe contrary, the time is now at hand whena final decisionmust be made as to the course whichthis country is to pursue ; and inasmuch as my purposeis not solely to describe what is being done , bri t topoint out what (in my opinion) should be done, thepresent i s the proper time to speak .A surveyor

,who wishes to determine the distance of

aninaccessible Obj ect, measures a convenient base-lineand Observes the direction of the object as seenfromeither end of the line . He thus has the base andthe two base-angles of a triangle ; and the simplestgeometrical considerations teach that the other twosides of the triangle canthence be determined. Thesesides are , of course, the distances of the inaccessibleobj ect from the two ends of the base-line . Now this isthe fundamental method employed by astronomers todetermine the distances of the celestial bodies . It i sapplied directly to the moon. Anobserver at Greenwich (let us say), notes the directionof the moonwhenat her highest, or due south ; another at Cape Town

(let us say), does the like ; thena line j oining Greenwich and Cape Townis a base-line of known length

,

and the two directions give the base-angles . Thetriangle is a very long one , its vertical angle (that is ,the angle opposite the base) being one ofabout a degreeand a half, or about the angle swept out by the handof a clock or watch during a quarter of a minute ; butsuch a triangle is quite within the methods of treatment available to astronomers .

C

I8 LIGH T SCIENCE F OR LEISURE H o URS

Inapplying this method to the sun, a serious diffi

culty comes in He i s so far Off that, instead of a

triangle with a respectable vertical angle, there is atriangle having a vertical angle of about the 24othpart of a degree (under the most favourable conditionswhich can be conveniently obtained). To know howsmall such anangle is, let the reader note the minutehand of a clock or watch , and Observe how little itshifts around its centre ina single second of time ; yetthis angular shift is twenty-four times as great as thatwe have mentioned.It must not be forgotten that, inall such cases, thequestionis not whether the astronomer can recogni s e

such and such aneffect, but whether he canmeasure

i t. It is not the whole quantity about which astronomers are troubled. Unquestionably the observer at

Greenwich canrecognise the depressionof the mid-daysun

,

l due to the fact that Greenwi ch lies above (o rnorth of) the earth

’s centre . For this depressioni s anelement which he has to take into account inhis Observati ons . The corresponding depression, even in thecase of bodies far more distant than the sun, as theplanets Jupiter and Saturn, is announced systematicallyinour national astronomical almanac . But the directmeasurement of the depressioni s altogether out of thequestion.

If the stars which really bestrew the heavens beyondOnly Obs ervations of the mid-day sunwould avai l

,becaus e the only

ins truments having the requi s ite delicacy of adjus tment are meridional.There i s aninstrument suitable for obs erving the moonwhenshe i s notonthemeridian but it i s quite unfit for the purpos e we are cons idering .

THE COMING TRANSIT OF VENUS. 1 9

the suncould be seen, the case would be di fferent, forthey would serve as index points, by means ofwhich toestimate the sun’s displacement. But although starsnot lying near the sun’s place onthe heavens canbes eenby day with powerful telescopes, those close aroundhim are quite invisible . This method failing, theastronomer has to look for other means of solving theproblem . The planet Venus , which comes at timesmuch nearer to the earth thanthe sun i s, and infactnearer than any celestial body except the moon, naturally claims attention as a suitable obj ect for theastronomer’s purpose . For it i s to be remembered thatthe p roporti ons of the solar system have long beenaccurately determined ; so that as soonas the distanceofany one planet is ascertained, the scale of. the wholesolar system becomes also known.

Venus, however, whenat her nearest, is lost inthesun’s light, and, though discernible in powerful telescopes

,is quite unsuitably placed for the delicate obser

vati ons which would alone avail to determine herdistance .This brings us at once to the recognition of theimportance ofa transit of Venus . WhenVenus passe sbetween the sun and earth, in such a way as not tocross the sun’s face ,— that is, when she passes aboveor below the long and almost linear portion of spacelying actually betweenthe earth and sun,— she cannotbe well observed ; but when, in making the passage,she comes so close to the line j oining the earth and sun

as actually to be seen on the sun’s face, she can bee 2

2 0 LIGHT SCIENCE FOR LEISURE HOURS.

observed to great advantage . For she is thenseenasa round black spot onthe sun’s face ; this face is thus

as a sort of dial-plate onwhich the black disc ofVenus isas anindex. The sharply-defined edge of this blackdisc presents the same advantage which a neatlyn cut

index possesses, enabling the observer to measure satisfactori ly the place of the planet. All the circumstancesare favourable, except two —first, the index, —thatis,the black disc,—is not evenfor an instant at rest ;

and, secondly, the index-plate, —that is, the sun’s disc,

—is itself displaced by any difference inthe positionof

the terrestrial observers .Nothing can be done to remedy the latter circumstance. Its efiects are easily seen. Suppose anobserverat some northernstationsees Venus inreality depressedby a third ofaminute of arc, which is about the hundredth part of the sun’s apparent diameter. Thenthe

'

sun, being farther away in the proportion of aboutten to three, i s depressed by about the tenth of aminute. Accordingly, Venus only s eems to be depressedby the deference of these amounts

, or by little morethan a quarter of a minute . Nevertheless it is fareasier to measure this reduced displacement on thesun’s face, than to measure the larger displacementwithout his face as anindex-plate .The other circumstance has beendealt wi th intwoways .

First, inaccordance with a suggestion of Halley’s ,instead of attempting to measure the positionofVenusonthe sun’s face, the astronomer may simply time her

THE COMING TRANSI T OF VENUS. 2 1

as she crosses that face, and so judge how long thechord is which she has traversed . This shows hownearly the chord approaches the sun’s centre , and thusgives a determinationas satisfactory as anactual mea

surement. Of course,there are many details to be

takeninto account : for instance , the apparent path ofVenus is not a straight line in reality, because theobserver’s stationis not at rest, but carried round theaxis of the rotating earth . But the mathematicianfinds no difficulty in taking such considerations fullyinto account.Secondly, Delisle proposed that astronomers should

note the actual moment (of absolute, not local time)whenVenus seems to enter or leave the sun’ s face , asseenfrom di fferent stations on the earth . It will bemanifest, on a moment’s consideration of the actualcircumstances of the case

,that the transit will not

seem to beginor end at the same instant, as seenfromdifferent parts of the earth . There is the great globeof the sunat one side, and the smaller globe of theearth

'

onthe other ; and Venus passes between. Now,

in order to show more clearly what must happen, letus take an illustrative case drawnfrom anevent whichina few weeks from the present time will interest alarge proportion of our population. Suppose that onone side of the river Thames there i s a long buildingwhose extremeti es we call A and B . Suppose that justopposite there is a barge whose corresponding extremeties we call a and b. Now suppose the winning boat tobe coming along so as to pass betweenthe house and

2 2 LIGHT SCIENCE FOR LEISURE H oURS.

the barge (coming first betweenthe ends A , a) . And

for simplicity of descriptionlet us confine our remarksto the little fiag carried at the bow of the boat. It i smanifest that anobserver at a will see the little flagcross his l ine of vision towards A before anobserver atb sees the like . And the observer at a wi ll in likemanner see the light blue flag (I beg pardon, I shouldsay the blue flag simply) crossing his line of visiontowards B before an observer at b sees the like . Theflag will traverse the range A B as seenboth from a

and from I) , but both its ingress onthis range and itsegress from it will be earlier as seen from a than as

s eenfrom 6. Now our earth may be compared to thebarge ; the sunto the building A B ; and Venus to theboat. There is one spot on the earth at which Venuswill seem to enter earli est on the sun’s face , and an

other spot (on the opposite side , just as b i s farthestaway from a) where Venus will seem to enter latest ;and in like manner there is one spot at which Venuswill seem to leave the sun’s face earliest

, and another(onthe opposite side) at which Venus will seem to leavethe sun’s face latest.And as our illustrative case explains the nature ofDelisle’s method, so also it illustrates the rati onale ofthemethod. Of course, the two cases arenot exactly similarbut they are sufficiently so to make the illustration instructive. Suppose that the length of the barge a b isknown(as the dimensions of the earth are known)thus, say that it is 24 yards inlength . Now supposethat the course of the boat is known to be inmid

24LIGHT SCIENCE FOR LEISURE H OURS.

method,two stations are absolutely necessary, there i s

but a single observationto be made at each , whereas inHalley’s the beginning and end of the transit must beobserved at both stations . This introduces a doubledifii culty. For first, there is the necessity for a longercontinuance of clear sky, since the transit may lastseveral hours ; and, secondly, there is the difficulty ofsecuring a stationwhere the sun is well placed on thesky

,both at the beginning and end of the transit. It

will not suffice, in applying Halley’s method , to havethe sunwell above the horizonat the moment of ingressif he is low downat the moment of egress, or to havethe sunhigh at egress if he is low at ingress . Accordingly, the conditionhas to be secured that at stationswhere the day is short (that is, inDecember, at northerly stations) the middle of the transit shall occurnearly at mid-day. Thi s limits the choice for northernstations considerably.Onthe otherhand, Delisle’s method has this di sad

vantage, that the exact moment at which ingress o regress occurs must be known. A mistake

,even of a

second or two , would be of serious moment. So thatthe clocks made use of at each station where thismethod is applied, must not only have good rates , butmust show absolutely true time at the moment of theobserved phenomenon. Moreover

,the latitude and

longitude of the place of observationmust be known,the latter (the only difi‘i cult point) with especialaccuracy, since onits determinationdepends the changeof local time into (say) Greenwich time ; and this


change must be accurately effected before two observations made indifferent longitudes canbe compared asrespects the absolute time of their occurrence . Onthe

contrary, Halley’s method

,while only requiring a rela

tively rough determination of the'

longitude,can be

satisfactorily applied when the clocks employed are

simply well rated ; for it depends only onthe durationof the transit as seen at different stations . A clockmust be badly rated indeed—utterly unfit, in fact, forany astronomical use whatever—which should lose asingle second infour or five hours .But the most important point to be noticed is , that

both methods ought to be employed, if possible, apartfrom all nice considerations of their relative value . It iscertainthat astronomers will place much more confidenceinclosely concordant results obtained by the applicationof these two methods, differing wholly as they do inprinciple, than in as many and equally concordantresults all obtained by one method . A third method isindeed to be applied, —viz . , a method based on theingenious use of photography. But as yet too little isknownrespecting the chances of success by this methodto warrant too implicit reliance uponit.Let us inquire what preparations are being made by

astronomers , and especially by the astronomer s ofEngland, to make adequate use of the opportunities presented by the coming transit.It has fir st, unfortunately, to be noted, that, so far

as this country is concerned,no p rovi s i onwhatever hasbeen hitherto made for the employment of Halley

’s

2 6 LIGHT SCIENCE F OR LEISURE H O URS.

method . If this resulted from the simple preference ofDelisle’s method, there would be little to say. Mostassuredly, speaking for myself, I should be very loth tourge the advantages of Halley’s method, if I foundagainst such a view the practical experience of thoseastronomers who are continually testing the value of

various methods of observation. But the rej ection'

ofHalley’s method for the transit of 1874 was not originally, and is not now,

based on any objection to theprinciple of the method, but on certainmathematicalconsiderations , which appeared to prove that the methodcould not be advantageously applied in 1 874 , while itcould be applied successfully in1 882 . It was accordingly reserved for the latter transit, and all the stationsfor observing the transit of 1874 were selected withspecial reference to the method of Delisle .Now it happened that early in 1 8 69 I was attractedto the examinationof the subj ect of the coming transits, by the circumstance that the investigationappliedto the matter by the Astronomer Royal had struck meas imperfect inmethod. I was interested, viewing thematter merely as a mathematical problem

,to inquire

what corrections might occur if all the niceties of research of which the questionadmitted were appliedthroughout the investigation. Working with this soleobject in view, I analysed the whole matter in twoindependent ways, viz . , first as a problem of calculation,and secondly as a geometrical problem. The results ,perfectly concordant, differed so remarkably from thosepublished by the Astronomer Royal , that I was con

THE COMING TRANSIT OF VENUS . 7

strained (inmere fealty to the cause of s ci ence) to submit them to the examinationof the scientific world .To beginwith Halley’s method, ofwhich , -in1 8 5 7 ,

and aga1nin 1864 , and yet againin 1 8 68 ,— the Astro

nomer Royal had said that it is totally inapplicable in1 8 74 , was found to be applicable under circumstancesaltogether more favourable thanthose which will existin It was found not only to be applicable with

1 The o rigin of thi s mis take on the As tronomer Royal’

s part i s

thus explained inanarticle in the ‘ Spectator’

for March 1,1873

Everyone i s asking whether it i s po s s ible that an as tronomer s o emi

nent and s o ski lful as Sir G eorge Airy—for the time i s pas t when

names need be concealed- canhavemade any s er ious mis take inamatterof thi s importance . And again, everyone i s anxious to know preci s elywhat mis take i s imputed, and how it aros e (granting that ami s take hasbeenmade).

‘ To thi s last ques tion the reply is easy. It chanced unfortunatelythat in185 7 the As tronomer Royal delivered a lecture onthe subject ofthe now approaching Trans its . In that lecture hi s great mi stake hadits or ig in. Intent onpres enting themore s tr iking and popular featuresof hi s subj ect, and ina way which would be clear and convincing to

everyone, he was led to adopt amethod of reas oning which onthe faceof it s eems convincing enough (and which, indeed, i s s ound inits elf)but the conclus ions der ived fromwhich may be, and inthe actual cas e

are,dependent on certaindetai ls into which the As tronomer Royal

neither thenentered nor has ever entered s ince . It i s the palpably con

vincing nature of the evidence at a firs t view which led to all the

mis chief. We will endeavour to give a br ief but sufficient sketch of

the line ofargument.‘ Let it be premi s ed that, for applying Halley

’

s method—o r the

Engli sh method, as it i s o ftencalled—with advantage, what i s wanted

is that at s ome s tationthe trans it shall las t as long as pos s ible, whileat another it shall last as short a time as po s s ible . It matters nothingwhether the increas e or reductionofthe time be obtained by a s eemingchange inthe length of the line travers ed by Venus , or by a change in

the rate at which she s eems to move during trans it. So much premi s ed,let it be noted that in1874 Venus wi ll cros s the sun

’

s face ona line

placed s omewhat as a line from the figure ! to the figure I ona

clock-face. As s eenfromnortherns tations , the line of trans it wi ll be

2 8 LIGHT SCIENCE F OR LEISURE HOURS.

advantage,but even more advantageously than De

lisle’s .lowered

,and therefore manifes tly will be lengthened. From s outhern

s tations,the line w ill be rai s ed, and therefore shortened. We therefore

s et an obs erver at as northerly a s tation as we can, to get as great a.

lengthening as we can, and that i s onepo int gained. We s et anobs erverat as s outherly a s tationas we can, and s o get as great a shortening as

po s s ible, and that i s a s econd po int gained. But it i s eas ily shown(wedo not trouble our reader s wi th the proof) that our norther ly obs erveri s s o shifted by the earth

’

s rotationwhi le the trans it i s inprog res s that

Venus i s s eemingly hastened onher cour s e intrans it. Thi s s hortens

the time of trans i t at the northern s tation, and i s dis cordant with the

leng thening obtained by s etting anobs erver as far north as pos s ible.

Here, then, i s one point agains t us . Lastly , the s outherns tation can

be taken s o as to g ive either a hastening or a retarding of Venus’

s

motion, s imply becaus e the trans it occurs inthe s outhernsummer, when

places far s outh have no night, s o that we cans et the obs erver ei ther

where he will have the sunmoving from eas t to west dur ing the trans it.

or where he will have the sunmoving fromwes t to eas t. We s et our

obs erver s o thatVenus i s has tened (which i s s ecur ed by taking a s tationwhere, during the trans it, the sunmoves from eas t to wes t). Thi s

has tening is manifes tly accordant with the shortening of her path at

s outhern s tations , and thus we get a thi rd po int in our favour . We

have, then, three points in our favour and one against us , or abalance

of only two favourable po ints .

Now, in1882 , Venus cros s es the lower part of the sun’

s face, or s omewhat as from figure VII to figure IV ona clock-face. Inthi s cas e, thenorthern s tationg ives the lowes t or shortest cours e, while the s outhern

gives the highest or longes t cours e . As before, we get two po ints inour

favour by s etting an obs erver far to the north and another far to the

s outh. As before . the northern obs erver s ees Venus has tened onhercourse but now thi s i s a favourable point, s ince it manifes tly accordsw ith the shortening of the northernline of trans it. Thi s mak es p ointthree inour favour . And again, as before, we cans et our s outhernobs erver where the motionofVenus can be has tened or retarded as we

pleas e. We as s ignhima s tationwhere she Wi ll be retarded (which i ss ecured whenthe sunmoves from wes t to eas t dur ing the trans it) : thi smanifes tly accords with the lengthening of her path. Thus we have

four favourable po ints inall in1 882 ; whereas in 1874 we cans ecure

only three or (one being unfavourable) , amajority of only two favourable po ints .

It s eems manifes t, then, that the trans it of 1882 i s twice as favour

THE CO IMING TRANSIT OE VENUS. 2 9

On this point all doubts should have been veryquickly removed . For, almost simultaneously with

able for applying Halley’

s method as the trans it of 18 74. So the

As tronomer Royal concluded. He did not enter into details , but aftersumming up the evi dence much as we have done above , he said “ the

observable deference of durationin1874 willp robably not be half of thatin It was in1 85 7 that he thus spoke ; and he has never said a

word or writtena line s ince that time implying that he had gone into

the details ofthematter . Whenhe next touched onthe subject (in1864)he referred to the lecture of 185 7 as showing the suitabili ty ofHalley’

s

method in 1882 , and he left the trans it of 1 874 wholly unnoticed.

Again, inDecember 1 868 , he touched onthe matter , s imply saying that

Halley’

s method fails totally in1874 . That fatal lecture, or rather the

error sugges ted inthe proces s ofpopularis ing the subjectfor that occas ion,led to s o establi shed a conviction as to the us eles snes s of Halley

’s

method in1 874 , that it hadnever s eemed worth whi le to re-examine thematter . But now letus cons ider details a little , and s ee how the matterwill thenappear .

‘ In the firs t place , the trans it of 1 882 at once los es its apparent

super iority . The s outhernob s ervermus t have the sunmoving fromwestto eas t dur ing the trans it,—or inother words , he mus t have the sunon

the night s ide (s o to s peak) of the sky. There i s,of cours e , no night

near the Antarctic Pole onDecember 6 , but at nominal midnight the

suni s at its lowes t ; and the sunmus t be towards thi s part ofhis diurnal

cours e , if the obs erver i s to get the advantagewe are cons ider ing . There

i s no knownAntarcti c s tation where thi s can be, the sun being als o

fairly high at the beginning and end of the trans it. Thi s at once di s

pos es of the super iori ty of the trans it of 1882 . IfanAntarctic s tation

i s s ought at all, there will be a has tening ins tead ofa retarding of the

planet’

s trans it,—or anunfavourable point, as inthe cas e ofthe earlier

trans it. In reality , the lo s s thus accruing i s found to be much more

s erious in1882 than the corresponding los s in1874, whenwe inquire

into actual details .

But in1874 , as we have s een, theremus tbe anunfavourable has teningofVenus

’

s motionas s een fromanortherns tation and this has tenings eems to cancel the effect due to the lengthened trans it path. Whenweinquire , however , to what extent thi s cancell ing takes place, we at onces ee that the As tronomer Rcyal was frightened away from Halley

’

s

method without sufficient reas on. He manifes tly (s ee the italici s ed re

mark quoted above) suppos ed that the duration would scarcely be in

creas ed at all at the northern s tation. Let us s ee, however , whether

Mr . Proctor has beenr ight or not insaying that the duration i s con

30 LIGHT SCIENCE FOR LEISURE HOURS.

the announcement of my result, the news arrived thatthe French astronomer, Puiseux, had obtained almost

I

s iderably increas ed at a suitable northerns tation, no twiths tanding the

undoubted partial cancelling which tak es place fromthe caus e indicated.

Not to favour one s ide or the other , we go direct to the Nautical Alma

nack for 1874 . We take Nertchinsk , the place pointed to byMr . Proc

tor eu far back as March 1869 and we note that he thenas s igned to

thi s stationalengthening of the durationof trans it by 1 55 minutes (a

very cons iderable amount, muchmore infact thanat the mo s t favourables tationin Now, what says theNautical Almanack fo r 1874 ? Atpage 434, it states that the meandurationof trans it i s 3 hours 42 min. 2

s ec. At page 20 of the appendix,it s tates that atNertchinsk the duration

is 3 hours 5 7 min. 6 s ec. , exceeding the former duration by 1 5 min.

4 s ee. Thi s i s very clo s e indeed to Mr . P roctor’

s result, and shows

how nearly the values obtained by hi s graphic cons tructions accord

wi th thos e deduced by rig id calculation. (Moreover , a part evenof the

s light difference i s due to a difference in the adopted value ofVenus’

s

diameter . ) Here, then, ins tead of that complete cancelling of the value

of the northern s tationwhich Sir G . Airy too has tily as sumed, we have

a lengthening of the trans it per iod by more than 15 minutes , which in

thi s problem i s an unus ually large amount. T0 show that thi s i s s o ,

and how s lightly the northerns tationi s affected by the peculiar itywhich

Sir G . Airy had hastily regarded as introducing a fatal objection, wehave only to remark that at Po s s es s ion Is land, the mo s t favourables outherns tation(where the two conditions consp ire, ins tead of opp os ingeach other ), the shortening of the trans it amounts only to 175 minutes .

Combining thi s shortening with the lengthening at Nertchinsk , we havea difference of duration of fully 32s minutes . And now obs erve howgreatly thi s result differs from Sir G . Airy

’s anticipation! He thought

the difference ofdurationwould probably not be half of that in1882but hi s ownestimate of the greates t difference ofdurationin1 882 (tobe obtained only by s eeking aninacces s ible s tation, where the sunwillbe but four degrees high at egres s ) amounts only to 28 minutes . In

s tead of being les s thanhalf, the difference of duration in 1 874 i s

greater inthe proportionofabout 7 to 6 . Add to this that in 1874 thes olar elevation, both at ingres s and egres s w ill exceed twenty degrees ,and the importance ofhaving a s tationat Pos s es s ion Is land becomesmanifes t. Rus s ia has occupied Nertchinsk , and it i s G reat Br itain

’

s

duty (and that of no other country) to occupy Po s s es s ionIs land. If

she shrinks from thi s duty, it will be no answer to the reproach which

she will hereafter incur , that she occupied s tations inother respects ad

vantageous . Other countries are occupying thes e s tations , the Papelotte

3 2LIGHT SCIENCE FOR LEISURE HOURS.

where the transit beganseveral minutes late, and endedseveral minutes ear ly, we should have a transit lasting

for a shorter time thanas seenfrom the earth’s centre :

and then, comparing what was observed at such a

stationwith what was observed atNertchinsk , we shouldhave Halley’s method applied under effective and

favourable conditions . But the southern stations to

whichEngland sends observing parties areRodriguez and

Christ Church (NewZealand) ;1 and at the former station

the transit begins late and ends late , while at the latter i tbegins early and ends early so that at neither is therethe combinationof a late beginning and anearly end

ing , required for the effective application of Halley’smethod .Now there i s a station—a stationwhich this countryought unquestionably to occupy—where the transitwould be evenmore shortened thanit is lengthened atNertchinsk . This station is an Antarctic island on

which Sir James Ross landed a party in 1 846 , and towhich he gave the name of PossessionIsland . It liesdue south of the southernmost extremity ofNew Z ea

land, close by the rugged shore-line of Victoria Land ,and within 1 8 degrees of the south pole . At thi sstationthe transit will begin 6 minutes late and end

1 1 45 minutes ear ly, or be shortened altogether no lessthan171

2 minutes . Adding to this the lengthening ofthe transit by 1 5 ; minutes at Nertchinsk , we obtaina

1 There has beena change as to the s tations elected inNew Z ealand,from Auckland to Chri s t Church. The change i s inaccordance withmy own suggestions , s o far as the application of Delis le

’

s method i sconcerned.

THE COMING TRANSIT OF VENUS. 33

difference of duration of fully 3 3 minutes . Nothinglike this difference was available in the transit of

1769 ; nothing like it will be available in 1 882 . I

do not know the circumstances of the transits of

2004 and 2012 , but it i s altogether unlikely that theOpportunity of applying Halley’s method will be sofavourable during either ofthese transits as in1 874 . Bethat as it may, however , it i s absolutely certainthatno opportunity equal to that which will be affordedduring the transit of 1 874 will recur for one hundredand thirty-two year s, nor has such anopportunity beenever before offered to astronomers . Absolutely thebest Opportunity ofapplying Halley’s ingenious methodwhich has ever been afforded, or will be afforded formore thana century and a quarter, i s available to astronomers during the approaching transit. The duty ofseizing this opportunity belongs assuredly to our

country, which alone has colonial possessions close tothe station inquestion, and which alone also has s ea

men still living who have actually set their foot on

PossessionIsland .I must confess that when, four years ago , I indicatedthis opportunity, I thought that it would have beenseized at once . I thought that reconnoitring expeditions would quickly have been prepared, and that bythe present time complete arrangements would havebeenmade for landing an observing party onPossessionIsland indue seasonfor the required observations .It would have been a matter of complete indifferenceto me whether this had been done with or wi thout

D

34LIGHT SCIENCE F OR LEISURE H o URS.

acknowledgment of the source whence the suggestion

had come . But assuredly I hoped that some stepswould have been taken without delay to seize an

opportunity so important, the loss ofwhich could notbut reflect some degree of discredit upon the scienceof this country.For up to that very time— the spring of 18 69— theimportance of an Antarctic expedition fo r observingthe transit of 1882 by Halley’s method had beeninsisted upon over and over again by leading astronomical and geographical authorities . Nay, thi s verystation, Possession Island, had been selected as themost suitable . The feasibility of reaching it and landing on it had been insisted upon. The superiormeteorological chances presented by the station

,as

compared with other southernstations , had beendweltonstrongly. Everything promised that before long anAntarctic reconnoitring expeditionwould set forth toprepare the way. It was in the full height of theseanticipatory inquiries that I pointed out the inex

pediency of any attempts to apply Halley’s method at

anAntarctic stationin 1882,dwelling earnestly onthe

fact that whenthe transit began at PossessionIsland,in 1 882 , the sun would be barely five degrees abovethe horizon, anelevation utterly unfit for exact observati ons . Upon this all the plans for an Antarcticexpedition in 18 82 were abandoned . But althoughthis was as it should be (for the lives of our seamenare not to be endangered without the prospect of

valuable results), there was no necessity for abandon


ing all idea -of anAntarctic expedition. The schemesset afoot for observing the transit of 1 882 shouldhave been transferred to the transit of 1 874 . Not a

single argument which had beenargued intheir favourwas wanting in the case of the latter transit. Themain argument was greatly strengthened for thedifference of duration in 1 88 2 would only be twentyfour minutes, if Possession Island were the selectedstation; whereas we have seenthat in 1 874 , the corre !

sponding difference will be fully thirty-three minutes .And the fatal obj ection to Possession Island as a sta

tionin18 82 , has no existence inthe case of the transitof 1 874 . Instead of the utterly insufficient solar elevationoffive degrees j ust mentioned, there will be , in1 874 , a solar elevation of thirty—eight and a halfdegrees when the transit begins, and of twenty-fivedegrees when the transit ends . And necessarily allthe considerations which had been urged as to theimportance of Antarctic expeditions, p er s e, and

especially of the interest which would attach to theexperiences of awintering party near the south pole ofthe earth , remainunchanged .While there is still apossibility of retrieving matters,

I would earnestly appeal to -all who canassi st inbringing about such a result to spare no pains lnthe endeavour. I believe the scientific credit of this country tobe seriously imperilled. Hereafter, the very argumentsused infavour of the now abandoned scheme for observing the transit of 1882 from PossessionIsland

,will

be urged,—even as now (for a better purpose) I am

i ) 2

36LIGHT SCIENCE FOR LEISURE H o URS .

urging them,—to show that the importance of suchobservations (if feasible) had not beenoverlooked. It

has been shown, and i s now admitted, that they arefeasible in1 874 . What, then, I ask , will be thought

of this country i f the task which is her duty shall be

neglected ? It was sufficiently unfortunate that theopportunity had beenso long overlooked. But it willbe nothing less than a national calamity, if, havingbeen recognised inample time to be employed, thatopportunity be altogether neglected .

Now ,after four years’ delay, time runs short indeed .

It is essential that any party intended to observe thetransit, should be landed before the Antarctic summerof 1873-74 draws near its end— certainly before themiddle of February 1 874 . There may not be time forsending a suitably provided expedition from England .Onthis point it is for others to speak . I should say,

however,that unquestionably there i s time for sending

anexpeditionfrom Tasmania or New Zealand . It was

infact proposed in1 868 by CaptainRichards (Hydrographer to the Admiralty) thatNew Zealand should bemade the head-quarters of the expedition then beingplanned for observing the transit of 1 882 from Possession Island . One can see no reason why this planshould not now be resumed for securing the morevaluable observations which can be made during thetransit of 1 874 .

Ifwe inquire what has beendone towards preparingfor observations by Delisle’s method

,we shall s ee

'

that

by a very slight modification of the Government


arrangements, Possession Island might be taken as a

stationwithout any great additional expense .The transit begins ear li es t at a place innorth latitude 3 9° and west longitude 143° Woahoo

has been selected as a suitable stationnear this spot ;and infact the transit begins more than 1 1 minutesearly at Woahoo , while the sunhas anelevationat thetime of about 20 degrees . Nothing could be moresuitable than the station selected by England inthi sneighbourhood. France takes the Marque sas, whileRussia has a stationnear the mouth of the AmoorRiver.The transit begins lates t at a place in44° 27’ southlatitude

,and 26

°

27 east longitude . The best stationhereabout is Crozet Island, so far as astronomical conditi ons are concerned but bad weather very commonlyprevails here . England will send an observing partyto Kerguelen

’

s Land, and will also occupy the Man

r itius and R odriguez Island, which are not s o wellplaced ; since the transit begins 12—5 minutes lateat Crozet, 1 1—3 minutes late at Kerguelen, only 1015minutes late at Mauritius , and only 10 minutes late atRodriguez . The party at Mauritius will be that whichLord Lindsay is preparing at his ownexpense ; and itwill be amply provided with all that is required for thepurposes of exact observation. Why should not theGovernment expedition to Rodriguez be given up ?Its cost will certainly not be well repaid

,since the cir

cumstances of the transit at Mauritius and Rodriguezare almost identical ; and if the money thus saved were

38 LIGHT SCIENCE F OR LEISURE H o URS .

devoted to anexpeditionto Possession Island , a goodstep would have beenmade towards providing for thecost Of such anexpedition.

The transit will end earli es t at a place in southlatitude 64° and west longitude 1 14° Thebest stationinthis neighbourhood is that very place ,Possession Island, which affords the most favourableopportunity for applying Halley’s method . For at

Possession Island the transit will end 1 1—12minutes

early. Next in value come several islands betweenNew Zealand and Victoria Land . It was originallyproposed to have anEnglish observing party at Auckland or Wellington

, New Zealand ; but the stationat present selected is Christ Church , where thetransit will end 95 minutes early. It i s , in myOpinion, most unfortunate, that whenPossessionIslandaffords the best station for the application of Delisle’s method as well as Halley’s

, a station inferiorin both respects should be selected . Here againexpense might be saved which would go far towardsthe preparation of an expedition (from New Zealand,ifneed be) to winter inPossessionIsland .Lastly, the transit will end lates t at a place innorthlatitude 62° and east longitude 48° Here theRussians are in great force

, as Orsk, Omsk , Tobolsk,and other Russian towns are very suitably placed .The selected station for anEnglish Observing party isAlexandria, where the transit begins late by about 10minutes . The sunwill only be about 14 degrees highat the time , and agreater elevationwould be preferable .

LIGHT SCIENCE FOR LEISURE HOURS.

whereas the reputationOf the eminent man of sciencewho stands at the head of the astronomy of this countrywill inno degree be affected if the proposed expeditionbe undertaken somewhat later than was desirable , itwill suffer seriously hereafter if that expeditionshouldnot be undertakenat all.

Fraser’s Magazine for March 1873 .

THE EVER WIDENING WORLD OF STARS.

As the science of astronomy has advanced , the ideasmenhave formed respecting the extent of the universehave gradually become more and more enlarged . In

far-off times , whenastronomers were content to judgeof the conformationof the universe by the appearancesdirectly presented to their contemplation, the ideasformed respecting the celestial bodies were singularlyhomely. We read that Theophrastus looked upontheM ilky Way as the fastening of the stellar hemispheres ,which are so carelessly knitted together

,that the fiery

heavens beyond them can be seenthrough the spaces .’

Anaximenes believed the heavens to be made Of a kindof fine earthenware, and that the star s are the heads ofnails driventhrough the domed vault formed of thismaterial. And evenLucretius. whose views of naturewere so noble, has referred without disapproval to thebizarre theory of ! enophanes that the stars are fieryclouds collected inthe upper regions ofai r .

THE E VER WIDENING WORLD OF STARS. 4 1

While the Ptolemaic system of astronomy was ac

cepted there were no means of forming any trustworthyviews respecting the extent of the stellar universe . If

the earth were ever at rest we could never know howfar the stars are from us ; and therefore the old astronomers were free to invent whatever theories they pleasedas to the scale on which the sidereal scheme is constructed. It was only when the earth was set free byCopernicus from the imaginary chains which had beenconceived as holding it inthe centre of the universethat it became possible to form any conception of thedistances at which the stars lie from us . Indeed TychoBrahe immediately pointed this out as an overwhelming Objection against the new theory. Are we tosuppose ,

’ he argued, that the stars are placed at suchenormous distances from us as to seem wholly un

changed inpositionwhile the earth sweeps round thesuninanorbit millions ofmiles indiameter ? If thisamazing theory were true , the stars would be hundredsofmillions of miles from us, a view which is utterlymonstrous and incredible .’

But strange as this new view appeared, it graduallygained ground . It became presently so well establishedthat whenCassini discovered that the earth travels inamuch wider orbit thanTycho Brahe had supposedso that the stars were at once thrownmany hundredsofmillions ofmiles farther from us—astronomers stillheld to the new order of things . With Briareanarms ,’ as Humboldt has described their labours , thefellow-workers of Cass ini thrust farther and farther


away the heavenof the fixed stars,’ until the immensityof the universe grew so great beneath their labours ,that new modes of expressing its dimensions had to beadopted . They were not satisfied with the Obviouscircumstance that the stars seem to remainunchangedinpositionas the earth sweeps round the sun. Theytested this apparent fixi ty of positionwith instrumentsof greater and greater power ,— yet always with thesame result. They made observations ten, twenty,evenfifty times more exact than Tycho Brahé’s , andthe fact that they still detected no change of positionsignified nothing less than the universe of the fixedstars is ten, twenty, even fifty times farther from usthanTycho Brahe’ had imagined.Thus whenSir W. Herschel began the noble series

ofresearches amid the stellar depths which has renderedhis name illustrious, the world Of stars was alreadyof inconceivably enormous extent. Yet so widely didhe increase our appreciation of the vastness of theuniverse, that it has been thought no exaggerationtosay of him, that he broke through the barriers of theheavens Caelorumperrupit claustra,

’ says his monument at Upton, and every student of astronomy whohas carefully examined Hersche l’s labours understandsthe justice of the expression.

’A

For consider whatHerschel did . When he began his survey of theheavens, astronomers had proved indeed that the nearestof the fixed stars lie at enormous distances from us, andsome of the more advanced thinkers had begun to formnoble speculations respecting the relations of the stars

THE EVER W'IDENING WORLD OF STARS. 43

which lie beyond the sphere of those visible to us .

But it was reserved for Sir W . Herschel to apply exactobservations to the unseen star-systems . He literallygauged the celestial depths . With a telescope whoselight-gathering power extended the range of visiontoabout eight hundred times its natural limit, he sweptthe whole of the northernheavens . He estimated the.depth of the system of stars in every direction by asimple and natural process . For, like all great thinkers,he struck out modes of inquiry which, the moment theywere presented to the world, seemed s o Obvious , thatthe wonder was how they could have remained solongundetected . He said that precisely as the quantity ofwater passed through by the sailor’s lead-line marksthe depth of the s ea, so the number of stars which canbe seen when a telescope of given power is turnedtowards any part of the heavens i s a measure of thedepth of the sidereal system in that direction. Inin

dividual cases . indeed, the lawmay not be true, just asthe sailor’ s lead-line may light on the peak of somesunkenrock, and so give no true measure of the generaldepth of the s ea inthe neighbourhood . But whentheaverage Of a great number of such ‘ star—gaugings ’ istaken, then we may feel tolerably certain that on

applying the simple rule devised by Herschel we shallform no inaccurate estimates of our system’s extent inany direction.

Thence arose his great theOIy of the stellar system .

He showed that our sun is but one of an immensenumber of suns, distributed ina reg10nof space resem

44 LIGHT SCIENCE F OR LEISURE H OURS .

bling a cloven di sc in figure. .When'we look alongthe thickness of the disc we see the enormous beds ofstars, which lie round us in that directionas a cloudof milky light, which so comes to form a cloven ringround the heavens . But whenwe look out towardsthe sides of the disc, where the stars are less profuselyscattered, we see between them the black backgroundof the sky.

“

Then Herschel extended his researches to thosestrange objects called the nebulae. He showed thatwhere astronomers had reckoned tens of these obj ectsthere were inreality thousands . And he found that alarge proportion of the nebulae can be resolved intostars . He held that these, therefore , may be lookeduponas external universes, resembling that great systemof stars of which our suni s amember. We need not,at this point, dwell uponthe distinctionwhich Herscheldrew betweennebulae of this sort, and those obj ectswhich he held (and as we now know, justly) to be trueclouds, formed of some vaporous substance, of theactualnature of which he forbore to express anopinion.

Let it suffice to remark that inwhatever mode thosevaporous nebulae might be supposed to be formed, itwas clear to Herschel that they cannot be held to lieneces sari ly beyond the system of the fixed stars, as heheld to be certainly the case with the stellar nebulae.Since Herschel’s day amultitude of important dis

coveries have been made . His s on, the present SirJohnHerschel, carried the system of star-gaugings overthe southernheavens, having first trained himself for

THE EVER WIDENING WORLD OF STARS. 4 5

the work by verifying Sir William’s northernstar-gaugings . The eminent astronomer Struve and others haveapplied a serie s of tests to the basis ofHerschel’s theoryof the universe . Increased telescopic power has beenapplied to the examinationof the nebulae. And lastly

,

amode of research more wonderful than the boldestpioneers of science had ventured to hope for has beenapplied to determine what the stars and nebulae reallyare , nay eventhe very elements of which they are con

stituted .Therefore we stand ina positionso far inadvance ofthat to which it was inHerschel’s power to attain, thatthe attempt to modify his theories need no longer bethought to savour of undue boldness . Half a centurydoes not pas s without bringing a vast extension of

knowledge, and certainly the last half-century has beenno exception to this rule ; insomuch that could thegreat astronomer take his place againamong us

, and

become cognisant of the vast strides which his favouritescience has made since he left us , he would be the firstto point out that many of his views require to bemodified or evento be wholly abandoned.For instance, let us consider the meaning of thefollowing observation made by the younger Herschel .While ‘ sweeping ’ the southern heavens , this eminentastronomer noticed occasionally the existence of faintoutlying streamers belonging to the M ilky Way, not

only irresolvable into stars , but so exceedingly distantthat he could scarcely speak of them as really visible .Hewas s ens ible of their existence

,but when the eye

46 LIGHT SCIENCE F OR LEISURE HOURS.

was turned di rectly uponthem they vanished, insomuchthat, he says, the idea of illusionhas repeatedly arisensubsequently,

’ yet when he came to map down theplaces where these phantom star-streams had beendetected, he found that they formed regular branchesof the galactic system .

Now these outlying star-streams prove first of allthat the star-system is not disc-shaped, but spiral infigure . Between the stars which form the ordinarystreams of the M ilky Way, and those which form thephantom streams , there must lie regions inwhich starsare either altogether wanting or strewn with muchless profusion than in either the nearer or the fartherstream.

But this is not the only nor the chief conclus ionwhich may be drawnfrom the existence of the almostevanescent star-streams . According to Herschel’s viewsthe stars which compose those streams are only faintthrough enormity of distance . They may be as largeas our sun, many of them perhaps far larger. And

between them there may yawn distances as large asthose which separate us from Arcturus or Aldebaran.

Now , this being s o— the outlying parts of, our own

sidereal system being removed so far from us as to beall but evanescent inHerschel’s splendid reflector

how much greater ought to be the faintness of thesidereal systems which lie outside ours ! If the nebulaeare really such systems, and made up of suns like ourown, thennot only ought Herschel’s great reflector tofail in rendering them visible

,but even Lord Rosse’s

48 LIGHT SCIENCE F OR LEISURE H o URS .

and the idea is at once precluded that the eye i s w i thin

the clus ter , of whatever nature that cluster may be .Therefore the theory that the sunforms one ofa systemof stars spread pretty uniformly over a disc-shapedspace must be gi ven up ; for were it true , the approach to the M ilky Way would always be gradual.Whenwe add that in the southern skies the M ilky

Way presents the most fantastic configuration, hereexpanding into fan-shaped masses, there winding aboutinamultitude of strange convolutions , here suddenlynarrowing into a bright neck or i sthmus, there exhibiting anearly circular vacancy, it becomes clear thatthe galaxy cannot have the figure assigned to it by SirW. Herschel . It must consist of streams and spraysof stars at different distances . Such streams by theirfantastic convolutions serve to explain all the peculiariti es of the galaxy’s structure .And next, have we any evidence that the nebulae

are not really beyond the galaxy, but are mixed upwith the sidereal system ? It appears to me that wehave .Sir William Herschel noticed that there are placeswhere the nebulae are much more densely crowded thanelsewhere , and he was disposed to suspect that preci s ely as the stars by their aggregation form the zoneof the M ilky Way, so there is a zone of nebulae. ButwhenSir JohnHerschel had completed the survey ofthe heavens it was found that a very different law of

distribution made its appearance . Instead of beingcollected in a zone or band around the heavens, the

THE EVER WIDENING WORLD OF STARS. 49

nebulae are arranged in two distinct but irregularclusters, separated by a well-marked zone almost enti rely free from nebulae. And thi s zone coincides

almos t exactly w ith the Mi lky PVay.

What are we to understand by so special anarrangement as this ? A modern astronomer says it clearlyproves that the nebulae 'do not belong to the starworld ; but I can see no escape from anexactly opposite view. A simple illustrationwill s erve to exhibitthe nature of the case . Suppose a person found aspace of ground onwhich gravel was arranged in theform of a ring, and that rough stones were thicklyspread over the whole space except the gravel ring

,

would he conclude that there was no association betweenthe arrangement Of the gravel and the arrangement of the stones, because few stones were to befound on or near the gravel ? Would he not ratherfind in thi s peculiarity distinct evidence that therewas some as sociation? He would, we think , arguethat the gravel had been collected into one place andthe stones into another, inpursuance of s ome one p ar

ti cular s cheme. The corresponding conclusioninthecase of the stars and nebulae would clearly be that thestars had beendrawntogether inone directionand thenebulae inanother, out of a commonworld of cosmicalmatter.

,Inother words we should look onthe nebulae

as members of the same system or scheme that thestars belong to .

And here it may be asked how the conclusion thusdeduced from the arrangement of stars and nebulae can

E


be said to tend to enlarge our views of the world Of

stars . Onthe contrary, it might be urged, the viewswhich had prevailed before, presented us with noblerconceptions of the universe . For we were able torecognise in the thousands Of nebulae which fleck thedark background of the sky, sidereal systems as nobleas that of which our sun i s a member ; and in the

existence of countless star-systems we had a spectacleto contemplate before which the human intellect wascompelled to bow in its utter powerlessness and ins ignificance : whereas it seems as though the new viewswould reduce the scope of our vision to a singlegalaxy of stars, unless some few members of the nebularsystem may still be looked onas outer star-schemes .But ona closer inspection of the vi ews I have beenmaintaining, it will appear that they largely enhanceour conceptions of the scale on which the world of

stars i s constructed . Until now it has beenheld thatthe telescopes which man has been able to constructenabled us to scan the limits of our sidereal system ,

and to pass so readily beyond those limits as to becomesensible of the existence of thousands of other schemesas noble as our own or nobler . But if the new viewsshould be established, we should be compelled to recognise in the world of stars a system which our mostpowerful instruments are not ful ly able to gauge . Theclusters of stars , whose splendour has so worthily excited the admi ration of the Herschels, the Rosses, theStruves, and the Bonds, must be looked uponas amongthe glories of our own system

, and indicative of the

THE E VER I/VIDENING WORLD OF STARS. 5 1

multiplied forms Of structure or of aggregation to befound within its boundaries. As of late our conceptions of the wealth of the solar system have beenenhanced by the discovery of numberless new Objectsand new forms of matter existing within its range ,and co-ordinating themselves inregular relations withthe earlier knownmembers of the system, so we seemnow called on to recognise inthe stellar world anunsuspected wealth of material , a hitherto unrecognisedvariety of cosmical forms, and anextensioninto regionsof Space to which our most powerful telescopes havenot yet beenable to penetrate .But now I would call attention to a peculiarity ofthe southern skies which, while apparently affordingconclusive testimony in favour of the new vi ews, hasunaccountably (inmy opinion) been urged as anargument tending in quite another direction. There areto be seeninthose skies two mysterious clouds of li ght,which were called by the first Europeans who sailedthe southern seas the Magellani c clouds

, and are now

commonly spoken of by astronomers as the Nubeculae.

Examined by the powerful telescope of Sir JohnHers chel, these objects have beenfound to consist of smallfixed stars and nebulae, grouped together without anyevidence of special arrangement, but

i

still obvious lyintermixed,—not merely seen proj ected on the samefield of view.

These strange objects have givenrise to many speculati ons ; and among the definite views put forwardrespecting them is one recently expressed in a most

E 2


valuable communication to the Royal AstronomicalSociety from the pen of Mr. Cleveland Abbe , an

astronomer who has laboured inthe sound s chool of thePoulk owa Observatory. Having recognised in thepeculiar arrangement of stars and nebulae above re

ferred to, anargument that the nebulae lie beyond our

s ystem,Mr . Abbe suggests that the Magellanic clouds

are two of the nearest of the nebular systems, whichthus exhibit larger

,

di mensions than their fellowschemes.The converse ofthis , whichmay be termed the positivetheory of the Nubeculae, is the hypothesis which may betermed the negative theory. Whatever these obj ectsmay be, astronomers have said, they are quite distinctfrom the sidereal system , nor are the nebulae seenwithinthem to be looked upon as fellows of the othernebulae. For in the Nubeculae we see what we recognise nowhere else , the combinationnamely of clusteringgroups of stars and freely scattered nebulae . It i s thecharacteristic (still I am quoting the theory) of thesidereal system that where its splendours are greatestnebulae are wanting ; it i s the characteristic of nebularaggregationthat it withdraws itself inappearance fromthe neighbourhood of clustering star groups . But inthe Magellanic clouds neither of these characteristics i sto be recognised ; therefore these obj ects are distinctfrom either system.

Nor has another argument beenwanting to indicatethe distinctionthat exists betweentheMagellanic cloudsand the other splendours of the celestial vault. Sir

THE EVER WIDENING WORLD OF STARS. 5 3

JohnHerschel, sweeping overtheir neighbourhood withhis 18-inch reflector, was struck with the singular barrenne s s of the skies around them . With that express ive verbiage which gives s o great a charm to hi s astrom

nomical descriptions, he forces onour attention, againand again, the poverty of the regi ons which lie aroundthe Nubeculae. Oppressively

“

barren he describesthem inone place ; the access to the Nubeculae onall

sides i s through a desert,’ he says inanother. And this

peculiarity, thus established by the certainevidence ofanobserver so able and trustworthy, has been held bymany to imply inthe clearest and most distinct mannerthat there i s no connectionbetweenthe Nubeculae and

the stellar system.

To me the evidence afforded by the barrenness of theregions round the Magellani c clouds points irresistiblyin the Opposite direction. Why should some outersystem , free as i s assumed of all associationwith ourown, occupy that peculiarly barrenSpace which so attracted the attention of Sir. John Herschel ? But ifwe look onthe coincidence as striking in the case ofone , how much more remarkable will it appear whenthe only two outer systems of the sort, thus broughtwithinour ken

,are associated inthis way with the most

s ingularly barrenregioninthe whole heavens ! Surelythe more natural conclusion to be drawn from thephenomenon i s that the richness of the Magellanicclouds and the poverty of the surrounding districtss tand to each other in the most intimate correlation.

Is there not reason for concluding that those districts


are poor because of the action of the same process ofaggregationwhich has attracted withinthe Nubeculaea larger share thanusual ofstellar andnebular gloriesIt need hardly be mentioned that the former argument, onwhich the distinction betweenthe Nubeculae

and other celestial obj ects has beenfounded, i s disposedof at once i f we recognise the stellar and nebularsystems as inreality forming but a single scheme . Notonly so , but the Nubeculae afford a striking argumentinfavour of the latter view. To returnto the somewhathomely illustrationmade use ofabove . Our conceptionsof the original associationbetweenthe stones and thegravel arranged inthe manner indicated would certainlybe strengthened, or would even be changed into absolute certainty, ifwe perceived ina part of the groundtwo heaps inwhich stones and gravel were intermixed .When I add that there are two distinctly markednebular streams leading towards the Nubeculae, as wellas several well-marked star-streams tending in the

same direction, the evidence of associationseems greatlystrengthened.If these views be accepted, we shall have to look uponthe world of stars as made up ofall classes of clusteringaggregations, besides strange wisps and sprays extending throughout space inthe most fantastic convolutions .Thenalso, while dismissing the idea that the nebulae as

Sir WilliamHers chel has recorded a peculiar ity respecting nebulaewhi ch 18 worthy of mention in connection with the facts above cons idered. I have found, ’ he says , that the spaces preceding nebulae weregenerally quite deprived of s tars , so as often to afford many fieldswithout a s ingle s tar .

’


apparent movements of the stars , taught any otherlesson. It has , indeed, shown that the stars are evenmore steadfast than they seem, inso far as it teachesthat their diurnal and annual motions are but apparent,while the great precessional swaying of the star

-sphereis but the reflexi onof the earth’s gyration. M ore andmore just

,so far as these motions are concerned, has

appeared the title of the fixed stars,’assigned by

astronomers to the suns which people space .Yet the depths di splayed to our View inthe stillness ofthe calmest and clearest night are , inreality, astir withthe most stupendous activity. The least of the orbs wesee—some star so faint that it is only discerned bymomentary gleams— is the abode of forces whose actionduring a single instant surpasses ineffect all the forcesat work uponthe earth during a decade of years . Allthe wonderful processes taking place withinand aroundthe globe of our ownsun have their analogues in thatdistant orb . Let it be remembered also that our sunhimself presents an aspect which inno sense suggestshis real condition. If we would picture him as heactually is , we must consider the uproar and tumultwhich prevail where , to our ordinary perceptions, allseems at perfect rest. The least movement on thatglowing photosphere represents the actionof forces sotremendous that they would be competent to destroyinaninstant this earth onwhich we live . The mosthideous turmoil, outvying amillion-fold the roar of thehurricane or the crash of the thunderbolt

,must prevail

for ever inevery part of the solar atmosphere . And in

M OVEMENTS IN TH E STAR-DEPTHS. 5 7

whatever respects other suns may differ from our own,

inthis at least we know that they resemble him. It i sthe very charter of their existence as suns— as realliving centres of energy to schemes of circling worldsthat they should thus continually pulsate with theirown vitality. Each i s the central engine on whoseinternal motions the life ofa system ofworlds dependsand each must, with persistent activity, work out itspurpose , until the fuel which supplie s its forces shall beexhausted.All the evidence as yet obtained points to the conclusionthat our ownsun, wonderful as i s his structur e andstupendous his energy, is yet very far inferior insplendour and power to most of his fellow suns . Placed whereSirius is, the sunwould appear but as a third-rate star ,less bright than hundreds of the stars visible to theunaided eye . But removed to the distance of Aldebaran, or Castor, or Betelgeux, our sunwould certainlynot shine more brightly than the four th-magnitudestars, while it i s probable that his lustre would be soreduced that he would be barely discernible . Therecanbe little doubt that of all the star s seen on theclearest and darkest night, there are scarce fifty whichare not far larger suns thanours

,and consequently the

scene of more tremendous processes of change .But when we turn from the consideration of the

energy and vitality of individual stars to inquire intothe movements taking place withinthe star system, we

are yet more startlingly impressed by the contrast between the apparent rest prevailing inthe star-depths

58 LIGHT SCIENCE FOR LEISURE H O URS.

and the inconceivable activity really present there . It

seems incredible that all those orbs which look so stillare speeding through space with a velocity comparedwith which every form of motion familiar to us onearth must be regarded as almost absolute rest . Thisappears even more surprising whenwe consider thatduring all those centuries with which history deals

,

during the rise and fall of the nations of antiquity,

during the darkness of the M iddle Ages,during the

more familiar scenes of recent centuries,the stars

have presented anaspect so constant that if the Chaldaeanastronomers could be restored to life

,they would

recognise scarce any change inthe positions of the starsforming the ancient constellations . Yet there are noastronomical facts more thoroughly established thanthose which relate to the motions of the stars . Thegiant orb of Sirius , exceeding our sunathousand timesinvolume, Capella and Procyon, the glories of Orion,the clustered Pleiads , Arcturus, Vega, and Aldebaran,all the stars knownto the astronomer , are urging theirway with inconceivable velocity, each onits owncourse,though doubtless all these motions are subordinated tosome as yet unexplained system of movements wherebyall the star s of the galaxy are made to form parts ofone harmonious whole .Until lately it had only been by one method of

observationthat the astronomer could assure himselfthat these motions were taking place .

’ That method isthe simplest conceivable . If a star’s place were accurately determined, either with respect to neighbouring

M OVE ZIIENTS IN THE STAR-DEPTHS. 59

stars or to the imaginary circles and points on thesphere which are determined by the earth ’s movementsof rotation and revolution, then, if the star be reallyin motion

, a change of place must in the long run

manifest itself, not indeed to ordinary vision, but tothe pierc ing scrutiny and to the yet more remarkablemeasuring powers of the astronomical telescope . Ahundred years may elapse before the motionis measureable, yet the astronomer cannone the less certainlyassure himself that the motioni s taking place , since hehas the records of those who have gone before him , and

the means of satisfying himself that those records aretrustworthy.It had long been felt, however, that there was an

unfortunate gap in the evidence respecting stellarmotions . The astronomer

.

could tell how much or howlittle the stars were shifting on the heavens , but hecould obtainno measure whatever of other motionswhich nevertheless must exist among the stars . If a

star were receding or approaching, no trace whateverof such motioncould be recognised . No instrumentalmeans could enable the astronomer to measure thechange of brightness due to the star’ s change of distance, since such changes must needs be infinitely smallcompared with the actual lustre of the star.So that it seemed as though the astronomer must forever remainignorant of one most important portionof

the stellar motions . All he could do, as it appeared ,was to watch the aspect of the heavens

, and, as it slowlychanged, to infer inwhat way the stars were moving

60 LIGHT SCIENCE F OR LEISURE H 0 URS.

athwart the line of vision; and eventhis he could onlydo most imperfectly, since hi s knowledge of the di stances of the stars i s so limited that he can form butinexact notions of the rate at which

.

the stars are s o

moving. Theymay be very far away and moving veryswiftly

, or theymay be at a less (though still enormous)distance and moving with a correspondingly reducedvelocity.This source ofdifficulty was very strikingly illustratedwhenthe subject of the stellar motions was treated inconnectionwith the ideas respecting the sidereal universe promulgated by Sir W. Herschel. In the hypothes i s which regarded the stars as spread with a

certaingeneral uniformity through a stratum or sliceof space, there was no feature which afforded any promise that by the study of the stellar motions themysteries of the sidereal universe might be interpreted .The very basis of Sir W. Herschel’s ownresearches intothe subj ect is the vague suppositionthat it is as likely61.p ri ori that any givenstar will move inone directionas in another. Later we find Struve presenting hisresults in the following form: One may wager fourhundred thousand to one that a portionof the seemingmotions of the stars i s due to the sun’ s motion, and itis anevenwager (onp eutp ar i er wa centre an) that thelatter motiontakes place at the rate of between1 3 5 and175 millions of miles per annum.

’ The whole questionhad become one of probabilities

,based onmore or less

trustworthy assumptions . We cannot wonder greatlythat, when Sir Cr. Airy undertook the complete rev

M OVEMENTS IN THE STAR-DEFTHS. 6 r

examinationof the matter twenty years ago , the resulthe obtained, while indicating the general probabilityof the inferences before obtained,nevertheless exhibitedthe whole problem as oneneeding further investigation.

‘

It will be seen presently that we cannot too attentively regard those earlier researches , if we would fullyestimate the importance of the results which haverecently been obtained. Let it be carefully noticedthat the earlier results flowed directly from the hypothesis respecting the stars which have so long maintained their ground in our text-books of astronomyIf these hypotheses are sound, the results flowing fromthem , eventhough only based onthe general principlesof probability as applied to those hypotheses, might beexpected to be somewhat near the truth . If, on thecontrary, an independent and trustworthy serie s of

results should show that those earlier results are notcorrect—are indeed very far from correctness— then

p ro tanto the hypotheses which led to those earlierresults would be invalidated .Let it thenbe clearly understood that, according tothe results in question, the stars were held to be inmotionat rates comparable ingeneral with the velocityof our sun, this velocity being estimated at about fourand three-quarter miles per second . We do not includehere the result that the sunis moving towards Hercule s,because that may be regarded as established , whatever

Thi s part ofmy subj ect i s fully dis cus sed ina paper called The

Sun’

s Journey through Space,’ which appeared inFraser’

s Magaz ine for

September 1869, and will be found among my Es says onAstronomy .

’

6 2 LIGH T SCIENCE FOR LEISURE H o URS.

opinionwe may form as to the distributionof the stars

inspace .Before proceeding to indicate the bearing of recentobservations on these theoretical conclusions, I wouldinvite some degree of attention to the circumstancethat the view I am here advancing as to the bearing ofnew facts on the old hypotheses , is not a new one

framed to account for the new facts inaway agreeingwith my owntheories respecting the stars . M ore thanthree years ago inFras er

’

s Mctr/amine, and earlier stillinthe proceedings of scientific societies, I indicated mybelief that the real facts are precisely such as have nowbeendemonstrated .Already whenI so wrote, promise had beenaffordedthat the astronomer might come intime to know,1 notmerely whether certain stars are approaching or receding, but at what rate (in miles per second) thesemotions are taking place . I need not here enter intoanexplanationof the method by which this was to beaccomplished, inasmuch as a full account of the principle onwhich the method is based is given in thepaper called News from Sirius,

’inmy Essays onAstro

nomy. Suffice it to say, that it depends onthe observeddisplacement of some knowndark line 111 the rainbowtinted streak forming the spectrum of a star, and thatwhensuch a line is displaced towards the red end of thespectrum it is known that the star is receding, while

See the clos ing words of the last paragraph but three inthe es saymentioned.

64 LIGH T SCIENCE FOR LEISURE HOURS.

star-motions observed and measured by Dr. Huggins ,i s the amazing velocity with which some of the starsare moving . Astronomers had ascertained that Si ri usi s moving athwart the line of visionmuch more rapidlythanthe sun i s travelling through space . But Siriusis so exceptional both inhis brightness and inhis estimated bulk, that his enormous velocity did not appearaltog ether surprising. It did not lead the generalityof astronomers to consider that the sun’s velocity andthe average velocity of the stars had beengreatly underestimated . Butnow we learnfrom amethod of researchwhich is far more trustworthy than any applied to themeasurement of thwart motions, that some of the starsare moving from or towards the earth with a velocityfar exceeding that of Sirius . If we take the thwartmotionof Sirius at twenty-five miles per second, andhis motionof recessionat twenty miles (this being thevalue assigned by the latest and best measurements),we find for this absolute motion the amazing velocityofabout thirty-two miles per second. But Dr . Hugginsfinds that Arcturus i s receding from the sunat the rateof 5 5 miles per second, Vega at the rate of about 5 0miles, Arided (the chief brilliant of the Swan) at therate of 3 9 miles, Pollux 49 miles, and Dubhe of theGreat Bear at the rate of from 46 to 60 miles persecond . Beside such motions as these, our sun

’s estimated velocity ofabout Al i miles per second, which hadseemed so imposing when it was considered that hebore with him at this enormous rate his whole familyofplanets, sinks into relative ins ignificance. We here

M OVEMENTS IN THE STAR-DEPTH S. 6 5

recognise stellar rates of motionnearly equalling thatat which

.

our earth circuits around the sun. But avelocity which , considered with reference to a minuteorb like the earth , i s intelligible, becomes altogetherstartling inthe case of orbs like Arcturus and Vega,which undoubtedly exceed our own sunmany times involume . I use the word ‘ intelligible with a purpose ;for I am not considering here what is conceivable orthe reverse . We can in reality unders tand why theearth should be possessed of the velocity she actuallydisplays . We know that the sun’s attractionis competent to generate such a velocity, o r amuch. greatervelocity. But inthe case ofa star these swift motionscannot be thus explained . The stars are too far apartto be so influenced by their mutual attractions thatgreat velocities would be generated . And thus thethoughtful mind cannot but recognise in the stellarmotions a subject of contemplationfar more impressivethanthe subordinate , though even swifter motions ofthe Earth , Venus , or M ercury. Whence sprang thatamazing energy which is represented by the propermotions of the suns ? If we admit the possibility thatforces of eruption or expulsion could account for theobserved motions , we shall have to answer the startlingquestion

,Of what order are the orbs whence the giant

suns are expelled ? and the yet more difficult questions ,Where are these orbs ? and, How i s it that, inordinatelylarge though they must be , we are yet unable to distinguish them from ordinary suns ? If

,on the other

hand, we prefer to regard the stellar velocities as geneF

6 6 LIGHT SCIENCE F OR LEISURE H o URS.

rated by the attractions of larger orders of bodies than

the stars (as planetary velocities may be regarded asgenerated by their parent suns), we still have the lasttwo questions to answer ; and, so far as canbe judged,these questions are at present unanswerable .

l

Another striking feature inthe results announced byDr . Huggins i s the absence of any systematic agreement between the stellar motions he has recognised ,and the motion of our sun towards Hercules . It i smanifest that if our sun were alone in motion, theactual rates of approach and recessionof all the starsinthe heavens would be at once determined whentherate of the sun’s motion was determined . If, for example

,he were moving at the rate of twenty miles per

second towards the star Lambda of Hercules, he wouldbe approaching every star lying in that direction at

Inpas s ing , however , I would venture to touch onthi s ques tionof

central suns , or of central but opaque orbs round which the s tars mayrevolve , inorder to remove a very prevalent mis conception. It s eems

to be commonly suppo s ed that we cannot imag ine such orbs to lie farenough away to account for their not being dis cernible ei ther as orbs oflight or by hiding more dis tant s tars , w ithout depr iving them of the

attractive influence neces sary to sway the motions of the s tars . Thi s ,

however,i s not the cas e . An orb look ing as bright as Sirius , but ten

times as far away, if of equal dens ity and inherent brightnes s , would bea thousand times more mas s ive , whi le the effect

’

of dis tance would onlybe to reduce its attraction one hundred times . It would, therefore,attract our suntentimes as s trongly as Si r ius actually doe s . In likemanner, anorb one hundred times as far away as Sirius , but s o large

as to appear as br ight, would attract our sun one hundred times as

s trongly, and s o on. So that it cannot be pos itively as s erted that

among the s tars vi s ible to us there may not be the central sun of the

s idereal s cheme— inordinately large and mas s ive compared with the

res t, but re duced by di s tance to the same order of br ightnes s .

M OVEMENTS IN THE STAR-DEPTHS. 6 7

the same rate ; he would be receding from all starslying inthe opposite directionat the same rate ; andhe would be approaching o r receding from stars lyinginopposite directions at a less rate (readily calculable ).A certain half of the heavens would contain all thestars which the sun was approaching ; the other halfwould containall the stars fromwhich he was receding ;and the circle separating these halves would mark theplace of stars which the sunwas neither receding fromnor approaching. But nothing of this sort can berecognised in the obs erved stellar rates of approachand recession. Sirius (which lies nearly opposite to

Hercules) i s receding at the rate of about 20 miles persecond ; but Vega (which lies Close to Hercules), insteadof approaching at about the same rate

,is actually

approaching at the rate of about 5 0 miles per second .Castor , which is very near the border line betweenthetwo hemispheres just mentioned, and should thereforeneither be approaching nor receding, is infact receding at the rate of about 2 5 miles per second ; whilePollux

,though similarly placed, i s approaching the sun

at the rate of about 49 miles per second. Again, of theseven bright stars forming Charles’s Wain, six areapproaching (five of them at the rate of about 20 milesper second), while the seventh i s receding at a rateprobably exceeding 5 0 miles per second .Thus we s ee that the sun cannot be regarded as an

orb moving withinthe scheme of stars,and by his own

movement causing the chief apparent motions of thesurrounding orbs. His motionis but part of a grand

F 2

6 8 LIGHT SCIENCE FOR LEISURE H 0 URS.

scheme of motions, whose laws are as yet unknowntous . We may recogni se inthe method of research whichhas now beenso successfully applied, the sole means ofdetermining what those laws may be. We cannow tellthe very rate, in miles per annum, at which the sunsare approaching or receding from us ; and though wehave no reasonfor believing that our sun occupies inany sense a central position—so that we have yet tolearn at what rate and in what way the stars movearound the true centre of their system ,

—yet it is farfrom unlikely that if we canbut ascertainthe motionsof a sufficient number of stars, we shall have themeans of judging where the centre lies round whichthese motions are taking place .The astronomer may well look wi th doubt, however,

onthe efforts which are being made to solve this stupendons problem. If we may judge from the analogyof our ownsolar system, we can see that in the farmore complicated scheme of the stars there must exis tinnumerable features to perplex the observer. If weimagine abeing placed inthe midst of the solar system

,

and enabled to study the various apparent motionsvisible from hi s stand-point, and if we further suppose

from him, thenwe know that if his scrutiny were butcontinued long enough

,he could not fail to recogni se

the laws which exist withinthat system and regulateall those motions . Where at first all had seemed confusmn, our imaginary observer would recognise inthe

M OVEMENTS IN THE STAR-DEFTH S. 69

course of time abeautiful harmony ; motions whi ch hadappeared discordant would be found to be inrealitysubordinated into one grand scheme . But ifwe supposeour observer to occupy hi s imaginary stand-point for afew hours, or evenfor a few days only, how imperfectwould be his ideas of the harmony of the celestial motions ! He would see the primary planets movingapparently in diverse directions and at inconsistentrates ; the secondary planets apparently travelling withnon-accordant motions and on different paths ; theasteroids would perplex him by their wide range ofapparent distribution; meteoric systems would appearto conform to no recognisable law ; and the movementsof comets would seem altogether inexplicable .Yet the terrestrial observer of the infinitely morecomplicated sidereal system i s in reality even lessfavourably circumstanced thanour imaginary observerof the planetary scheme . The motions which comewithin his k en are more minute, compared with thereal dimensions of the stellar paths, thanthe motionof

Saturnor Jupiter ina single second compared with thewide orbits traversed by these planets . We cannot tellwhether the observed motionofa star IS that by whichit is carried onsome vast independent orbit ; o r i s itsmotionwithinsome subordinate scheme ; or , lastly, isfor the most part due to the sun’s ownmotionwithinthe sidereal system . When we see the stars of thesame constellation carried in different directions

,we

cannot tell whether the real motions are diverse incharacter, or whether the diversity is but apparent,

7c LIGHT SCIENCE F OR LEISURE H o URS.

like the apparent advance and retrogression of planetswhich , nevertheless, are travelling ina commondirectionaround a commoncentre .But precisely because the difficulties which sur roundthe problem of the stellar motions are so stupendous,we must so much the more Carefully examine everyfeature which observationmay reveal to us . To dootherwise were to abandon the problem as altogetherhopeless .Now it cannot but be recognised that inthis respectthe new method of research is peculiarly promising.For whereas all former methods have dealt only withapparent motion, this method tells us of the real rateof stellar displacements . We have seen how it hasdisposed of the inferences which had beenformed as tothe sun’s velocity, and the average velocities of stellarmotion; let us inquire what has been its bearing onthe views of astronomers respecting the stellar universeregarded as a scheme or system .

Other methods of dealing with the motions of thestars had related chiefly to the question of the sun’sj ourney through space, until Madler was led to inquirewhether the motions of the stars might not afford themeans of determining where the centre of the stellarsystem may lie . Limiting his range of inquiry, inthefirst instance , by certainpreliminary considerations, heproceeded to examine the direction of the apparentstellar motions ina particular region of the heavens .It seemed lik ely to him that the centre of the universewould be near the M ilky Way, and probably onthat

72 LIGHT SCIENCE FOR LEISURE H o URS.

brightest of the P leiades, as the central o rb aroundwhich all the stars revolve .Now to such a problem as thi s—a problem whosegrandeur cannot but be recognised evenby those whoreject the conclusions adopted by Madler— the newmethod of research is applicable with peculiar force.For instance, if the stars of Taurus are circling rounda particular orb also inTaurus, it will be manifest, ona moment’s consideration, that they can have only aslight motion either of recessionor approach with re

spect to the sun. When from our s tationonthe earthwe see Venus or Mercury nearly inthe same directionas the sun, we know that at the moment either planethas only a thwart motion, be ing then either at itsgreatest or least distance from us . So that if the newmethod were applied to stars in Taurus, and showedthat swift motions of recessionor approach are there inprogress, it would at once dispose of the attractive buttoo speculative theory of the Germanastronomer.This has not yet been accomplished ; in fact, since

Dr . Huggins’ instrument was mounted and in order,the constellationTaurus has not beenwell placed forobservationby the new method . But inthe meantime,evidence of the most convincing nature has beenob

tained to Show that Madler’s theory is unsound .We have seenthat the theory was based, inthe main,

on a certain general community of apparent motionamong the stars inTaurus . Madler took it for grantedthat this community of motioni s exceptional. It didnot occur to him to examine the motions of stars in

M OVEMENTS IN THE STAR-DEPTH S. 73

other parts of the heavens , to see whether perchance alike feature might not present itself elsewhere .Having been myself led by other inquiries than

Madler’

s to the conclusi on that the stellar motionsmight afford useful information as to the structure ofthe heavens, I thought it desirable to make a chartshowing all the known stellar motions in such a waythat wherever a community of directionexists it wouldbe at once apparent inthe chart. Little arrows afli xedto the star-discs onthe map, showed by their directionand length the nature and amount of the stellar thwartmotions . When the map was completed, it was easyto s ee that the community of motion in Taurus wasonly one instance, and by no means the most strikingwhich could be recognised, of a phenomenonwhich Ihave since called s tar-draft. Certainsets of stars areseento be moving athwart the heavens, nearly in thesame direction, and nearly at the same rate , in suchsort as to show that they form distinct families of suns ,travelling onwards— each family as a single groupthrough the celestial spaces.If this view is just, Madler’s theory is at once shown

to be unsound ; since the stars inTaurus thus appearas simply a drifting family of stars

, one among severalsuch families .All that was required to make the proof convincing

was , that one of these sets of drifting star s should beshown to be either approaching the earth or recedingfrom it as a single group .Now, among the instances of star—drift, there was

74 LIGHT SCIENCE F OR LEISURE IIO URS.

one in the Great Bear which presented some verystriking features . Five stars in this constellation,known as Beta

,Gamma, Delta, Eta, and Zeta, were

seen to be travelling, not merely at the same rate andin the same direction, but ona course precisely opposite to that which they would have bad if their apparentmotion had been due to the sun’s motion in space .M oreover, all these stars are large and conspicuous ;while one of them, Zeta, i s distinguished by havingtwo companions , one very close to it, and the other sofar away that its motionaround Zeta is only completed

(according to Madler’

s computation) in a period of

about years ; so that, if all the five large starsform a single system

,the cyclic revolutions of the

system must require millions of millions of years fortheir completion.

I selected this family of stars as affording a con

veni ent means of testing (crucially) the accuracy ofmy theory of star-drift. If that theory is j ust, allthese stars must be either approaching or receding ata commonrate . If the theory is unsound

,the chances

are enormous against their possessing acommonmotionofapproach or recession. I expressed a strong feelingof confidence that whenever Dr. Huggins applied thenew method of research to these stars, he would findthat they are either all approaching or all receding,and at one and the same rate . WhenI expressed thisopinion, I knew that before many months had passed,the matter would be decided one way or the other.Nothing could be more complete thanthe confirma

M OVEMENTS IN THE S TAR-DEPTHS. 75

tion of my views by Dr. Huggins’ observations . In

his table of stellar motions, Dr. Huggins bracketstog ether the five stars inquestion as possessing a com

monmotion of recession at the rate of about twentymiles per second . M oreover he finds, from the natureof their spectra, that they are all alike in physicalconstitution.

It i s hardly necessary to insist upon the importanceof this result. It proves , first, that inthis instanceand therefore presumably in the other instances—of

app ar ent star-drift, there is a distinct family or groupof stars , travelling bodily onwards amidst the stardepths . It i s shown that the motions taking placewithin this star-family are small compared with thecommonmotionof the group . It canbe inferred thatthe group i s relatively isolated, since otherwise weshould find other stars in the Great Bear sharing inthe motion of these five ; and also , if there had beenadisturbing orb at amoderate distance from the group,the members of the family would ere this have losttheir uniformity of motion. Whatever may be thecentre around which these five stars are moving as asingle group, the distance of that centre must exceedenormously the dimensions of the group, pr ecisely asthe distance of the sunfrom Jupiter’s satellite familyenormously exceeds the dimensions of that system.

Yet the distances separating the stars of the GreatBear are themselves amazingly vast. The distancebetween Beta and Zeta of the Great Bear cannot beless than times the distance separating our

76 LIGH T SCIENCE F OR LEISURE H o UR S.

earth from the sun, and is probably far vaster. Whatthenmust be the distance of the centre of motion, asseen from which this enormous space i s reduced to analmost evanescent arc !It seems not unlikely that we ought to regard thefamily of stars here recognised as bearing the samegeneral relationto the stellar universe (or to that portion of it to which our sun belongs) that a group ofmeteors bears to the solar system . All the driftingstar-families may not indeed travel around one and thesame centre ; or there may be no true centre , but onlya central region, round which these movements takeplace : but it is impossible to consider thoughtfullyany instance of community of stellar motions withoutfeeling that it implies a common influence affecting inthe same or nearly the same way each member of thedrifting star-family. If there is but one such centre ,whether it be asingle orb, or acentral regionof thicklyclustering stars, there now seems to be at least a poss ibi lity that we may find where this centre lies . Whenonly a few more star-families have been recognised,and their motions ofapproach or recessiondetermined,it will be a problem of no inordinate difli culty todeduce the position in space of the regions roundwhich these motions are taking place , or else to prove(which would equally be a solutionof the problem nowbefore us) that no such regionexists , and that the starsdrift around more centres thanone.

Whatever success may attend the efforts made toexplainthe stellar motions, there canbe no doubt that

‘ M OVEMENTS IN THE STAR—DEPTHS. 77

the problem is well worthy of the most thorough investigati on. There is, indeed, something startling inthe thought that man, placed as he is ona tiny orbanorb rotating swiftly onits axis, carried swiftly roundthe sun, and borne along with him inhis swift motionthrough space—man, shortlived and weak, and unableby hi s unaided vision to perceive a thousandth part ofthe star-system, should yet attempt (and not unhopefully) to master the secret of its structure and motions .It may be that what has hitherto beendone is but thebeginning of the series of labours by which

,i f ever,

that end will be accomplished ; or it may be that weare nearer to the mastery of the problem thanwe atpresent imagine : but, in any case, there i s but onecour se by which success can be achieved . Piece bypiece the facts on which our reasoning is to dependmust be gathered together ; while at every stage of theinquiry, the full meaning of observed facts must be asfar as possible evolved. Success will not be obtainedby observation alone , nor by theorising alone ; but bythat combination only of observation and theory towhich we owe all the most important discoverieshitherto effected by astronomers .

Fraser ’s Magazine for November 1872 .

78 LIGHT SCIENCE FOR LEISURE H 0 URS.

THE GREAT NEB ULA IN ORION.

DURING the first four months of the year, the constellation Orion is very favourably situated for observation

in the evening. This magnificent asterism i s moreeasily recognised than the G reat Bear, Cassi opeia

’sChair, or the fine festoon of stars which adorns theconstellationPerseus . There is, indeed, a peculiarityabout Orion which tends considerably to facilitaterecognition. The other constellations named above ,gyrate round the pole in a manner which presentsthem to us incontinually varying positions . It i s notso with Orion. Divided centrally by the equator, themighty hunter continues twelve hours above and twelvehours below the horizon. His shoulders are visiblesomewhat more , his feet somewhat less, than twelvehours . Whenhe is inthe south , he is seenas a giantwith uprai sed arms, erect, and having one knee bent,as if he were ascending a height. Before him

,as if

raised onhis left arm, i s a curve of small stars , formingthe shield, or target of lion’s skin, which he 'i s represented as uprearing inthe face of Taurus. WhenOrionis inthe east, his figure i s inclined backwards : whenhe is setting, he seems to be bent forwards, as if rushing downa height ; but he is never seeninaninvertedposition, like the northernconstellations .And we maynote inpassing, that the figure of Orion,

80 LIGHT SCIENCE FOR LEISURE H o URS.

In an Opera-glass this phenomenon i s yet more easilyrecognisable . A very small telescope exhibits thecause of the peculiarity, for it is at once seen, thatwhat seemed a star i s inreality amass of small stars

intermixed with a diffused nebulosity.It is a very remarkable circumstance that Galileo ,whose small telescope, directed to the clear skies of

Italy, revealed so many interesting phenomena, failedto detect

That marvellous round ofmilky light

Below Orion.

It would not, indeed, have beenvery remarkable if hehad simply failed to notice this object. But he wouldseem to have directed his attention for some timeespecially to the region in the midst of which Orion’snebula is found. He says : ‘ At first I meant to delineate the whole of this constellation; but onaccountofthe immense multitude of stars— being also hamperedthrough want of leisure -I left the completionof thisdesign till I should have another opportunity.’ Hetherefore directed his attention wholly to a space ofabout ten square degrees, between the belt and sword,inwhich space he counted no less thanfour hundredstars . What is yet more remarkable

,he mentions the

fact that there are many small spots onthe heavensshining with a light resembling that of the M ilkyWay

(comp lures s imtlts color i s ar eolce sp ar s im p er cethera

subfulgeant) ; and he even speaks of nebulae of thissort in the head and belt and sword of Orion. Heasserts, however, that by means of his telescope , these

THE GREAT NEB ULA IN ORION.8 I

nebulae were distinctly resolved into s tars —a circumstance which

,as we shall see presently, renders hi s

description wholly inapplicable to the great nebula.

Yet the very star around which (inthe naked-eye view)this nebula appear s to cling, i s figured in Galileo’sdrawing of the belt and sword of Orion!It seems almost inconceivable that Galileo shouldhave overlooked the nebula, assuming its appearancein his day to have resembled that which it has at present. And as it appears to have beenestablished , thatif thenebulahas changed at all during the past centuryit has changed very slowly indeed

, one can scarcelybelieve that inGalileo’s time it should have presenteda very different aspect. Is it possible that the viewsuggested by Humboldt is correct— that Galileo di dnot see the nebula because he did not w i sh to see it ?Galileo ,

’ says Humboldt, was disinclined to admit orassume the existence of starless nebulae.’ Long afterthe discovery of the great nebula inAndromedaknown as ‘ the transcendently beautiful queen of thenebulae —Galileo omitted all mentioninhis works ofany but starry nebulae. The last—named nebula was

discovered in 16 14 , by SimonMarius, whose claims tothe discovery of Jupiter’s satellites had greatly angeredGalileo , and had called forth a torrent of invective , inwhich the Protestant Germanwas abused as a hereticby Galileo , little aware that he would himself beforelong incur the displeasure of the Church . If we couldsuppose that an unwillingness, either to confirm hisrival’s discovery of a starless nebula, or to acknowledge

G


that he had himself fallen into an error onthe subject

ofnebulae, prevented Galileo from speaking about thegreat nebula inOrion, we should be compelled to formbut a low Opinion of his honesty. It happens toofrequently that

The manof s cience hims elf i s fonder ofglory , and vainAneye well practis ed innature, a spir it bounded and poor .

That Hevelius, the star-cataloguer,’ should have

failed to detect the Orionnebula i s far less remarkable ;for Hevelius objected to the use of telescopes inthework of cataloguing stars . He determined the positionof each star by looking at the star thr ough minuteholes or pinnules , at the ends of a long rod attached toaninstrument resembling the quadrant.The actual discoverer of the gr eat nebulawas Huyghens, in1 65 6 . The description he gives of the discovery is so animated and interesting, that we shalltranslate it at length . He saysWhile I was observing the variable belts of Jupiter,

a dark band across the centre of Mars, and some indistinct phenomena on his disc, I detected among thefixed stars an appearance resembling nothing whichhad ever been seen before, so far as I am aware .This phenomenon can only be seen with large telescopes such as I myself make use of. Astronomersreckonthat there are three stars inthe sword of Orion,which lie very close to each other . But as I was looking , inthe year 1 65 6 , through my 2 3-feet telescope, atthe middle of the sword, I saw, in place of one star,no less thantwelve stars—which indeed is no unusual

THE GREAT NEB ULA IN ORION. 83

occurrence with powerful telescopes . Three of thesestars seemed to be almost in contact, and with thesewere four others which shone as through a haze, sothat the space around shone much more brightly thanthe rest of the sky. And as the heavens were sereneand appeared very dark, there seemed to be a gap inthis part, through which a view was disclosed of

brighter heavens beyond . All this I have continuedto see up to the present time [the work inwhich thes eremarks appear— theSys temaSaturntnm—was published in so that this singular object, whatever it i s ,may be inferred to remain constantly in that part ofthe sky. I certainly have never seen anything resembling it in any other of the fixed stars . For otherobjects once thought to be nebulous , and the M ilkyWay itself, show no mistiness whenlooked at throughtelescopes, nor are they anything but congeries of starsthickly clustered together.’

Huyghens does not seem to have noticed that thespace between the three stars he described as closetogether is perfectly free fromnebulous light— insomuchthat if the nebula itself is rightly compared to a gap inthe darker heavens, this spot resembles a gap withinthe nebula. And indeed, it i s not uninteresting tonotice how comparatively inefficient was Huyghens’

telescope, though it was nearly eight yards in focallength . A good achromatic telescope two feet longwould reveal more thanHuyghens was able to detectwith his unwieldy instrument.Dominic Cassini soon after discovered a fourth star

G 2

84 LIGHT SCIENCE FOR LEISURE H O URS.

near the three seen by Huyghens . The four form thecelebrated trap ezium, anObject interesting to the poss es sors of moderately good telescopes , and which hasalso beena subject of close investigation by professedastronomers . Besides the four stars seen by Cassini ,there have been found five minute stars within and

around the trapezium. These tiny objects seem toshine with variable brilliancy ; for sometimes one wi llsurpass the rest, while at others it will be almostinvisible.After Cassini’s discovery, pictures were made of thegreat nebula by Picard , Le Gentil, and Messier . Thesepresent no features of special interest. It is as weapproach the present time, and find the great nebulathe centre ofquite a little warfare among astronomers—now claimed as an ally by one party, now by theiropponents—that we beginto attach analmost romanticinterest to the investigationof this remarkable object.In the year 1 8 1 1 , Sir W. Herschel announced thathe had (as he supposed) detected changes inthe Orionnebula. The announcement appeared in connectionwith a very remarkable theory respecting nebulae generally—Herschel’s celebrated hypothesis of the conversion of some nebula into stars . The astronomicalworld now heard for

[the first time of that self-luminousnebulous matter , distributed in a highly attenuatedform throughout the celestial regions, which Herschellooked uponas the material from which the stars havebeen originally formed . There is an allusion to thistheory inthose words of the Princess Ida

THE GREAT NEEULA IN ORION. 85

There s inks the nebulous s tar we call the Sun,If that hypothes i s of theirs be s ound.

And inthe teaching of comely PsycheThis world was once afluid haze of light,Ti ll toward the centre s et the s tarry tides ,And eddied into suns , that wheeling cas t

The ,planets .

Few theories have met with a stranger fate . Receivedrespectfully at first onthe authority of the great astronomer who propounded i t—then in the zenith of hisfame— the theory gradually found a place innear ly allastronomical works ; But

,inthe words ofa distinguished

living astronomer, The bold hypothes is did not receivethat confirmation from the labours of subsequent in

quirers which is so remarkable inthe case of many ofHerschel’s other speculations .’ It came to pass at lengththat the theory was looked uponby nearly all Englishastronomers as wholly untenable . In Germany it wasnever abandoned, however , and agreat moderndiscoveryhas suddenly brought it into general favour, and has inthis, as inso many other instances , vindicated Herschel

’sclaim to be looked upon as the most clear-sighted, aswell as the boldest and most original of astronomicaltheorisers .Herschel had pointed out various circumstance swhich, inhis opinion, justified a belief inthe existenceof a nebulous substance—fire-mist or star-mist, as ithas been termed— throughout interstellar Space . Hehad discovered and observed several thousand nebulae,and he considered that amongst these he could detecttraces of progressive development. Some nebulae were ,

86 LIGH T SCIENCE FOR LEISURE H O URS.

be supposed, comparatively r

young ; they showed nosigns of systematic aggregationor of central condensa

tion. Insome nebulae he traced the approach towardsthe formation of subordinate centres of attraction;while in others, again, a single centre began to benoticeable . He showed the various steps by whichaggregationof the former kind might be supposed toresult inthe formation of star-clusters, and condensationof the latter kind into the formati on

’

of conspicuoussingle stars .But it was felt that the strongest part of Herschel’scase lay inhis reference to the great nebula of Orion.

He pointed out that amongst all the nebulae whichmi ght be reasonably assumed to be star-systems, a cer

tain proportionality had always been found to existbetweenthe telescope which first detected anebula andthat which effected its resolutioninto stars . And thiswas what might be expected to happen with starsystems lying beyond our galactic system. But how

'

diflerent is this from what was seen inthe case of theOrionnebula. Here is anobject so brilliant as to bevisible to the naked eye, and which is found onexami

nationto cover a large region of the heavens . And

yet the most powerful telescopes had failed to show theslightest symptom of resolution. Were we to believethat we saw here a system of suns so far off that notelescope could exhibit the separate exi stence of thecomponent luminaries ,and therefore (considering merelythe observed extent of the nebula

,which is undoubtedly

but‘ a faint indicationof its real dimensions) so incon

88 LIGHT SCIENCE F OR LEISURE H O URS.

that . which he had applied to the northernheavens .

Inthe clear skies of the southernhemisphere the nebulashines with a splendour far surpassing that which it hasinnorthernclimes . It is also seenfar higher above thehorizon. Thus the drawing which Si r J. Herschel wasable to execute during his three years’ residence at theCape

'

is among the best views of the great nebula thathave ever beentaken. But evenunder these favourablecircumstances, Sir John records that the nebula,through hi s great reflecto r, showed not a symptom of

re solution.

’

ThenLassell turned hi s powerful mirror, two feet indiameter, uponthe unintelligible nebula. But thoughhe was able to execute a remarkable drawing of theobject, he could discernno trace of stellar constitution.

In 1 845 Lord Rosse interrogated the great nebulaWith his three-feet mirror . Marvellous was the com

plexi ty and splendour of the object revealed to him,but

not the trace ofa star could be seen.

The end was not yet, however . Encouraged by thesuccess of the three-feet telescope

, Lord Rosse com

menced the constructionof one four times as powerful.After long and persistent labours, and at a cost

,it is

said , of thirty thousand pounds, the great Parsonstownreflector, with its wonderful six-feet speculum

, was

directed to the survey of the heavens. At Christmas,1845 , while the instrument was yet incomplete

, and in

unfavourable weather, the giant tube was turnedtowards the Orionnebula. Professor Nichol was thefirst who saw the mysterious obj ect as pictured by the

THE GREAT NEE ULA IN ORION. 89

great mirror. Although the observationwas not succes sful so far as the resolutionof the nebula was concerned, yet Ni chol

’

s graphic account of the telescope’sperformance is well worth reading :Strong ly attracted inyouth by the lofty conceptions

ofHerschel [he writes], I may be apt to surround theincident I have to narrate with feelings in so far of apersonal origin and interest but, unless I greatlydeceive myself, there are few who would view it otherwise than I. With an anxiety natural and profound,the scientific world watched the examinationof Orionby the six-feet mirror ; for the result had either toconfirm Herschel’s hypothesis

,in so far as human

insight ever could confirm it ; or unfold among theste llar groups a variety of constitutionnot indicatedby those inthe neighbourhood of our galaxy. AlthoughLord Rosse warned me that the circumstances of themoment would not permit me to regard the decisionthen given as abs olutely final, I went in breathlessinterest to the inspection. Not yet the veriest trace ofa star ! Unintelligible as ever

,there the nebula lay ;

but how gorgeous its brighter parts ! How countlessthose streamers branching from it onevery side 1 Howstrange , especially that large horn onthe north , risinginrelief from the black skies like avast cumulous cloudIt was thus still possible that the nebulawas i rresolvable by humanart ; and so doubt remained. Why theconcurrence of every favourable conditionis requisite forsuccess insuch inquiries -may be readily comprehended .The object in view is to discern, s ing ly, sparkling

90 LIGHT SCIENCE FOR LEISURE H OURS.

points, small as the point of a needle, and close as theparticles of a handful of sand ; so that it needs but thesmallest unsteadiness inthe air, or imperfection intheshape of the reflecting surface, to scatter the light ofeach point, to merge them into each other, and presentthem as one mass.Before long Lord Rosse , after regrinding the greatmirror

,obtained better views of the mysterious nebula.

Evennow,however, he could use but half the power of

the telescope, yet at length the doubts of astronomersas to the resolvability Of the nebula were removed .‘We could plainly see ,

’ he wrote to Professor Nichol,that all about the trapezium was amass of stars, therest of the nebula also abounding with stars, and exhibiting the characteristics of resolvability stronglymarked .’ These views were abundantly confirmed bysubsequent observations with the great mirror .

It will surprise many to learnthat evenLord Rosse’sgreat reflector is surpassed incertainrespects by some ofthe exqui site refractors now constructed by Opticians .As alight-gatherer the mirror is

, of course, unapproachable by refractors . Evenif it were possible to constructanachromatic lens six feet indiameter

,yet

,to prevent

flexure, a thickness would have to be givento the glasswhich would render it almost impervious to light andtherefore utterly useless . But refractors have a powerof definitionnot possessed by large reflectors . With arefractor eight inches only in aperture

, for instance,Mr. Dawes has obtained better views of the planets (andspecially of Mars ), thanLord Rosse

’s six-feet speculum


s olvable by any but the most powerful telescopes , shouldcover so large a space onthe heavens . Onthe contrary,I do not believe that a galaxy resembling our own

would be resolvable at all, unless it were so near as toappear much larger thanthe Orionnebula. I believeastronomers have beenwholly mistaken inconsideringany of the nebulae to be such systems as our own.

There may be millions of such systems inspace, but I

am very certain no telescope we could make wouldsuffice to resolve any of them. But what I do considerinconceivable, is, that anebulaextending so widely, andplaced (as supposed) beyond our system, should yetappear to cling (as the Orionnebula undoubtedly does)around the fixed stars seen in the same field with it.So strongly marked is this characteristic, that Si r JohnHerschel (who failed, apparently, to see its meaning)mentions amongst others no less thanfour stars, —one ofwhich is the bright middle star of the belt—as involvedins trong nebulosity,

’ while the intermediate nebulosityis only just traceable. The probability that thisarrangement is accidental i s so small as to be almostevanescent.However, as I have said, English astronomers, almostwithout a dissentient voice, accepted the resolution of

the nebulaas a proof that it represents a distant starsystem resembling our own galactic system,

but farsurpassing it inmagnitude .The time came, however, when a new instrument,more telling eventhanthe telescope

, was to be directeduponthe Orionnebula, and with very startling results .

THE GREAT NEB ULA IN ORION. 93

The spectroscope had revealed much respecting theconstitutionofthe fixed stars . We had learned that theyare suns resembling our own. It remained only toshow that the Orionnebula consists of similar suns, inorder to establish beyond all possibility of doubt thetheories which had beens o complacently accepted . Avery different result rewarded the attempt, however.WhenDr. Huggins turned his spectroscope towards thegreat nebula, he saw, inplace ofa spectrum resemblingthe sun’s , three bright lines only ! A spectrum of thissort indicates that the source of light is a luminous

gas , so that whatever the Orionnebula may be , it i smost certainly not a congeries of suns resembling our

own.

It would be unwise to theori se at present ona resultso remarkable . Nor can we assert that Herschel’ssp eculati ons have beenconfirmed, though his generalreasoning has beenabundantly justified . Astronomershave yet to do much before they can interpret themy sterious entity which adorns Orion’s sword . Oneverys ide, however , observations are being made which givepromise of the solutionof thi s and kindred difficulties .We have seenthe light of comets analysed by the samepowerful instrument ; and we learnthat the light fromthe tail and coma is similar inquality (so faras observation has yet extended) to that emitted from theOrionnebula. We see , therefore , that inour ownsolarsystem we have analogues ofwhat has beenrevealed inexternal space . I would not, indeed , go so far as toassert that the Orion nebula is a nest of cometic


systems ; but I may safely allege that there i s now

not a particle of evidence that the nebula lies beyondour galaxy.Nor need we doubt the accuracy of Lord Rosse’sobservations. More than a year before his death,indeed, he mentioned to Dr. Huggins that the matter

of the great nebula inOrionhad not beenresolved byhis telescope . Insome parts of the nebula he observeda large number of exceedingly minute red stars . Thesered stars, however, though apparently connected withthe irresolvable blue material of the nebula, yet seemedto be distinct from it.’

The whole subject seems to be as perplexing as anythat has ever been submitted to astronomers . Time,however

,will doubtless unravel the thread of the

mystery. We may safely leave the inquiry inthe handsof the able observers and physicists whose attentionhasbeenfor a long time directed towards it. And we needonly note, in conclusion, that in the southern hemisphere there exists an object equally mysterious— thegreat nebularound 77 Argus—which has yielded similarresults whentested with the spectroscope . The examinationof this mysteriousnebula

,associated with the most

remarkable variable inthe heavens—a star which at onetime shines but as afifth magnitude star, and at anotherexceeds eventhe brilliant Canopus in splendour—may,for aught that is known, throw anew light onthe constitutionof the great Orionnebula.

FromFraser ’s Magazine for February 1869.


able, that it is with a sense ofwonder one hears thatArago called it inquestion. To use the words of SirJohn Herschel, ‘ the fact i s so palpable that it i s amatter of some astonishment that it could ever fai l tostrike the most superficial observer.’ And, again, notonly the light but the heat of the outer portions of thesun’s image has been estimated. Inthis case we donot depend upon the perhaps fallible evidence of theeye

,but on that of heat-measuring instruments . Fr.

Secchi, measuring the heat of different parts of the solarimage, has found that of the part near the centre nearlydouble that from the borders . Lastly, photographygives unmistakable evidence onthe subject.Now,whenKirchhoff discovered the meaning of the

solar spectrum, it seemed clear to him that he haddetermined the nature and constitution of the solaratmosphere . Let us consider the nature of Kirchhoff ’sdiscovery.He found that the dark lines across the rainbowtinted streak forming the background (as it were) ofthe solar spectrum

, are due to the action of absorbingvapours . The vapours necessarily lie outs ide the sourceof that part of the sun’s light which produces the rainbow-tinted streak . If those vapours could be removedfor a while, we should see a simple rainbow-riband of

light. Or if the vapours could be so heated as to beno less hot than the matter beneath them which produces the rainbow spectrum

,they would no longer

cause any dark lines to appear ; but being cooler, andso giving out less light than they intercept, they cut

THE SUN ’S TR UE ATM OSPHERE . 9 7

out the dark spaces corresponding to their Specialabsorptive powers . To use M r. Lockyer

’

s striking,though perhaps not strictly poetical, description of

their action, these vapours gobble up the light on i tsway to the observer, so that it comes out with a

balance on the wrong side of the account.’ Eachvapour produces its ownspecial set of lines, occupyingpreci sely those parts of the spectrum which the vapour’slight would illuminate if the vapour shone alone . Forthese vapours, notwithstanding their action in intercepting or absorbing portions of the sunlight, are

themselves in reality glowing with a light s o intensethat the humaneye could not bear to rest uponit. If

we could examine the vapours we supposed just nowremoved from the sun, we should obtainthe very linesof light which are wanting in the spectrum of the

sun.

When Kirchhoff had recognised in this way the

presence of absorptive vapours around the real lightglobe of the sun, he judged that they form the solaratmosphere . Because , although his mode of observationwas not such as to assure him that these vapourscompletely envelope the sun, yet the telescopic aspectof the sun, and especially that darkening near the edgeto which I have just referred, s eemed to leave roomfor no other conclusion. But at this stage of the

inquiry Kirchhoff fell into amistake . He judged thatthe solar corona was the atmosphere which producedthe solar dark lines, as well as the darkening of thesun’s disc near the edge . The mistak e is one which,

98 LIGHT SCIENCE F OR LEISURE H OURS.

as it seems to me, he would have avoided had he takeninto account the enormous pressure at which anatmoSphere so extensive as the corona would necessarilyexist under the influence of the sun’ s mighty attractiveenergies . It may easily be shown that if the outerparts of the corona were as rare as the contents of ourso-called vacuum-tubes, or evena thousand times rarer,yet according to the laws which regulate atmosphericpressure

,the density even at vast heights above the

sun’s surface would attainto many hundred times that ofour heaviest gas es . The pressure would, indeed, be sogreat that we can

,see no way of escaping the conclu

s ion that, despite the enormous heat, the gases composing the imagined atmosphere would be liquefied orevensolidified .When the observers of the Indian eclipse of 1 868found that the coloured prominences are masses ofglowing hydrogen, with other gases intermixed, andwhen the prominence-spectrum was found to show thehydrogen lines as these appear when hydrogen existsat very moderate pressures, Kirchhoff’

s View had to beabandoned as altogether untenable . Wherever thevapours exist which produce the solar dark lines

,they

are undoubtedly not to be looked for inthe corona.

But there the lines are . The absorptive action i sexerted somewhere . The question i s—Where are theabsorptive vapours ?At this stage of the inquiry

, a very strange view wasexpres sed 'by Mr . Lockyer—a view which appears tohave been founded ona slight misapprehensionof the


shows beyond all question that, like the prominences ,this regi on consists of glowing hydrogen, mixed up

with a few, and at times with several other gases, butcertainly not capable of accounting for the thousandsof dark lines in the solar spectrum. It seems quiteclear, also, that the sierra is not of the nature of an

envelope at all.Over the narrow layer whi ch Secchi supposed toexist betweenthe sun’s surface and the coloured sierra,began, and presently waxed warm, the controversyabove referred to . Fr. Secchi was positive that hecould see the narrow continuous spectrum onwhich hefounded hi s view ; Mr. Lockyer was equally positivethat the worthy father could see nothing of the kind.Fr. Secchi urged that hi s telescope was better thanMr. Lo ckyer

’s , and that he worked ina better atmo

sphere ; Mr. Lockyer retorted that hi s spectroscopewas better than Fr. Secchi’s , and that the imaginedsuperiority of the Roman atmosphere was a myth .Something was said, too , by the Londonobserver abouta large speculum, which was to decide the question,though this mirror does not seem to have beenactuallybrought into action. Both the disputants expressedfull confidence that time would prove the j ustice oftheir several views .Soonafter, anobservationwas made byMr. Lockyer,which seemed to prove the justice of Fr. Secchi’sOpinion; for, onavery favourable day for observations,Mr. Lockyer was able to detect, not the narrow rainbow-tinted spectrum seenby Secchi

,but anarrow strip

THE S UN ’S TRUE ATM OSPHERE. 1 0 1

of spectrum belonging to the region just outside thesun’s edge , which showed hundreds of bright lines .

Here seemed to be conclusive evidence of that shallowatmosphere of glowing vapours in which Fr. Secchibad faith . But Mr . Lockyer interpreted hi s observation differently. The presence of these vapours on

this particular occasion he regarded as wholly excepti onal, and the cause of the exception he held to bethe energetic inj ection of vapours from beneath thesurface of the sun.

At about this stage of the controversy I had occas ionto consider the problems associated with the physicalconditionof the sunand hi s surroundings ; and althoughI took no part inthe discus sionbetweenFr. Secchi andM r . Lockyer, I expressed (inpapers which I wrote uponthe subj ect) Opini ons which agreed with the views ofthe Italianastronomer. It is neces sary for me to present inthis place my ownreasoning onthe questionatissue

,because it not only serves to introduce the special

observationmade last December, by which the problemhas been finally s olved, but also presents certaincon

s iderations which must be attended to ininterpretingthat observation.

Inthe first place, I noted that the darkening of thesun’s disc near the edge, or rather the marked natureof that darkening, instead of showing (as had been so

often stated ) that the sunhas a very deep atmosphere,proves, on the contrary, that hi s atmosphere must beexceedingly shallow by comparison with the dimens ions of his globe . It i s easy to Show why this i s ; and

1 0 2 LIGHT SCIENCE FOR LEISURE HOURS.

although the considerations onwhich the matter depends are exceedingly simple, yet the case i s by no

means the first inwhich exceedingly simple considera

tions have been lost sight of by students of science .Suppose we have a brightly-white globe encased symmetrically withina globe of some imperfectly transparent substance— as green glass . Now, if the whiteglobe is aninch indiameter and the green glass globea yard indiameter, the brightness of the white globewill be more or less impaired according to the trans

parency of the glass ; but it will not be much moreimpaired at the edge of the inner globe’s disc thannear the middle . For clearly, when we look at themiddle, we look through a foot and a half of glas s

(wanting only half an inch) and whenwe look at theedge of the inner globe’s disc

,we also look through a

foot and a half of glass (wanting only a small fractionof an inch) . Neither the half inch in the one case,nor the small fraction of an inch in the other , can

make any appreciable difference, so that the enclosingglobe of glass cuts off as much light whenwe look atthe centre of the inner globe’s disc as whenwe look atthe edge. But now suppose that the enclosing globeforms amere shell around the inner one . Suppose, forinstance, that the inner globe is a yard in diameter,and the shell of glass only half aninch thick . Thenin this case, as in the former, the brightness of theinner g lobe will be more or less impaired according tothe transparency of the glas s but it will no longer beaffected equally whether we look at the middle or at

I0 4 LIGHT SCIENCE F OR LEISURE H OURS.

anatmosphere relatively so shallow as this . Let i t beremembered

, inpassing, that the average height of thesierramay be set at about five thousand miles so thatthe atmosphere we are dealing with would be at theoutside but one-fifth as high as that fine rim of redlight with saw-like edge which astronomers detectedaround the eclipsed sun in the total eclipses of 1842 ,1 85 1 , and 1860. Still it might be thought thatpatience only would be needed to detect the s igns ofsuch anatmosphere, shallow though it be . But thereis a peculiarity of telescopic observationwhich rendersthe recognitionof such anatmosphere, if of less thanacertaindepth , not difficult merely, but impossible . It

may be well to exhibit the nature of the peculiarity atlength , because it is of considerable interest to all whopossess or use telescopes . I take an illustrative case,which seems, at first, to have little connectionwith mysubject.Every reader of this work has heard of the doublestars, and I dare say most Of those who read this particular article have seenmany of these beautiful obj ects .It is known that some double stars are much closerthanothers, and we commonly hear it menti oned as aproof of the excellence ofa telescope that it will dividesuch and such a double star. But it might seem thatif a telescope of a certain size were constructed withextreme care , it should be capable of dividing anydouble star ; because we might use aneye-piece ofanymagnifying power we pleased, and so, as it were, forceapart the two star-images formed by the object-glass .

THE SUN ’S TRUE ATMO SPHERE. 1 0 5

Instead of this being the case, howe ve r, there is a limitfor every obj ect-glass , beyond which no separationi spossible ; for this reason, simply, that the star-imagesformed by the obj ect-gl ass are not points of light, asthey would be if they correctly represented the stars ofwhich they are the O ptical images . The larger theobj ect-glass (assumed to be perfect inc onstruction) thesmaller is the star-image but it has always a definitesize

, and if this size is such that the two images of thestars forming a pair actually touch or overlap, we cannot separate them by using highly-magnifying eyepieces .

Now what i s true of a star is true of' every point ofany obj ect we examine with a telescope . The imageof the point is always a circle of light, which , thoughminute , has yet appreciable dimensions . The imageof the obj ect is ‘made up of all these circle s , whichnecessarily overlap . Nor let the reader s uppose thatonthis account telescopic obs ervat1on i s untrus tworthy .Precisely the same peculiarity affects ordinary vision.

There is no such thing as a perfect optical image ofanObject ; though neither eyesight nor teles copic visionneed be regarded as deceptive on this account. Ourpower of seeing minute details is limi ted by thispeculiarity, but we are not actually deceived. If

A curi ous illustration of thi s i s g iven by the fact that a certain

as tronomer of old, having reduced the aperture of his telescope to a

mere pin-hole , announced that he was thus enabled to measure the realglobes of the s tars , for , ins tead of s eeing the s tars through hi s teles cope

as minute po ints of light, he now saw themwith discs like the planets .

He thought he was improving the defining qualities of his telescope,

ins tead ofaltogether des troying them.


mi crosc0pi c writing be shownus, for instance , we may,

find ourselves, after poring over it for some time,unable to make out its meaning, the letters seemingall blended together ; but we know what our failurereally means , and do not fall into the mistake of

concluding that there are no details because the actualdetails are inscrutable .Let us apply this considerationto the sun, and moreparticularly to the appearance presented by the edgeof the sun’s disc. The image of every point of thisedge is a small circle the combination of all thesesmall circles must produce a ring of light all roundthe true outline of the disc . If the sun’s atmospheredid not reach beyond this ring, then no contrivancewhatever could render the atmosphere discernible

,let

the telescope be ever so perfect and the observerever so clear-sighted or skilful . Now,

the actual extensionof this ring will be greater or less according asthe objectf glas s of the telescope i s less or greater. It

may readily be shown that neither Mr . Lockyer’

s telescope nor Fr . Secchi’s could p os s ibly Show any signs ofa solar atmosphere under two hundred miles indepth ,while in all probability an atmosphere four or fivetimes as deep would escape their scrutiny.Are we then to remain altogether in ignorance of

such anatmosphere, supposing that it actually exists,and that the dark lines in the solar spectrum are dueto its absorptive power ? Is there no way of obviatingthe difficulty which has just beendealt withSo far as the method of observing the sun when

r0 8 LIGHT SCIENCE FOR LEISURE H OURS.

thing existing close by the '

sun’s globe can be recogni s ed.

But then, unfortunately, no total eclips e canpresentthese desirable features . If a total eclipse is to beworth seeing at all, the moon

’s disc as seenat the timemust be appreciably larger than the sun’s . Whentotality begins the outlines of the two di scs just touchat a single point, and whentotality ends the two discsjust touch at another point ; but during all the rest ofthe totality the two outlines do not touch at all, thatof the moonsurrounding without touching that of thesun. The outlines of the two discs do twice touch ,however, ineach cas e for one moment and at one point .What Professor Young determined to do

,therefore, was

to bring under special examination that one pointwhere the outlines touch at the exact moment whentotality begins . Inother words

,he directed his Special

attentionto the point where the last trace of the sun’sdisc was about to disappear . It is perhaps scarcelynecessary to say that he did not trust to the powers ofhis telescope, but that he employed a powerful spectro scope . And further, he did not depend onhis ownobservations alone, but had adjusted a spectroscope forthe use of Mr. Pye

, anEnglish gentleman residing inthe part of Spainwhere the eclipse-observing partieswere stationed, so that that gentlemanalso might makethe required observations .Inhis account, Professor Young does not mentionwhat he expected to see. It is probable that he hadin hi s thoughts the observations of Fr. Secchi, and

THE S UN ’S TR UE A TM OSPHERE. I0 9

hoped to obtain evidence respecting that shallow atmo spheric envelope which Secchi believed in and

Lockyer rejected though it i s qui te possible he merelydesired to ascertain whether the constitution of thelower part of the s ierra differed inany marked respectfrom that of the upper portion. As the momentapproached whenthe las t fine s ickle of sunlight was tobe obscured, the solar spectrum which was vis ible inthe spectroscopic field of view grew rapidly fainter.The regionactually examined by Profes sor Young wasinreality a narrow, almost linear space, touching theedge ofthe sun’s di sc ; so that before totality had commenced he had the light from our own illuminatedatmosphere, and not direct sunlight, to deal with

“

.

Thus he had jus t such a solar spectrum as i s seenwhena spectroscope i s directed to the sky in the daytime .But as the moment of totality drew near, the illuminationof the atmosphere, and with it the brightness ofthe rainbow-tinted streak

,rapidly diminished. At last

the solar spectrum vani shed ; and then—What was itreplaced by ? What was found to be the spectrum of

the solar atmosphere close by the sun’s surface ? In

place of the rainbow-tinted riband cros sed by thousandsand thousands of dark lines, there appeared anew and

most beautiful spectrum—a r iband of rainbow-tinted

lines , thousands innumber and ofall degrees of thicknes s, -hundreds of red lines, and then

,in order

,

hundreds of orange lines, hundreds of yellow, green,indigo , and violet lines, like coloured cross-threads onablack riband, only infinitely more beautiful . A charm

I IO LIGH T SCIENCE F OR LEISURE '

HOURS.

ing spectacle , truly, but so short-lived that no mancan

ever hope,though he lived to four-score years and ten,

to let his eyes rest inall his life for more than tenortwelve seconds onthe beautiful array of coloured lineswhich two menonly have as yet beheld. We may in

crease the dimensions and power of our telescopes untilthe existence of these lines can be recognised withoutthe aid of eclipse-darkness, but the lines cannever beseen, save during eclipse, as Young and hi s colleaguesaw them last December. And these observers tell usthat in a second or two the lines vanished, the advancing moonhiding the shallow solar atmosphere . If

it should ever be given to any man to see six totaleclipses (which has never yet happened to any), and tosuccessfully apply in each instance the method employed by Professor Young, theninall, during his life,that manwould have seenthe beautiful line-spectrumto perfectionfor some ten or twelve seconds but nototherwise canevenso long atotal period of observationbe secured . No single observer

,then, can hope to

learnmuch about the thousands of lines which havestill to be mapped during eclipse Opportunities .But now let us consider the import of the observation. What are these myriads of coloured lines ?Every dark line of the solar spectrum

,says Professor

Young, seemed to have its representative in thisbright-line spectrum. Many of the groups of lineswhich had flashed so quickly into view and enduredbut so brief a period, were familiar to him ; inotherwords, hi s study of the solar Spectrum had made him

I 1 2 LIGHT SCIENCE FOR LEISURE H OURS.

Inthe above estimate, I have supposed the measurement to be made from the sun’s visible surface . Butit is very unlikely that that surface i s the true lowerlimit of the atmosphere . It seems far more probablethat the surface we see is merely a layer of clouds (asSir William Herschel suggested so long ago ) in the

solar atmosphere , and that the actual depth of the

atmosphere is more truly indicated by the appearancesseenwhenlarge sun-spots are examined . That the sespots are cavities has been abundantly established.That they are openings through layers of solar cloudshas not beenindeed demonstrated, yet it i s difficult toconceive how they can otherwise be interpreted . As

to the way inwhich the spots are formed, theorists areat issue, some urging that there is an uprush fromdepths beneath the solar surface ; others, that there isa downrush of matter from without. But neither ofthese views is inany way incompatible with Herschel

’stheory

,that the spots are openings in s olar cloud

layers.We mi ght thus be led to compare the solar atmosphere with our own

, though it will of course beobvious that there are many marked points of difference . But in our own atmosphere we have at leasttwo distinct cloud-levels , the region, namely, where thecumuhts or wool-pack clouds are formed

, and thatwhere the ci rrus or feathery clouds

,make their ap

pearance . There is ai r above the cirrus clouds,ai r be

tween the cirrus and cumulus layers , and air betweenthe cumulus clouds and the earth . And precisely in

THE SUN’S TRUE ATM OSPHERE. I I 3

the same way we may conceive that there exists at alltimes a solar atmospheric region beneath as well asabove the cloud-layer which forms the sun’s vi siblesurface, and beneath and between the other cloudlayers revealed by telescOpi c observations .

But passing from the very difficult question suggested by the consideration of regions below the sun’svisible surface

,let us di scuss briefly the bearing of

Profes sor Young’ s discovery uponour views respectingthose outer regions—the coloured prominences andsierra, the corona itself, and, in fine, all the portionsof space which lie above the true atmosphere .Inthe first place , it seems to me that the observations made during the late eclipse dispose finally ofthe theory that the coloured s i erra is anatmosphericenvelope , properly s o-called . I had long since beenled to questionwhether the sierra could be s o regarded .Let me remind the reader that the sierra i s nothingmore nor less than the region which Lockyer rediscovered in1 8 68 . It had, in fact, been recognised bytelescopists since 1 806 , the name s i erra having beengivento it by the observers of the eclipse of 1 842 . It

i s a red region, having (as its name implies) a serratedupper surface, as seen in the telescope, and seeminglyextending all round the sun’s disc . The red prominences appear to spring from i ts upper surface .Strangely enough , when Lockyer made his ingeniousobservations of the coloured prominences, he had notheard of this discovery, or had forgotten it. Accordingly, finding traces ofprominence-matter all round the


sun, he concluded that there was a continuous envelopeof hydrogen(mixed with some other gases) surrounding the whole of the sun

’s globe . It was probablythrough being misled by this suppositionthat he gaveto the sierra a new name—entitling it the chromo

sp here—announcing at the same time that i ts upper

surface was smooth inoutline . Respighi , the eminentItalian spectroscopist—also working, it would seem,

inignorance or forgetfulness of the prior recognitionof the layer—announced presently that the uppersurface of the so-called chromo sphere

l was altogetheri rregular—more irregular, in fact, than the surface ofa tempest-tossed s ea. On re-examining the sierra,M r. Lockyer found this to be the case . But perhapsthe most striking evidence as to the real aspect of thesierrawas afforded during the eclipse of last December,when Fr. Secchi, towards the close of totality, saw

around the western half of the moon’s disc a completesemicircle of sierra, and noted that this beautifulcoloured crescent was formed of multitudes of minute

It affords strange evidence of the caution wi th which new namesshould be suggested, that thi s name, embodying , as we s ee, anerroneoustheory , and als o perpetuating the remembrance of ami s takenclaim, i s

s carcely yet beginning to fall into di sus e . Perhaps its G reek originandi ts lengthmay have s omething to do with thi s for although as tronomy—at least des criptive astronomy—has hi therto not beendi sfigured bythe hideous nomenclature which botani s ts and geologi s ts s eem to rejoicein, yet there i s always a large clas s of s cience s tudents who delight in,

s esqui pedal names , as g iving an air of profundity to their di s cours e.

It may evenbe dangerous to hint that the true form ofthe compoundfor a colour-spher e i s not chr omo-sphere, but chromato-sp here, s ince theextra syllable wi ll multiply tenfold the favour wi th which the compoundi s accepted. Whenwill the tyro learnthat the true lover of s ci ence

Proj icit ampullas et s esquipedalia verba

r 1 6 LIGHT SCIENCE FOR LEISURE H O URS.

aninner and brighter portion, which the sesquipedalianshave proposed to call the leucosp here, —apparently onthe lucus a non lucendo principle, for it i s neitherwhite nor spherical . And there i s the outer portion,much less brilliant, and much more strikingly radiated .

Neither one part nor the other presents a single featuresuggestive of an atmospheric nature ; I and the certainty that the two portions belong to a single Obj ectaffords yet more conclusive evidence against this interpretati onof the corona. But the rays of the coronaare of a somewhat remarkable nature . Whenwell seen,as during the eclipse of 18 68 , they are pointed ; andeven during so unfavourable an eclipse as that of

December last, the dark spaces between the rays are

seento widenrapidly wi th increased distance from thesun. These pointed radiations serve to show thatcoronal rays must be, in reality, shaped somewhat ascones

,having their bases towards the sun. The idea

is startling enough , but, admitting the accuracy of thepictures made during well-seen eclipses, and of theAstronomer-Royal’ s account of the corona duringthe eclipses of 1 85 1 and 1 860, there is no escapefrom the conclusionhere stated. It is not more certainthat the sun i s a globe, and not a flat disc as heseems to be, thanthat the coronal radiations are not

flat pointed rays, but cone-shaped. Yet no one wi llsuppose that there are a number of monstrous cone

I amhere referring to the pos s ibi lity that the coronamay be due tos ome species of s olar atmo sphere. The theory that the corona i s dueto light inour ownatmosphere has now at length beendefinitely abandoued by all as tronomers .

THE S UN’S TR UE A TM OSPHERE. I I 7

shaped masses—atmospheric or otherwise— standing,as it were

,uponthe sun’s surface . I cansee no other

way ofaccounting for these conical extensions thanbyregarding them as phenomena indicating s ome form of

repulsive actionexerted by the sun.

But whatever Opinion we may form on this andkindred problems, it seems clear that we must regardthe envelope discovered by Professor Young as the onlytrue solar atmosphere : and a very strange and complex atmosphere it i s . Nothing yet learned respectingthe sun’s surroundings surpasses in interest this fieryenvelope, inwhich some of the most familiar of our

metals appear as glowing vapours . Ifanything couldadd to the interest attaching to the coloured prominences and sierra, it i s the fact now revealed thatthey are propelled through this wonderful envelope,over which they float for a while with strangely changing figure . Truly the study of solar physics, whichtwenty years ago seemed at a stand-still, is advancingwith rapid strides ; and i t.

seems scarcely possible toexaggerate the interest either ofwhat has beenalreadyrevealed, or of the discoveries which are likely to beeffected during the approaching eclipse .

From the St. Paul’s Magazine for May 1871 .

ADDENDUM.—Doubts were urged, for some time after

this paper appeared,as to v the reality of Young’s dis

covery. But during the total eclipse ofDecember 187 1 ,and yet again during the annular eclipse of 1872 ,

decisive evidence was obtained in its favour, and it i snow received by all.

r r8 LIGHT SCIENCE EOR LEISURE H O URS.

SOMETHING WRONG WITH THE S UN.

WHENwe consider the intense heat which has prevailedinEurope during July, and the circumstance that inAmerica also the heat has been excessive, insomuchthat inNew York the number of deaths during theweek ending July 6 was three times greater thantheaverage, we are naturally led to the conclusion thatthe sun himself is giving out more heat thanusual .Though not endorsing such anopinion

,which , indeed,

is not warranted by the facts, since terrestrial causesare quite sufficient to explainthe recent unusual heats,we cannot refrain from noting, as at least a curiouscoincidence, that at the very time when the heat hasbeenso great, the great central luminary of the solarsystem has been the scene of a very remarkable di sturbance—an event, in fact, altogether unlike anywhich astronomers have hi therto observed.Now certain Italian spectroscopists—Respighi

, Sec

chi , Tacchini, and others— have set themselves thetask ofkeeping a continual watch uponthe chromatosphere . They draw pictures of it, and of the mightycoloured prominences which are from time to time

velope . They note the vapours which are present, aswell as what canbe learned of the heat at which thesevapours exi st, their pressure , their rate of motion, and

I 2 0 LIGHT SCIENCE F OR LEISURE HOURS.

magnesium vapour, but complete expulsions‘ . Onlywe would venture to substitute for the word expul

sion’ the expression outflow ’or uprising,’ s ince it

may well be that these vapours rise by a quiet processresembling evaporation, andnot by any actionso violentthat it could properly be regard ed as expulsive .In whatever way, however, the glowing vapour ofmagnesium thus streamed into the envelope of the sun,it would seem that the aspect of our luminary wasmodified by the process—not indeed ina very strikingmanner, or our observers inEngland would have noticedthe change, yet appreciably. M ore than one person,

’

says Tacchini, ‘ has told me that the light of the s unhas not at present its ordinary aspect ; and at theObservatory we have judged that we might make thesame remark. The change must be attributed to magneS1um.

’

It 1 s 1mpo s s ible to consider attentively the remarkable occurrence recorded by Tacchini without beingstruck by the evidence which it affords of solar mutabi lity. We know that during thousands of years oursun has poured forth his light and heat upon theworlds which circle around him

, and that there hasbeenno marked intermittence of the supply. We hear,indeed, of occasions when the sun has been darkenedfor a while ; and we have abundant reasons for believing that he has at times been so spot covered thatthere has beenanotable diminution of the supply oflight and heat for several days together .

Yet wehave had no reasons for anticipating that our sun

SOMETHING WRONG WITH THE S UN. I 2 I

might permanently lose so much of his heat and lustrethat the inhabitants of earth would suffer. Tacchini ’s

observation reminds us, however , that processes are atwork upon the sunwhich admit of being checked orincreased, interrupted altogether or exaggerated soviolently

,that the whole aspect of the sun, his condi tion

as the fire and lamp of the planetary system, maybe seriously affected .Ifwe only remember that our suni s one of the stars ,

not inany way distinguished, unless perhaps by relativeins ignificance , from the greater number of the starswhich illuminate our skies at night or are revealedby the telescope , we shall learn to recognise the possibi lity that he may undergo marked changes . Thereare stars which after shining with apparent steadinessfor thousands of years (possibly for millions of yearsbefore astronomy was thought of), have become suddenly much reduced in brightness, or after a fewfli ckerin

‘

g s (as it were ) have gone out altogether .There are other s which have shone with equal steadiness, and have then suddenly blazed out for a whilewith a lustre exceeding a hundredfold that which theyformerly possessed . It would be equally unpleasantfor ourselves whether the sun suddenly lost the best

part of his light, and presently went out altogether, orwhether he suddenly grew fiftyfold brighter and hotterthanhe now i s . Yet inthe present positionof siderealastronomy, it i s quite impossible to assert confidentlythat one event or the other might not take place atany time.


Fortunately, we may view this matter (just asastronomers have learned to view the prospect of mischievous collisions with comets) as a questionof probabi lities . Among so many thousands of stars there havebeen so many sudden outbursts of light and fire

,so

many sudden defalcations Of splendour. Our sun isone of those thousands , and so far as we know takeshi s chance Wi th the rest.

From the Spectator for August 1872 .

FROM IIERSCHEL’S PLANET.

SATURN the alti s s imus p laneta of the ancients—re

mains still the most distant planet respecting whosephysical condition astronomers can obtainsatisfactoryinformation. The most powerful telescopes yet con

structed have been turned in vain towards those twomighty orbs which circle outside the path of di stantSaturn: from bevond the vast depths which s eparate usfrom Uranus and Neptune

,telescopists canobtainlittle

intelligence respecting the physical habitudes of eitherplanet. Nor need we be surprised at the failure ofastronomers

, when we consider the difficulties underwhich the inquiry has been conducted . Incomparingthe telescopic aspect of Uranus with that of Saturn(forexample) we must remember that Uranus is not onlytwice as far from the earth but also twice as far from

LIGHT SCIENCE F OR LEISURE HOURS.

planet sending us altogether some sixty times as muchlight as Uranus .

But what the telescope had hitherto failed to aecom

pli sh, has just beenachieved by means of that wonderfulally of the telescope, ‘ the spectroscope, inthe able handsof

l

the eminent astronomer and physicist, Dr. Huggins .

News has been received about the constitutionof theatmosphere ofUranus, and news s o strange (apart fromthe strangeness of the mere fact that any informationcould be gained at all respecting a vaporous envelopeso far away) as to lead us to speculate somewhat curiously respecting the conditions under which the Uranians , if there are any, have their being.Before describing the results of Dr. Hugg ins

’

s latestudy of the planet, it may be well to give a briefaccount of what is knownrespecting UranThe question has been raised whether Uranus wasknown to the astronomers of old times . Thereis nothing altogether improbable in the supposition that in countries where the skies are unusuallyclear, the planet might have been detected by i tsmotions . Even in our latitude Uranus can be quitereadily seenonclear and moonles s nights, whenfavourably situated . He shines at such times as a star ofabout the fifth magnitude—that is, somewhat morebrightly than the faintest stars visible to the nakedeye . Inthe clear skies of more s outherly latitudes

.

hewould appear a sufficiently conspicuous obj ect, though,of course, it would be wholly impossible for eventhemost keen-sighted observer to recognise any difference

NEWS FROM HERSCHEL ’S PLANET. I 2 5

betweenthe aspect of the planet and that ofa star ofequal brightness . The steadiness of the light of Saturncauses this planet to present a very marked contrastwith the fir st magnitude stars whose lustre nearlyequals hi s own. But although the s tars of the lowerorders of magnitude scintillate like the leading orbs ,their scintillations are not equally distinguishable bythe unaided eye . Nor i s it unlikely that if Uranuswere carefully watched (without telescopic aid) hewould appear to scintillate slightly. Uranus wouldonly be recognisable as a planet by his movements .

There seems little reason for doubting, however , thateven the motions of so faint a star might have beenrecognised by some of the ancient astronomers, whosechief occupation cons isted in the actual study of thestar groups . We might thus understand the Burmesetraditionthat there are eight planets , the sun, the moon,Mercury, Venus, Jupiter, and Saturn, and anothernamed Rahu which is invisible . IfUranus was actuallydiscovered by ancient astronomers , it seems far from un

likely that the planet was only discovered to be lostagain, and perhaps withinavery short time . For if anything positive had been learned respecting the revolution of thi s distant orb, the same tradition whichrecorded discovery of the planet would probably haverecorded the nature of its apparent motions .Be this as it may, we need by no means accept the

Opinion of Buchanan, that if the Burmese traditionrelates to Uranus, Sir William Herschel must bestripped of hi s honours .

’ The rediscovery of a lost

1 2 6 LIGHT SCIENCE F OR LEISURE I-I O URS.

planet, especially of one which had remained concealedfor so many centur ies , must be regarded as at least asinteresting as the discovery of a planet altogether unknown. Nor was there any circumstance inthe actualdiscovery of Uranus , which would lose its interest, eventhoug h we accepted quite certainly the conclusionthatthe Herschelianplanet was no other thanold Rahu . l

Let us turnto Herschel'

s ownnarrative of his detectionof Uranus . It i s inmany respects very instructive .In the first place, we must note the nature of thework he was engaged upon. He had conceived the ideaofmeasur ing the distances of the star s, or at least ofthe nearer stars, by noting whether as the earth circlesaround the sunthe relative positions of stars lying veryclose to each other seemed to vary inany degree . Tothis end he was searching the heavens for those objectswhich we now call double stars, most ofwhich were inhis day supposed to be not inreality pairs of starsthat is, not physically associated together—but seennear together only because lying nearly in the samedirection. The brighter star of a pair was infact sup

It i s , after all, at leas t as likely that Rahu—as suming there reallywas a planet known under this name- might have beenVes ta, thebrightes t of the small planets which circle betweenMar s and Jupiter ,as the di s tant and s low-moving Uranus . For although Ves ta i s notnearly s o br ight as U ranus , shining , indeed, only as a s tar of the

s eventh magni tude, yet she canat times be s eenwithout teles 00pic aid

by per sons of extremely good s ight ; and her movements are far morerapid thantho s e ofU ranus . In the high table-lands of tho s e eastermcountri es , where s ome place the birth of as tronomy, keen-s ighted ohs ervers might quite readily have di scovered her planetary nature,whereas the s low movements of Uranus would probably have es capedtheir notice .

1 2 8 LIGHT SCIENCE F OR LEISURE H 0 URS.

to be on a supposition of its not being a fixed star,while the diameters of the stars to which I comparedit were not increased inthe same ratio . M oreover , the

comet being magnified much beyond what its lightwould admit of, appeared hazy. and ill-defined withthese great powers, while the star s presented that lustre

and distinctness which from many thousand observa

tions I knew they would retain. The sequel has shownthat my surmises were well-founded .’

There are three points to be specially noted inthis

account. First, the astronomer was engaged in a

process of systematic survey of the celestial depths— so

that the discovery of the new orb cannot be properlyregarded as accidental, although Herschel was not atthe time onthe look—out for as yet unknown planets .

Secondly, the instruments he was employing were ofhis own construction and device, and probably noother inexistence in his day would have led him to

the di scovery that the strange orb was not a fixed star .And thirdly, without the experience he had acquiredinthe study of the heavens he would not have beenable to apply the test which, as we have seen, he foundso decisive . The fact that the stars are not magnifiedby increased telescopic power to the same extent asplanets or comets, is , as Professor Pritchard has justlyremarked, animportant result of the undulatory theoryof light, and was unsuspected inSir William Herschel’sday.

’ So that whether we consider the work Herschelwas engaged upon, the instruments he used, or the ex

peri ence he had acquired, we recognise the fact that he

NEWS FROM HERSCHEL ’S PLANET. 1 2 9

alone of the astronomers ofhis time was capable ofdi scovering Uranus otherwise thanby afortunate accident .Others might have lighted on the discovery—indeed ,we shall presently see that the real wonder i s thatUranus had not beenfor many years a recognised member of the solar system—but no one except Herschelcould withina few minutes of his first view of the planethave pronounced confidently that the strange orb (whatever it might be) was not a fixed star.I do not propose to enter here , at length, into theseries of researches by which it was finally demonstratedthat the newly-discovered body was not a comet but aplanet, travelling on a nearly circular path around thesun, at about twice Saturn’s distance from that orb.

With this part of the work Hers chel had very little todo . To use Professor Pritchard’s words

,having asoer

tained the apparent s ize, position, and motion of thestranger, Herschel very properly consigned it to thecare of those professional astronomers who pos sessedfixed instruments of precision in properly constitutedobservatories—to Dr . Maskelyne, for instance , who wasthen the Astronomer-Royal at G reenwich, and toLalande, who presided over the observatory in Paris .

’

As the newly-discovered body travelled onwards uponits apparent path , astronomers gradually acqui red themeans of determining what i ts real path might be .At first they were misled by erroneous measures of thestranger’s apparent size, which suggested that the supposed comet had inthe course of the fir st month afterits discovery approached to within half its original

K

.I30 LIGHT SCIENCE FOR LEISURE HOURS.

distance . At length, setting aside all these measures,and considering only the movements of the stranger,Professor Saronwas led to the belief that it was no

comet, but a member of the solar system. It was

eventually proved, chiefly by the labour s of Lexell,Lalande, and the great mathematician Laplace , thatthis theory fully explained all the observed motions ofthe newly-di scovered body, and before long (so completeis the mastery which the Newtonian system gives astronomers over the motions of the heavenly bodies) all thecircumstances of the new planet’s real motions becamevery accurately known. It was now possible, not onlyto predict the future movements of the stranger, butto calculate his motions during former years . This lastprocess was quickly applied to the planet, with theobj ect of determining whether among the records “

of

observations made on stars, any might be detected'

which related in reality to the newly-discovered body.The result will appear at first sight somewhat surprising . The new planet had actually been observed noless thannineteentimes before thatnight whenHerschelfirst showed that it was not a fixed star, and thoseobservations were made by astronomers no less eminentthan Flamstead, Bradley, Mayer, and Lemonnier.Flamstead had seenthe planet five several times, eachtime cataloguing it as a star of the sixth magnitude, sothat five such stars had to be dismissed from Flamstead’s lists . But the case of Lemonnier was evenmoresingular for he had actually observed the planet noles s thantwelve times, several of his observations having

1 3 2LIGHT SCIENCE FOR LEISURE HOURS.

letter ofHerschel's

'

name with a small globe attachedto the cross-stroke, still reminds us of the honour which

Continental astronomers generously proposed to renderto their fellow-worker inEngland.1 Lichtenberg proposed the name of Astraea, the goddess of justice—forthis ‘ exquisite reason,

’ that since justice had failed toestablish her reignuponearth, she might be supposedto have removed herself as far as possible from our

unworthy planet. Po ins inet suggested that Cybelewould be a suitable name for since Saturnand Jupiter,to whom the gods owed their origin, had long held theirseat inthe heavens, it was time to find aplace forCybele,the great mother of the gods .’ Had the supposedGreek representative of Cybele—Rhma—beenselectedfor the honour, the name of the planet would haveapproached somewhat nearly insound, and perhaps ins ignification, to the old name Rahu. But neitherAstraeanor Cybele were

'

regarded as of sufficient dignityand importance among the ancient deities to supply aname for the new planet.2 Pro sperinproposed Neptuneas a suitable name, becaus e Saturnwould thus have the

There is a certainincongruity, accordingly, among the symbols ofthe primary planets . Mercury is symboli s ed by his caduceus , Venus byher looking-glas s (I suppos e), Mars by hi s spear and shi eld, Jupiter byhi s throne, Saturnby hi s s ickle ; and again, whenwe pas s to the symbols as s igned to the planets di s covered in the pres ent century, we findNeptune symbolized by hi s trident, Ves ta by her altar , Ceres by hers ickle, M inerva by a sword, and Juno by a s tar-tipped sceptre. Uranus

alone i s repres ented by a symbol which has no relationto hi s pos itionamong the deities ofmythology .

2 Both thes e names are found among the as teroids , the fifth of these

bodies (in order of di scovery) being called As traea, the eighty-ninth

being named after the greatmother ofgods and goddes ses .

NEWS FROM HERSCHEL ’S PLANET. 1 33

eldest of his sons on one side of him ,and his second

sononthe other. Bode at length suggested the nameof Uranus, the most ancient of the deities and as

Saturn,the father of Jupiter, travels on a wider orbit

thanJupiter , so it was judged fitting that anevenwiderorbit than Saturn’s should be adjudged to Jupiter’sgrandfather. Inaccepting the name ofU ranus for thenew planet, astronomers s eemed to assert a belief thatno planet would be found to travel ona yet wider path ;and accordingly whena more distant planet was discovered, the suggestionof P ro sperin had to be recons idered ; but it was too late to change the acceptednomenclature, and accordingly the younger brother ofJupiter has had assigned to him a planet circling out

side the paths of the planets assigned to their father andgrandfather. It may be noted , also , that amore appro

priate name for the new planet would have beenCoelus ,since all the other planets have received the Latinnames of the deities .Herschel himself proposed another name . As Galileo

had called the satellites of Jupiter the Mediceanplanets ,while French astronomers proposed to call the spots onthe sunthe Bourbonian stars , so Herschel, grateful forthe kindness which he had received at the hands ofGeorge III . , proposed that the new planet should becalled Georgium' Sidus . On account of the interestattaching to all Herschel’s remarks respecting his discovery, I quote infull the letter inwhich he submittedthis propositionto Sir Joseph Banks , thenthe Presidentof the Royal Society. By the observations of the most

1 34 LIGH T SCIENCE F OR LEISURE HOURS.

eminent astronomers inEurope ,’ he remarks, it appears

that the new star, which I had the honour of pointingout to them inMarch 178 1 , is a primary planet of oursolar system . A body so nearly related to us by itssimilar conditionand situation in the unbounded expanse of the starry heavens, must often be the subj ectof the conversation, not only of astronomers , but ofevery lover of science in general . This consideration,then

,makes it necessary to give it aname , whereby it

may be distinguished from the rest of the planets andfixed stars . Inthe fabulous ages of ancient times, theappellations of Mercury, Venus , Mars, Jupiter, and

Saturn, were givento the planets, as being their princi

pal heroes and divinities . In the present more philosophical era, it would be hardly allowable to haverecourse to the same method

,and call onJuno, Pallas,

Apollo , or M inerva, for a name to our new planet .The firs t consideration in any particular event or re

markable incident seems to be its chronology ; if, inany future age it should be asked whenthis last-foundplanet was discovered, it would be very satisfactory tosay, Inthe reign of George III.” As a philosopher,then, the name ofGeorgium Sidus presents itself to meas an appellationwhich will conveniently convey theinformation of the time and country where and whenit was brought to View. But as a subj ect of the bestofkings, who is the liberal protector of every art andscience ; as a native of the country from whence thisillustrious family was called to the Briti sh throne ; asamember of that society which flourishes by the di s

1 36LIGHT SCIENCE FOR LEISURE H o URS.

length of time occupied by Uranus in circling roundhis orbit

,the astronomer labours under a difficulty

distinct in character from the difli culties which havealready beenconsidered. As Jupiter and Saturncircleon their wide orbits they exhibit to us—the formerinthe course of elevenyears, the latter in the courseof twenty-nine and a half years—all those varying presentati ons which correspond to the seasons of theseplanets . Jupiter, indeed, owing to the uprightness ofhis axis (with reference to hi s path) presents but slightchanges . But Saturn’s globe is at one time bowedtowards us , so that a large portion of hi s north polarregions canbe seen, and anon(fifteenyears later) i s sobowed, that a large portion of his southern polarregions can be seen; while between these epochs wesee the globe of Saturn so posed that both poles areon the edge of hi s disc, and thenonly does the shapeof his disc indicate truly the compres s ion or polarflattening of the planet.But although similar changes occur in the case ofUranus, they occupy no les s than eighty-four years inrunning through their cycle, or forty-two years incom

pleting a half cycle—during which, necessarily, all

possible presentations of the planet are exhibited.Now it i s commonly recognised among telescopists thatthe observing time ofanastronomer’s life—that is, theperiod during which he retains notmerely his full skill,but the energy necessary for difli cult researchescontinues but about twenty-five years at the outside .So that few astronomers can hope to study Uranus in

NEWS FROM HERSCHEL ’S PLANET. I 3 7

all his presentations, as they canstudy Mars, or Jupiter ,or Saturn.

When we add to this circumstance the extremefaintness of Uranus , we cannot wonder that Herschelshould have been unable to speak very confidently onmany points of interest. Hi s measures of the planet’sglobe were sufli ciently satisfactory, and, combined withmodern researches, show that Uranus has a diameterexceeding the earth’s rather les s than four and a halftimes . Thus the surface ofUranus exceeds that of ourglobe about twenty times , and his bulk i s more thaneighty times as great as the earth

’

s . His volume, infact, exceeds the combined volume ofMercury, Venus ,the Earth , and Mars , almost exactly forty times . ButSir W. Herschel was unable to measure the disc of

Uranus in such a way as to determine whether theplanet is compressed in the same marked degree asJupiter and Saturn. All that he felt competent tosay was that the disc of the planet seemed to him tobe oval, whether he used his seven-feet, or his ten-feet

,

or his twenty-feet reflector. Arago has expressed someSurprise that Herschel should have been content withsuch a statement . But in reality the circumstance i sin no way surprising. For as a matter of factHerschel had been almost foiled by the difl‘i culty of

measur ing even the planet’s meandiameter. The discordance between his earliest measures i s somewhatstartling. His first estimate of the diameter made itten thousand miles too small (its actual value beingabout thirty-four thousand miles) ; hi s next made it

1 38 LIGHT SCIENCE F OR LEISURE H o URS.

nearly three thousand miles too great ; while his thirdmade it ten thousand miles too great. His contem

porari es were even less successful. Maskelyne, after along and careful series of observations, assi gned to theplanet a diameter eight thousand miles too small ; theastronomers of M ilan gave the planet a diameter morethan twenty thousand miles too great ; and Mayer, of

Mannheim, was evenmore unfortunate, for he assignedto the planet a diameter exceeding its actual diameterof thirty-four thousand miles, by rather more thanfifty thousand miles . It wi ll be understood, therefore ,that Herschel might well leave unattempted the taskof comparing the diflerent diameters of the planet.This task required that he should estimate 'a quantity

(the difference between the greatest and the leastdiameters) which was small even by comparisonwiththe errors of his former measurements .But besides this, a peculiarity in the axial pose ofUranus has to be takeninto account. I have spokenof the uprightness of Jupiter’s axis with reference tohis path ; and by this I have intended to indi cate thefact that if we regard Jupiter’s path as a great levelsurface , and compare Jupiter to agigantic top spinninguponthat surface, this mighty top spins with anearlyupright axis . In the case of Uranus the state of

things is altogether different. The axis of Uranus i sso bowed downfrom uprightness as to be nearly inthelevel of the planet’s path. The result of this is thatwhen Uranus is in one part of his path his northernpole is turned almost directly towards us . At such a

he was unable to watch any of these satellites through

a considerable part of its path , or to identify any ofthem ondifferent nights . All he felt sure about wasthat certain points of light were seen which did not

remain stationary, as would have happened had theybeen fixed stars . No astronomer, however, has sinceseenany of these four additional satellites, though Mr.

Lassell has discovered two which Herschel could notsee (probably owing to their nearnes s to the body of theplanet). As Mr. Lassell has employed a telescope morepowerful than Herschel’s largest reflector, and has

given much attention to the subject, no one has a

better right to speak authoritatively on the subject ofthe four additional satellites . Since, therefore, he is veryconfident that they have no existence, I feel bound torepresent that view as the most probable ; yet I am

unable to pass from the subject without expressing ahope that one of these days new Uraniansatellites willbe revealed.The four known moons travel backwards ; that i s,they circle in a direction opposed to that inwhich allthe planets of the solar system, and all the moons ofJupiter and Saturn, as well as our ownmoon, are

observed to travel . Much importance has beenattachedto this peculiarity ; but in reality the paths of theUranian moons are so strangely s’ituated with respectto the path of Uranus, that the directioninwhich theytravel canhardly be compared with the commondircotion of the planetary motions . Imagine the path of

NEWS FROM HERSCHEL ’S PLANET. I4 I

Uranus to be represented by a very large wooden hoopfloating ona sheet of water ; then, if a small woodenhoop were so weighted as to float almost upright, withone half out of the water, the positionof that hoopwould represent the position of the path of one of theplanet’s satellites . It will be seen at once that if wesuppo seabody to travel round (and upon) the formerhoopina certaindirection, thena body travelling round thelatter hoop could scarcely be said to travel inthe samedirection, whether it circled one way or the other .

Or to employ another illustration— if a watch be laidface upward ona table, we should correctly say that itshands move from east through south to west ; but, ifit be held nearly upright and the face rather upwards,we should scarcely say that the hands moved fromeast through s outh to west nor if the face were tilteda little further forward, s o as to be inclined ratherdownwards, should we say that the hands move fromeast through north to west.The great slope or tilt of the paths is undoubtedly amore singular feature than the direction of motion.

Implying as i t does that the planet’s globe is s imilarlytilted, it suggests the strangest conceptions as to theseasonal changes of the planet. It seems impossibleto suppose that the inhabitants of Uranus, if there areany, candepend on the sun for their supply of heat.The vast distance of Uranus from the sun, althoughreducing the heat-supply

‘

to much less thanthe threehundredth part of that which we receive , i s yet aninsignificant circumstance by comparisonwith the axial

14 2LIGHT SCIENCE FOR LEISURE HOURS.

tilt. One can understand at least the p os s i bi li ty thatsome peculiarity in the atmosphere of the planetmight serve to remedy the effects of the former cir

cumstance ; precisely as our English climate is temperedby the abundant moisture with which the air i s ordi

nari ly laden. But while we can conceive that theminute and almost starlike sunof the Uranian skiesmay supply much more heat thanits mere dimensionswould lead us to expect, it i s difficult indeed to understand how the absence of that sunfor years from theUranian sky can be adequately compensated. Yet inUranian latitudes corresponding to the latitude of

Londonthe sun remains below the horizon for abouttwenty-three of our years in succession. Such is theArctic 1 night of regions in Uranus occupying a pos ition corresponding to that of places in our temperatezone.But the most important result of the discovery ofthe satellites has been the determinationof the massor weight of the planet, whence also the meandensityof its substance has beenascertained . It has beenthusdiscovered that, like Jupiter and Saturn, Uranus i s constructed of much lighter materials thanthe earth . Ourearth would outweigh almost exactly six times a globe as'

large as the earth butno denser thanUranus . It i s to be

1 It has been remarked that there i s s ome incongruity inthe nameArctic planets which I have as s igned inmy Other Worlds to U ranusand Neptune, whencons idered with reference to the theory} have enunciated that these planets still retainan enormous amount of inherentheat. Many s eem to imagine that the term arctic impli es cold. I have ,of cour se, only us ed the name as indicating the di stance ofUranus andNeptune from the sun.

1 44LIGH T SCIENCE FOR LEISURE H o URS .

Faint as i s the light ofUranus, yet, whena telescopeof sufficient size i s employed, the spectrum of theplanet is seenas a faint rainbow-tinted streak . Thepeculiarities of this streak, if discernible, are the meanswhereby the spectroscopist is to ascertainwhat is thecondi tion of the planet’s atmosphere . Now, FatherSecchi , studying Uranus with the fine eight-inch teles cope of the RomanObservatory, was able to detectcertainpeculiarities in its spectrum, though it wouldnow appear that (owing probably to the faintness ofthe light) he was deceived as to their exact nature .He says : The yellow part of the spectrum is wantingaltogether. Inthe green and the blue there are twobands, very wi de and very dark .

’ But he was unableto say what is the nature of the atmosphere of the

planet, or to show how these peculiarities might beaccounted for.Recently, however, the Royal Society placed in thehands ofDr. Huggins a telescope much more powerfulthaneither the Romantelescope or the instrument withwhich Dr. Huggins had made his celebrated observations onsunand planets, stars and s tar-cloudlets. It

is fifteen inches inaperture, and has a light-gathering

power fully three times as great as that possessed byeither of the instruments just mentioned .As seenby the aid of this fine telescope the spectrum

of Uranus is found to be complete, no part be ing

wanting, so far as the feebleness of its light permits itto be traced.’ But there are six dark bands, or s tronglines , indi cating the absorptive action of the planet’s

NEWS FROM HERSCHEL’S PLANET. I4 5

atmosphere . One of these strong lines corresponds inposition with one of the lines of hydrogen. Now itmay seem at a first view that since the light ofUranusi s reflected solar light, we might expect to find inthespectrum of Uranus the solar lines of hydrogen. Butthe line in question is too strong to be regarded asmerely representing the corresponding line inthe solarspectrum indeed, Dr. Huggins distinctly mentionsthat the bands produced by planetary absorptionarebroad and strong in comparisonwith the solar lines .’

We must conclude , therefore , that there exists in theatmosphere of Uranus the gas hydrogen, sufficientlyfamiliar to us as anelement which appears incombinationwith others, but which we by no means recogniseas a suitable constituent (at least to any great extent)ofanatmosphere which living creatures are to breathe . ‘

And not only must hydrogen be present inthe atmoSphere ofUranus, but insuch enormous quantities asto be one of the chief atmospheric constituents . Thestrength of the hydrogen line cannot otherwise beaccounted for. If by the action of tremendous heatall the oceans of our globe could be changed into theirconstituent elements, hydrogen and oxygen, it i s probable that the signs by which an inhabitant of Venusor M ercury could recognise that such a change hadtakenplace would be very much less marked thanthes igns by which Dr . Huggins has discovered that hydro

Traces of hydrogen cannearly always be detected inthe air ,—butthe quantity ofhydrogenthus shown to be pres ent i s almost infinites imally small compared w ith the amount of oxygenand nitrogen.

r46 LIGHT SCIENCE F OR LEISURE H o URS.

genexists inthe atmosphere of Uranus . It will indeedbe readily inferred that this must be the case, whenthe fact is noted that no signs whatever of the existence ofnitrogencanbe recognised in the spectrum ofUranus, though it is difficult to suppose that nitrogeni s really wanting in the planet’s atmosphere . Dr .Huggins also notes that none of the lines inthe spectrum of Uranus appear to indicate the presence of

carbonic acid. Nor are there any lines inthe spectrumof Uranus corresponding to those which make theirappearance inthe solar Spectrum when the suni s lowdown, and is therefore shining through the denseratmospheric strata. Most of these lines are due tothe presence of aqueous vapour in our atmosphere ,and it would seem to follow that if the vapour of

water exists at all in the atmosphere of Uranus itsquantity must be small compared with that of the freehydrogen.

Admitting that the line seen by Dr . Huggins isreally due to hydrogen—a fact of which he himself hasvery little doubt—we certainly have a strange discoveryto deal with . If it be remembered that oxygen, themainsupporter of such life as we are familiar with,cannot be mixed with hydrogenwithout the certaintythat the first spark will cause an explosion(inwhichthe whole of one or other of the gases will combinewith a due portion of the other to produce water), itis difficult to resist the conclusionthat oxygenmust beabsent from the atmosphere of Uranus. If hydrogencould be added in such quantities to our atmosphere

148 LIGHT SCIENCE F OR LEISURE H o UR S.

which afforded doubtful evidence respecting the qualityof the light we receive from comets , and thus allowedastronomers to form vague guesses respecting the strueture of these mysterious wanderers . But beyond theunsatisfactory indications of this instrument, astronomers had no means whatever of ascertaining the phys ical nature of comets .At present, however, aninstrument of incomparablyhigher powers i s applicable to the inquiry. The spectros cOpe has the power of revealing, not only thegeneral character of any substance which is a sourceof light, but even of exhibiting, in many instances,the elementary constitutionof such a substance. Theindi cations of this wonderful instrument of analysisare not affected by the distance or dimensions of theobject under examination. So long as the obj ect i sluminous the spectroscope will tell us with the utmostcertainty whether the light is inherent or reflected ;and if the light is inherent—that is

,if the object i s

self-luminous—the spectroscope will tell us with theutmost certainty what terrestrial elements (if any)exist inthe constitution of the obj ect. It is with therevelations of the spectroscope respecting Brorsen’scomet that I now propose to deal. I must make afew preliminary remarks

,however

,respecting the vari

I assume that my readers are familiar with thegeneral appearance presented by comets—at least bythose whi ch are visible to the naked eye . It may be

THE TWO COMETS OF THE YEAR 1868 . 149'

necessary to note, however, of the three features commonly recognised in comets—viz . the nucleus , coma,and tai l— the coma alone i s invariably exhibited. Acomet which has neither nucleus nor tail presentssimply a round mass of vapour slightly condensedtowards the centre . The nucleus, when seen, appearsas a bright point withinthe condensed part ofa comet .The tail, as every one knows , i s a long trainof lightis suing from the head .It was noted in very early times that comets are

almost perfectly translucent. This peculiarity hasbeenconfirmed bymodernandmore exact observations .Sir W. Her schel watched the central passage ofa cometover the fainter component of a double star ; and hecould detect no diminution of the star’s brilliancy.Similar observations were made by MM . Olbers andStruve . Sir John Herschel watched the passage of

Biela’ s comet over a small cluster of very faint teles cOpic stars . The slightest haze would have obliterated the cluster, yet no appreciable effect was produced by the interposition of cometic matter having athickness (according to Herschel

’s estimate) ofmiles . And there is another remarkable evidence oftenuity. From recognised optical principles , a starseen through the globular head of a comet

, shouldappear displaced from its true position just as anyobj ect seen(non-centrally) through aglobular decante rfull of water seems thrownout of its true place . Theastronomer Bessel made anobservationona star whichapproached within about eight s econds of the nucleus


ofHalley’ s comet, and he found that the place of thestar was not affected to anappreciable extent.Whether the nucleus ofa comet is solid or not had

long beena disputed point among astronomers . Withtelescopes of moderate power the bright point withinthe coma presents an appearance of solidity whichmight readily deceive the observer. But with an

increase of power the nucleus assumes a differentappearance. Instead of having a well-defined outlineit appears to merge into the coma by a somewhat rapidgradation—but not by an abrupt variation— of light .Good observers have reported the extinction of telescopic stars behind the nuclei of comets, but there arepeculiar difli cultie s about an observation of this sort ;and it is very difficult to determine whether a star i sreally concealed from view by the interposition of thenucleus or simply obliterated by the glare of light.The tail of a comet appears nearly always as an

extensionfrom the coma, and a dark interval i s usually

seenbetween the head and the tail. But there is animmense variety in the configuration of comets’ tails .

The comet of 1744 had six tails spread out like a fan.

The comet of 1807 had two tails— both turned fromthe sun. The comet of 1 823 had also two tails, butone was turned almost directly towards the sun. Othercomets have had lateral tails .The processes which seem to be passed through by

comets during their approach towards and recessionfrom the sun have proved very perplexing to astronomers and physicists . When first seena comet usually


comet when passing away from the sun increased in

volume upwards of fortyfold ina single week .The tremendous heat to which many comets are sub

j ected during perihelion passage is animportant pointfor consideration, inattempting to form anopinionof

the physical structure of comets . Newton calculatedthat the comet of 1 680 was subjected to a heattimes greater than that of red-hot i ron. But cometshave beenknownto approach the sunevenmore closely.

Sir JohnHerschel estimates that the comet of 1843

was subj ected to a heat exceeding inthe proportion of

24-5 to 1 the heat concentrated inthe focus ofPerkins’

great lens. Yet the heat thus concentrated had suf

ficed to melt agate, rock-crystal, and cornelian.

We cannot wonder that so great anintensity of heatshould have produced remarkable effects upon manycomets. The great wonder is that any comet shouldresist the effects of such heat without being dissipatedinto space .We learn from Seneca that Ephorus , an ancientGreek author, mentions a comet which di vided intotwo distinct comets . Kepler considered that twocomets which were seen together in 1 6 18 had beenproduced by the division of a single comet. Cysatusnoticed that the great comet of 1 6 18 showed obvioussigns of a tendency to break up into fragments . Thiscomet when first seen appeared as a circular nebulouscloud . A few weeks later it seemed to be divided intos everal distinct cloudlike masses . On December 20‘ it resembled amultitude of small stars .’

THE TWO COMETS OF THE YEAR 1868 . 1 5 3

We might doubt whether these observations wereentitled to credit were it not that, quite recently,Biela’s comet has been seen to separate into two distinct comets , each having a nucleus, coma, and tail,and each of which pursued i ts course independentlyuntil distance concealed both from View.

It i s clear that nothing but a long series of carefulobservations can put us ina position to theorise withconfidence, respecting the nature of comets, the processes of change which they undergo, and the functionswhich they subserve inthe economy of the solar system .

We may therefore dwell with particular satisfactionon

the fact that every comet which has appeared during thelast two years has been subj ected to careful observation, and that at length , by means of spectroscopicanalysis we are beginning to get hold of pos itive factsrespecting comets, and have promise of shortly beingable to form consistent theories with regard to theses ingular members of the solar system.

I have had occasion in other works to speak of

the principles onwhich spectroscopic analysis dependsbut I think it best briefly to restate the most important points . When the light from a luminousobject i s received upona prism, there i s formed whati s called the prismatic spectrum . According to thenature of the s ource of light this spectrum varies inappearance . If the source of li ght i s anincandescentbody the spectrum is a continuous , rainbow-tintedstreak . Where the light comes fromanincandescentmasssurrounded with cooler vapours, the streak of rainbow


coloured light i s crossed by dark lines whose positionindicates the nature of the vapours which the light hastraversed. When the light comes from luminousvapours the spectrum consists wholly of bright lines ;and these have exactly the same positionas the corresponding dark lines which are seen when the samevapours intercept light from an incandescent solidmass . Lastly, whenlight is reflected from an opaquesubstance

,the spectrum is the same as that which

would be presented by the light before reflection, unles sthe opaque substance i s surrounded by vapours , inwhich case the spectrum will be crossed by new darklines corresponding to the absorptive qualities exertedby those particular vapours .We see then the wonderful qualities of the new

analysis . Applied to the sunand stars it has enabledour physicists and astronomers to pronounce as con

fidently that certainelements exist inthese far distantorbs , as the chemist can pronounce onthe constitutionof substances submitted to his direct analysis . Thequestions , or some of them, which have beenat issuerespecting comets, will undoubtedly yield to the powersof the spectroscope . The great want, at present, is abrilliant comet to work upon. Donati’s cometor the great comet of 1 86 1 would have served thispurpose admirably, but the first came inthe very yearinwhich the principles of spectroscopic analysis werefirst discovered ; and the powers of the spectroscopewere only just beginning to be recognised when thecomet of 186 1 made its brief visit to our northernskies .

1 56 Lm11T SCIENCE F0R LEISURE IIo URS.

It appears inthe telescope as anearly round nebulos ity, inwh ich the light increases rapidly towards thecentre , where, onsome occas ions, I detected, I believe ,a small ste llar nucleus . G enerally

, this minute nucleuswas no t to be dis tinguished in the bright central partof the comet. The spectrum cons ists fo r the mostpart of three bright bands . The length of the bandsin the ins trument shows that they are not due aloneto the s tellar nucleus , but are produced by the lighto f the brighter portions of the coma. I took somepains to learn the preci se character ofthese luminousbands . When the sli t was wide they resembled the

expanded lines seen in s ome gases . As the slit wasmade narrow the two fainter bands appeared to fadeout wi thout becoming more defined . I was unable toresolve them into lines . The middle band, which i s somuch brighter than the others that i t may be cons idered to repres ent probably th ree-fourths, or nearlyso ; ofthe whole of the light which we receive from thecomet, appears to pos sess s imilar characters . In thi shand

, however, I detected occasionally two bright lineswh ich appear to be shorter thanthe band

, and may bedue to the nucleus itself. Besides these brightbands there was a very faint continuous spectrum.

’

Interpre ting these observations according to theprinciples which have beenalready s tated, we deducethe following interesting results .The nucleus of Bro rscn

’

s comet consi sts o fluminousgas . The coma is als o gaseous in the neighbour-howlo l

‘

the nucleus , but i ts outer portions are ofa different

THE TWO COMETS OF THE YEAR 1868 . 1 5 7

characte r and shine by reflecting the solar light. Thispart of the comamay be either liquid o r solid . Therei s nothing opposed to the supposition that it i s of

the nature of cloud—that is, that it i s produced bythe condensation of true vapour into minute liquidglobules .Returning to the consideration of the gaseous part

of the comet the questionwill at once suggest itselfwhat the gases may be which constitute the substanceof the nucleus and coma. Here our informationi s notquite so satisfactory as could be desired .The brightes t band i s inthe greenpart of the spectrum, and agrees very nearly with the brightest line inthe spectrum of nitrogen. The want of exact agreement prevents us from assuming that nitrogenreallyexists (in any form) in the substance of the comet.The other line s of the spectrum of nitrogenare not

present in the spectrum of the comet : but this peculiarity i s not so perplexing as the other, for it is wellknownthat certainlines will disappear fromthe spectraofhydrogen,nitrogen, and other gases , under particularcircumstances of illumination, temperature , and so

on.

Nor i s the circumstance that there are bands of lightinstead of well marked lines a peculiarity which needcause perplexity. For under certain circumstances oftemperature and pressure , the lines of the spectra ofvarious gases become expanded or diffused until theyappear as bands of light.The two fainter bands are yellow and blue

, re spec


tively. They cannot be identified with the lines seeninthe spectra of any knownterrestrial gases .Of whatever gases the nucleus i s composed it appearsthat conditions wholly different from any with whichwe are familiar on earth prevail inthis, and doubtlessinall other comets . The gases which form the nucleus,though self-luminous, are probably not incandescent.Remembering that comets are luminous whensituatedfar out inspace beyond the orbit of our ownearth, weare prevented from assuming the existence ofanintens ity of heat (due to solar action) sufficient to accountfor their inherent light. And if the light ofa cometwere due to a state of incandescence inthe componentgases, there would be a rapid consumption of thesubstance of the comet, and we should be quiteunable to account

‘

for the fact that Halley’s comethas continued to shine, with no appreciable loss of

brilliancy, for upwards of three hundred years . Weseem forced therefore to surmise that the gases whichform the substance of comets owe their light to a speciesof phosphorescence which is independent of the comet’stemperature, or else to some electrical properties thenature of which it would not be easy to divine .Our perplexity i s increased whenwe see the gaseswhich form the nuclei assuming either the liquid orthe solid form in the outer part of the coma. Thechange from gaseity to liquidity or solidity is an evi

dence of loss of heat, whereas one would expect theouter part of the coma, which i s exposed to the full

1 60 LIGH T SCIENCE F OR LEISURE H 0 URS.

a long-extended flight of cosmi cal bodies travelling inthe track ofTempel’s comet“.Now it appears clear that this flight of co smicalbodies may be looked upon as constituting an extension'

of the comet—an invisible train as it were .But for the accident that the comet’s track intersectsthe earth’s path in space, we should have remainedfor ever ignorant of the fact that the comet hasany other extent than

“ that which is indicated byits telescOpi c figure. Now,

however, that we knowotherwise , we recognise the probability that othercomets which have been looked upon as tailless mayhave invisible extensions reaching far behind them intoSpace, or evencompletely around their orbit.But the members of the November shooting-starsystem have been subjected to spectroscopic analys is .We know that they containseveral terrestrial elements ;and we recognise the probability that if we couldexamine one of them before its destruction (in tra

versing our own atmosphere) we should find a closeresemblance in its constitution to that of thos e aerolites or meteorites which have reached the surface ofthe earth .But here we encounter anew difficulty. One theory

respecting the tails of comets has accounted for themby the suppositionofa propulsive effect exerted by thesolar rays ; and another theory has ascribed them to theacti onof vapour s ascending in the solar atmosphere.But if the tails of comets really consist of solid mattervery widely dispersed, it must be quite evident that

THE TWO COME TS OF THE YEAR 1868 . i 6 1

neither of these causes could suffice to account for thegreat extension of these appendages . Thenthe rapidmanner inwhich the tails seem to be formed remainswholly mysterious . And we are also left without anyexplanationof the rapid change of positionexhibited bythe tail while the comet is sweeping around the sunat the time of nearest approach to that luminary.Sir JohnHerschel compared this motion to that of astick whirled round by the handle- the whole extentof the tail partaking inthe movement as if comet andtail formed a rigid mass .The difficulties here discussed seem in the presentstate of our knowledge wholly insoluble . In fact, itseems impossible even to conceive ofa s olutionto thelast mentioned phenomenon, so long as We look uponthe comet’s tail as a distinct unvarying entity . Forinstance, if the tail, a hundred millions of miles long,which extended backwards from Halley’s comet beforeperihelionpassage , consisted of the same matter as thetail which proj ected forwards to the same extent a fewdays later, then certainly there is nothing in our present experience of matter and its relations which canenable us to deal with so astounding a phenomenon.

It will be understood,of course , Sir JohnHerschel does

not say in so many words , that the tail of Halley’s

comet was brandished round in the manner describedabove, but that, although it appeared to move inthis manner, it i s impossible so to conceive of itsmotion.

1 6 2 LIGHT SCIENCE F OR LEISURE H o URS .

We refrain, however, from speaking further on a

point respecting which we have no means of reasoningsatisfactorily. Mere guess-work is an altogetherunprofitable resource in the discussion of scientific

matters .Now that we have so powerful an instrument of re

search as the spectroscope , there really seems hopethat eventhe hitherto inscrutable mysteries presentedby comets’ tails may one day be interpreted . Eachcomet which has been subjected to spectroscopicanalysis has revealed something new . Observations ,such as those which have beenmade onBrorsen’s comet,and on the two telescopic comets previously examinedby D r. Huggins, are not merely valuable inthemselves ,but as affording promise ofwhatmay be achieved when

“

some brilliant comet shall be subjected to spectroscopicanalysis . When we consider that all the comets yetexamined have beenabsolutely invisible to the nakedeye onthe darkest night, whereas several of the greatcomets have blazed forth as the most conspicuousobjects inthe heavens, and have even beenvisible inthe full splendour of the midday sun, we see goodreasonfor the hope that far fuller information will begained respecting the structure of comets so soon as

one of the more important members of the family shallhave paid us a visit.Whenever such aneventmay happenit i s not likely to

find our spectroscopists unprepared . It is probable that,before long, every important observatory will be supplied with spectroscopes . Already some of the most

1 64‘

LIGH T SCIENCE F OR LEISURE H o URS .

hypotheses which have beenput forward to account foithese peculiarities must now for a brief space claim our

attention. Although we are far from being ina position to theorise with any confidence respecting thenature of comets, and still less as to the purposes whichthey subserve inthe economy ofnature, yet the observations made uponthe second comet ofthe year 18 68 haveresulted ina positive discovery which may serve as astand-point, so to speak, whence we canexamine somewhat more confidently thanof old, the various theorieswhich have suggested themselves to those who havestudied cometic phenomena.

Inconsidering these hypotheses we have to distingui shbetweenthe views which have beenentertained respecting the nucleus and coma, and those which regard theless intelligible phenomena presented by the tai l. Thisremark may seem trite and obvious, but in realitythe two classes ofhypotheses are found singularly confounded together inmany works onpopular astronomy.Let it be understood then, that when, in speaking of

anhypothesis respecting comets no special mentioni smade of the tail, it i s to be assumed that the hypothesis applies solely to the head of the come t. The sameholds, by the way, with reference to the phenomenapres ented by comets. For instance

,when we said in

the paper on Comet I. that comets grow smaller asthey approach the sun, the remark was to be understood to apply to the volume of the head

,not to the

whole space occupied by the head and tail . Infact, itwould have been impossible to assert anything with

THE TWO COMETS OF THE YEAR 1868, 1 65

respect to the volumes of comets‘ tails, inasmuch as theapparent extent of these appendages varies accordingto the atmospheric conditions (humidity, clearness, andso on) under which the comet is observed, and alsoaccording to the light-gathering power of the observer’stelescope .To return, however, to the theories which have beenformed respecting comets .It has beencommonly admitted that the substance

ofwhich comets are composed is either wholly o r principally gaseous . Inno other way, it should seem, canthe remarkable variations of appearance which cometspresent as they approach the sunor recede from himbe reasonably accounted for .

Kepler held that comets are wholly gaseous , and

that they are liable to be dissipated in space by thesun’s action. He supposed that the process ofevaporationwhich thus led to the destructionof a comet wascarried onthrough the medium of the tail. It needhardly be said that modernobservations are completelyopposed to this view. Comets have beenseento returnagainand againto the neighbourhood of the sunwithout any apparent diminution of volume, although ateach return a tail of considerable length has beenthrownout. For a long time , indeed, it was thoughtthat Halley’s comet was gradually dimini shing involume ; but at the last returnthis magnificent obj ecthad recovered all its pristine splendour.Newtonheld, onthe contrary, that comets are partlycomposed of s olid matter. He supposed that only the

1 66 LIGHT SCIENCE FOR LEISURE H 0 URS.

gaseous matter was affected to any noteworthy extentby the actionof the sun’s heat. Raised from the solidnucleus the vaporised particles passed first into thecema, be imagined, and were thence carried offintospace to form the comet’s tail. Others so far modifiedNewton’s V1ews as to suggest that the vaporised matteris not wholly carried off but partially re-precipitateduponthe head of the comet, just as the vapours raised

‘ from the oceanare precipitated upon the earth intheform of rain.

We have seenthat a comet diminishes involume asit approaches the sun. It will be noticed that boththe theories which have beendescribed would accountsatisfactorily for the observed decrease of volume .But neither of them gives any satisfactory explanationof the fact that a comet recovers its original volume asit departs from the sun’s neighbourhood . Newton, indeed, put forward certainviews respecting the emissionof smoke from the nucleus during perihelion passage ,and he surmised that the true dimensions of the cometmight inthis manner be veiled to a certain extentbut this part of his theory has the disadvantage of

being almost unintelligible,besides being wholly in

sufficient to account for the regular diminution and

increase which attend the approach and recession of a

comet.A theory has lately been put forward by M . Valzwhich accounts for the variation of a comet’s volumeby the suppositionthat the solar atmosphere exerts apower of compression, which, varying with that atmoJ

1 6 8 LIGHT SCIENCE F OR LEISURE H O URS .

comet into invisible vapour. This action progressingfrom without inwards, of course produces anapparentdiminutionof volume . The diminution continues aslong as the comet i s approaching the sun, and for yeta few days after perihelionpassage but soonafter thecomet has begun to leave the sun’s neighbourhood thetransparent vapour begins to return to its originalcondition, the solar action being insufficient to keepthe whole of the vaporised matter inthe gaseous state .Thus the comet gradually re sumes i ts original apparentdimensions.There are few phenomenawhich have given rise to

more speculationthan those presented by the tails ofcomets. Astronomers who, in dealing with othermatters, have exhibited the soundest judgment, andthe most logical accuracy Of argument

,seem to feel free

to indulge inthe most fanciful speculations whendealing with this subj ect.A favourite theory with the earlier astronomers wasfounded on the Observed peculiarity that the tails ofcomets are usually turned directly from the sun. It was

supposed that the tail i s not a really existent entity,but merely indicates the passage of the solar raysthrough space, after their condensationby the sphericalhead of the comet. Just as a light received into adarkroom through a small aperture appears as a long rayextending in a straight line through the room, so ,according to this theory, the sun’s light , concentratedby the comet’s head, throws a long luminous beam intospace . Unfortunately for this View there is a want ’

Of

THE TWO COME TS OF THE YEAR 1868 . 1 69

analogy betweenthe two case s thus brought into com

parison. The light shining into a room produces theappearance ofa ray, becaus e it illuminates the ai r and

the small particles of floating dust which it encountersinits passage . There is nothing c orre sponding to thisin the interplanetary s paces. If there were, the skywould never appear black , since the sunwould alwaysbe shining onmatter capable of reflecting his rays.Kepler was the first to form a reasonable hypothesisrespecting comets’ tails . He supposed that the actionofthe solar heat dissipates and breaks upacomet’s substance.The rarer portions are continually swept away, heimagined, by the propulsive energy o f the solar rays,and are swept in this way to enormous distanc es fromthe comet’s tail. The denser portions remain aroundthe nucleus and form the coma.

The modern theory respecting light (according towhich there is no propulsion of matter from the sun,but a simple propagationofwave-like motion) , does notaffect Kepler’s hypothesis so much as might be ima

g ined. Whatever theory of light we adopt we areforced to assume an extreme tenuity in the matterwhich forms the tails of comets . And when once wehave made this as sumption, we are enabled to admitthat eventhe propagationofawave-like motionthroughthe ether which is supposed to occupy the interplanetaryspaces, might suffice to carry off the attenuated nebulous matter with tremendous rapidity.The defect of Kepler’s theory is that it appearsinsufficient to account for those anomalous tail-for

1 70 LI GH T SCIENCE F OR LEISURE H OURS.

mations which were referred to in our paper on

Comet I.

Newton’s hypothesis respecting comets’ tails wassomewhat different. He supposed that. the intenselyheated comet communicated its heat to the surroundingether

,which thus grew rarer and ascended inthe solar

atmosphere—that is, flowed away from the sun— preci sely as heated ai r ascends from the earth . The etherthus displaced would carry away with it the rarer portions of the comet’s substance , just as smoke is carriedupwards by a current ofheated a1r .

It will be seen at once that Newton’s theory,like

Kepler ’s , affords no explanationof lateral tails, or of tailsturned towards the sun.

Inmodern times a theory has been founded on thesupposition that cometic phenomena may be due toelectrical agency. The Germanastronomer Olbers wasone of the first to propound this view, and many eminent astronomers —amongst others the younger Herschelhave locked with favour upon the theory. As yet,however, we do not know enough respecting electricityto accept wi th confidence any theory of comets foundeduponits agency.The comet respecting which I now have to treat

was discovered in the middle of June 18 68 , by VVinnecke . At first it was a telescopic Object, but itgradually increased inbrilliancy until it became visibleto the unaided eye . In the telescope, at the end of

June, the comet appeared as a circular cloud ratherbrighter inthe middle, where there was a “roundish spot

1 72 LIGH T SCIENCE FOR LEISURE H OURS.

may exist in the direction of its length . But if theobject i s brighter insome parts of its breadth thaninothers, the spectrum will show corresponding variations

ofbrilliancy across its breadth . Hitherto we have been

assuming that all the light from the object i s of thesame kind, however it may vary inbrilliancy. Suppose,however

,that the light from the middle of the obj ect

gives one kind of spectrum, the light from the outerparts another ; thenthe spectrum will vary incharacter

as well as inbrilliancy across its breadth . Suppose forexample, that the middle of the obj ect is gaseous whilethe outer parts are solid or liquid, thenthe appearancepresented would be two thinstreaks of rainbow-tintedlight, separated by a dark space l across which wouldbe seenthe bright lines belonging to the gaseous centralpart of the luminous object.Now the breadth ofthe spectrum seenby Dr. Huggins ,corresponded with the breadth of the coma so far as

the widest parts of the tongue-shaped bands were concerned. But the narrower parts were about the widthof the nucleus . Therefore the first questionto be decided was 'this, —Is the narrowing of these bands oflight towards one extremity

, and of the other towardsboth extremities, to be cons idered as indicative of any

Our readers will, of cours e, unders tand that a s lice only ofthe objecti s brought under spectroscopic analys is at once. Ifthe whole ofa cir

onlar object, whos e centre was gaseous , were examined at once, themiddle s treak of the spectrumwould exhibit the compound spectrum of

the edg e and centre of the object. Such anarrangement would clearlybe unfavourable to the formationof clear views respecting the characterof the object’s light.


difference , incharacter , between the light emitted bythe nucleus and that emitted by the coma? At firstsight it seems that no other conclusion could be cometo . But a little considerationenabled Dr. Huggins toarrive at a different result. The tongue-shaped bandswere not only narrower but very much fainter towardsone end. They were also fainter along their outeredges , on account, of course, Of the faintness of thecoma as compared with the nucleus . Now it waspossible that the narrowing downof the bands mightbe only apparent, and due to the fact that their outerparts

,though really existent, became invisible at the

fainter end. And there were two modes of attackingthe question. First the Observer could determine by acareful inspection whether the light at the narrowerend of the tongues was so faint that it ought to di sappear at the edges merely by undergoing the same sortof reductionas the brighter light at the broader end of

the tongue : thi s would show that the coma does notdiffer in constitution from the nucleus . Secondly, ifthe strip brought under examinationwere narrowed byany contrivance , it is clear that any difference whichmight exist inthe constitutionof the coma and of thenucleus ought to be exhibited in a more markedmanner.Dr . Huggins applied both methods, and each resulted

inshowing that the nucleus has the same constitutionas the coma, excepting only that the exterior part ofthe coma seems to give a continuous spectrum . In

other words, the nucleus and all the coma except its

1 74 LIGH T SCIENCE F OR LEISURE H OURS.

outer shell consists of the same incandescent vapour ;but the outer shell of the coma either consists of m

candescent solid or liquid matter or shines by reflectingthe solar rays .SO far, however, there is little in the spectroscopic

analysis which differs incharacter from what had beenobserved respecting Brorsen’

s comet. But we have nowto record one of the most startling discoveries ever maderespecting comets .Dr. Huggins was reminded by the appearance of thecometic spectrum of a form of the spectrum of carbonwhich he had observed inthe year 18 64 . It must bepremised that the spectrum of anelement Oftenassumesa different form according to the circumstances underwhich it is obtained. Amongst the obj ects which havespectra thus variable is the element carbon. The parti cular form of carbon-spectrum which resembled thatof the comet, i s that obtained whenanelectric spark i stakenthrough olefiant gas—a substance which, as manyof my readers are doubtless aware , consists of carbonand hydrogen, and is one of the constituents of ordinarycoal-gas 1 Of course the spectrum of Olefiant gas

exhibits the bright lines belonging to hydrogen but asthese are well known, the part of the spectrum belonging to carbonalso becomes determinable .Having noticed, as we said, the resemblance betweenthe spectrum of the comet and a form of the carbon

The other cons tituent i s ‘ fire-damp ;’

also a compound of carbonand hydrogen. Olefiant gas i s commonly called heavy carburettedhydrogen, whi le fire-damp i s termed light carburetted hydrogen.

1 76LIGHT SCIENCE F OR LEISURE H O URS.

(one could suppose) to anextraordinarily high temperature

.There have beencases where comets have been

so near to the sun as ,to account for almost any con

ceivable change in the constitution Of their elements.Anintensity ofheat Of which we can form no concep

tion must have been experienced , for example, byNewton’s comet ; and a still fiercer heat dissipated thesubstance Of the comet Of 1843 . But Winnecke ’scomet at the time Of Observationwas at far too great adistance from the sun for us to assign to its mas s atemperature which under ordinary circumstances wouldaccount for the volatilisationOf carbon.

Nor does the rarity Of the atmosphere inwhich thecomet was moving serve to help us in our difi culty.

Doubtless we are little familiar with the effects whichterrestrial elements would experience if they were di stributed freely inthe ether occupying the interplanetaryspaces . But so far as our exper ience enables us tojudge

,we should rather look for intensity Of cold than

Of heat under such circumstances . We see the heightsof the Andes and Of the Himalayas clothed inperpetualsnow, though day after day the fierce heat Of the tropical sunpours downuponthem

, and though there is nowinter there (in our sense Of the word) during whichthe snows are accumulated . We know that the explanationOf this peculiarity lies inthe extreme rarity Of the.air at a great height. It seems

,therefore

,reasonable

to conclude that the cold Of the interplanetary spacesmust be far greater. Yet here we have anObj ect whoselight comes from the incandescent vapqur Of so fixed


and unchangeable a substance as carbon, and thus, inplace Of analmost inconceivable intensity Of cold wefind the evidence Of intense heat.It seems impossible , at present, to suggest any explanation Of the Observed phenomena. That carbonexists out yonder in space inthe state Of luminous gasor vapour, i s the one fact Of which alone we can

be certain. Dr . Huggins inhi s treatment Of this factsuggests the possibility that the carbonmay be dividedinto particles so minute , that as the comet approache sthe sun, more Of the sun’s heat i s gathered up, so tospeak, and that thus the carboni s volatilised . He alsopoints to phenomena of phosphorescence and fluores

cence in illustration Of the appearance presented bythe comet’s spectrum ; but without suggesting anyassociation between these phenomena and those presented by comets .One cannot help associating the new views thus

Opened out to us res pecting comets,with the discovery

recently made that the meteoric bodies which flashsingly or inshowers across our skies belong inrealityto the trains Of comets . We have now every reasontobelieve that there is not a single memb er Of the meteorie systems, not a single aérolite , bolide , or fire-ball,that has not belonged once upon a time to a comet.The evidence onwhich this View is founded

,though it

may seem insufficient at a first glance,i s almost irre

s i stible to those who can appreciate i ts significance .Let us briefly recapitulate the facts .

It has been proved that shooting-stars come from


the interplanetary spaces, that they form systems , andthat these systems travel in regular elliptical orbitsabout the sun. Two Of the systems produce strikingmeteoric displays, viz . the system encountered by theearth onor about August 10, and the system encoun

tered on or about November 1 3 . Now it had beensuggested that the members Of the former systemfollow the track Of the conspicuous comet which madeits appearance in the year 1 862 ; and it was provedthat, assuming the orbit Of the meteors to be veryeccentric, and assigning to them a period Of 147 years

(that Of the comet), their motions corresponded inthemost remarkable manner with the orbital track Of thecomet . In fact the agreement was so close that veryfew who had examined the question could bell eve it tobe accidental . But there were two Objections onwhichsome stress was laid . First

,it had beennecessary to

make assumptions respecting the motionOf the meteors ;secondly, those assumptions werenot rendered probableby anything which had been p r oved respecting anymeteoric system . The examination Of the Novemberstar-shower by a host Of eminent mathematicians in1866—7 led to results which at once removed theseObjections, and brought new evidence—and that Ofthe most striking character— in favour Of the theorythat comets and meteors are associated . It had beensupposed that the November meteors travelled in a

nearly c i rcular orbit withina period Of somewhat lessthanayear . Adams proved that they travel inanorbitextending far out into space beyond the orbit Of distant

( 80 LIGHT SCIENCE F OR LEISURE H O URS.

subjected to spectroscopic analysis , so that we cannotpronounce with any certainty respecting the structureOf these singular appendages . Some astronomers aredisposed to look onthe formationOf a track of meteorsall round the orbit of a comet as due to the actionOf

influences by which parts Of the comet’s mass arethrowninto orbits Of slightly longer period thanthatOf the head, though closely resembling that orbit infigure . Be this as it may, it is certain that the greatcontrast in character between the meteoric bodieswhich form the train Of a comet, and the gaseousnucleus and coma, remains yet among the mysterieswhich astronomers have beenunable to clear up .But so soonas it had beenshownthat acomet’s headis formed Of a certainwell-knownterrestrial substance,it was natural that the question should be askedwhether this substance is to be found in meteors .Hitherto no great progress has been made indetermining the elementary constitution Of meteors whichhave not actually fallenuponthe earth . It i s so diflicult to catch them during their brief transit across our.

skies that only a few substances, as sodium, phosphorus,magnesium, and so on, have been shown with anyappearance Of probability to exist in shooting-stars .Certainly carbon i s not among the number Of thoseelements which have been detected inthis way. But

at a recent meeting Of the Astronomical Society, it wasstated that several aérolites contain carbon in theirstructure , and Dr. De la Rue Offered a fragment Ofone of these to Dr. Huggins for analysi s. Certainly a

THE TWO COJIIETS OF THE YEAR 1868 . 1 8 1

strange circumstance that an astronomer who hadanalysed the structure Of a body millions Of miles awayfrom the earth , should take into his hands and subj ectto chemical analysis a fragment which had once inallprobability belonged to a similar comet !In conclusion, I must notice that there has beenaremarkable absence during the past few years Of thosebrilliant and long-tailed comets which alone seemcalculated to afford the spectroscopist the means Ofanswering some Of the difficult questions suggestedabove . The tail OfWinnecke’s comet was too faint togive a visible spectrum . In fact the comet itself wasonly just visible to the naked eye . When a blazingObj ect like Donati’s comet or the comet Of 18 6 1 comesto be subj ected to spectroscopic analysis, we may hopefor an amount Of information compared with whichthat hitherto Obtained is probably altogether ins ignificant.

FromFrazer ’s Magazine for February and June 1869.

COMETS OF SH OR T PERIOD .

IT is related by Apollonius the Myndian, that theChaldean astronomers held comets to be bodies whichtravel in extended orbits around the solar system .

The Chaldeans spoke Of comets ,’ he says, as Of travellers , penetrating far into the upper celestial spaces .

’

He adds, that those ancient astronomers were evenable

r8 2 LIGH T SCIENCE F OR J EEISURE H 0 URS.

to predict the return of comets . How far it may besafe to accept the statements ofApollonius is uncertain. He ascribed other powers to the Chaldeans, ofwhich we may fairly doubt their possession—for instance, the power of predicting earthquakes and floods.Infact, there is so marked a dispositionamong ancientwriters to exaggerate the acqui sitions of Chaldeanastronomers , that it becomes extremely difficult to distingui sh truth from falsehood. Still, there is sufficientevidence of their skill and patience as observers, torender it fully possible that they may have discoveredthe periodicity of one or two comets .But until the rise of modernastronomy, the opinionwhich was almost universally held respecting cometswas that ofAristotle, that they are of the same natureas meteors or shooting-stars

,existing either inthe air

not far above the clouds, or certainly far below themoon.

The discovery of the periodicity of Halley’s cometfollowing quickly onNewton’s announcement of thelaw of gravitation, led astronomers to examine theorbits of all the comets which became visible

,with the

hope of finding that some of these b ‘odies may be travelling in re-entering paths . But inasmuch as noneof the bri lliant comets of whose appearance records hadbeenpreserved seemed to have ever revisited the earthsave Halley’s alone , while evenHalley’s travelled inanorbit of enormous extent

, an orbit which reached outinspace more than three times as far as the orbit ofthe most distant known planet

, astronomers held that

r84 LIGH T SCIENCE F OR LEISURE H o URS.

ceived. Just as Halley’s comet, whenclose to the sun,sweeps rapidly round him— that is , ina sharply curvedpath—s o the new comet’s path was sharply bent aroundthe temporary focus formed by the great planet . But

just as Hall'ey’s comet, after perihelion passage, moves

away from the sun, so Lexell’s comet, after what may

be termed perij ovian passage, moved away from Jupiter

, and passed againwithinthe sun’s attraction. From

this time the comet beganto follow anew orbit aroundthe sun. This new orbit was an oval of moderateeccentricity, round which the comet travelled inaboutfive and a half years .At the next returnof the comet to perihelion, it was

not likely that astronomer s would obtaina View of it ;for , on account of the odd half-year in its period, itcame to perihelionwhenthe earth held a point inherorbit exactly opposite to that which she had occupiedat the comet’s former perihelion passage ; therefore,the comet, which before was favourably, was now un

favourably s ituate ‘

d‘

fOr observation.

As the period for the comet’s second return ap

proached, astronomers looked out eagerly for its advent.Again and again the heavens were swept ’ for thefaint speck of nebulous light which should have announced the return of the wanderer . But days, andweeks, and months passed , until it became certainthat either the comet had been shornof nearly all itsformer brilliancy, and had thus escaped unnoticed, orthat something had happened to deflect it from itscourse .

COMETS OF SHORT PERIOD . 1 85

The last alternative appeared so much the moreprobable one , that mathematicians began to examinethe path of the comet, to see whether it had approachedso near to any disturbing body as to have beendrivenfrom its recently adopted orbit. The examinationwass oonrewarded with success . Ifwe consider the natureof orbital motion, we shall at once s ee that, so long asLexell’s comet was subj ected to no new disturbingattractions , it was compelled, once inevery revolution,to returnto the scene of its former encounter with theplanet Jupiter. This returnwas fraught with dangerto the stability of the comet’s motions . So long asJupiter was not near that particular part of his orbitat which the encounter had takenplace, the comet wasfree to pass the point of danger, and return towardsthe sun but if ever it should happenthat Jupiter wasclose at hand when the comet approached his orbit,thenthe comet would be as liable to have its motionsdisarranged as at the original encounter. It happenedthat the period of the comet’s motionin its new orbitwas almost exactly one-half of Jupiter’ s period.This was unfortunate ; since it clearly follows that,when the comet had revolved twi ce, Jupiter had re

volved Once . round the sun. Thus the comet againencountered the planet, with what exact result hasnever become known; but certainly with this generalresult, that the comet’s movements were completelydisarranged. It has never returned to the neighbourhood ofthe earth .We may look upon Lexell

’

s as the first discovered


comet of short period for although it was never seenafter its first visit

,yet nothing. can be more certain

than that it did actually return once , and that itwent twice round its new orbit. Indeed, if i t hasnot been absorbed by Jupiter—a very unlikely contingency

—fi i t must still be revolving inSpace with anorbit which brings it, once in each revolution, to thescene of its former encounters . Thefigure of its orbitmay be altered again and again by encounters withJupiter ; but each new orbit mus t traverse this dan

gerous point. This follows directly from the laws oforbital motion around an attracting centre . A bodywill continue to revolve in any orbit along which ithas once begun to move

,unless it is acted uponby

some extraneous force . Accordingly,if at any point

of its path an extraneous force suddenly disturb itsmotion, the disturbed orbit cannot fail to pass throughthe point of disturbance . Thus the body may againfall under the influence of the disturbing agent, and becaused to move inyet another orbit through the samepoint. And inthe course ofmillions of years, a bodymight thus travel in a hundred different orbits, allpassing through a common point. There is, indeed ,one way inwhich Lexell’s comet might have escapedfrom Jupiter’s control . If after one of its encounterswith Jupiter, it happened to pursue a path whichbrought it verynearly into contact with Saturnor someother large planet, it might be compelled thenceforthto abandon its allegiance to Jupiter. But the probability of this happening to a comet which had once got


the private observatory of Sir T . M . Brisbane at Paramatta. In 1825 , the comet was detected by s everalindependent observers . It was seen again in 1828 ,

being detected by two observers—Harding at Gottin

g en, and Gambart at Marseilles . In 183 2 and 1 8 3 5 ,

it was seenfrom the observatory at the Cape of GoodHope.At the next return of the comet, which took place

onDecember 9, i t was visible to the naked eye for thefirst time since its discovery. At this passage, also, avery noteworthy peculiarity was remarked—or rathera pecul iarity which had been remarked by Encke in18 18 , was now, for the first time , placed beyond adoubt. Encke had suspected that the comet’s periodwas slowly diminishing. Each returnto perihelionoc

curred about two and a half hours before the calculatedtime . Such a discrepancy may appear very trifling,and in fact it might seem that no c ertainty could befelt respecting it ; and this i s the case so far as one or

two revolutions are concerned . But wheneach successive revolution shows the same discrepancy, the defic iency soonmounts up to a period respecting whichno doubt can be entertained . For example, betweenthe perihelion passage in1 789 and that of 1865 , thecomet has made twenty-three revolutions, and eachhas been less than the preceding by two and a halfhours (onthe average). Hence , the last revolutionof

the series occupied two days and a half less thanthefirst. But eventhis does not express the full effec t ofthe change ; for the comet having gained two and a

COMETS OF SHORT PERIOD . 1 89

half hours inthe first revolution, five inthe next, sevenand a half inthe next, and so on—it i s the sum of all

these gains (andnot the gainmade inthe last revolution)which expresses the total gainof the comet inpoint oftime . Hence the last perihelion passage occurredtwenty-nine days before the time at which it wouldhave taken place , but for some unknowncause whichhas interfered with the comet ’s motion. What thatcause may be, has not yet been certainly determinedbut it is at least highly probable that Encke has assigned the true cause in suggesting that so light asubstance as the comet may be retarded in its passagethrough the interplanetary spaces by the existence ofa thin ethereal medium ,

’ incapable of perceptiblyretarding the motionof the planets .At first s ight, it may seem strange that we shouldspeak of the accelerati on of the comet as being causedby the retarding influence of such a medium as has

!

been conceived to occupy the interplanetary spaces .Yet it i s strictly the case that, if a planet or comet becontinually checked in its onward course

,its velocity

will continually grow greater and greater. For instance,if our earth were s o checked, it would move ina Spiralwhich would gradually bring its orbit to that ofVenus,by which time i ts motionwould be as rapid as that ofVenus (which moves one-third faster thanthe earth) ;then 1t would continue revolving in a spiral till itreached the orbit ofM ercury, whenit would be movingas fast as this the swiftest of all the planets . And so


the earth would continue to approach the sun withcontinually increasing velocity.Returning to Encke’s comet, we have to notice yet

another important discovery which was effected by itsmeans . The comet passed so near to Mercury i n 18 3 5

as to enable astronomers to form amuch more satisfactory estimate of this planet’s mass thanhad hithertobeenobtained . It was found that the mass ofM ercuryhad beenlargely over-estimated.

No very long interval passed after the di scovery ofEncke’s comet before another comet of short periodwas detected. M . Pons, who had discovered Encke’scomet, it will be remembered, in18 1 8 , observed a faintnebulous object onJune 12 , 1 8 19. This object turnedout to be a comet ; and in this case, as inthe former ,Encke calculated the stranger’s orbit and period . Hefound that it moves inanellipse which extends slightlybeyond the orbit ofJupiter, and that it has a period ofabout five and a half years . This obj ect was not seenagain, however, until the year 1 8 5 8 , whenM .Winnecke

discovered it, and at first supposed i t to be anew comet.Calculationsoon showed the identity of the two obj ects

,

and confirmed the results which had beenobtained byEncke in 1 8 19.

The next comet of short period was discovered byM . Biela in 1 826 . Perhaps nothing inthe whole history of cometic observation is more surprising thanwhat has been recorded of th1s singular object. Wemust premise that the comet had beenseeninMarch1772 , and again inNovember 1805 . But it was not

1 9 2 LIGH T SCIENCE FOR LEISURE H OURS.

Padre Secchi , at Rome, detected a faint comet preceding it. If, as i s probable, this faint comet is thecompanion, we may assume that the two bodies arepermanently separated.At the two next returns the comet was not seen

,

and much interest was felt by astronomers respectingthe anticipated return in January 18 66 . It was

searched for systematically at the principal Europeanobservatories . In fact, so closely did Father Secchiexamine the calculated track of the comet, that hedetected several new nebulae in that region. But thecomet itself was not found . Astronomers are unableto assignany satisfacto ry reasons for its disappearance .It i s knownto have traversed the zone of the November meteors where that zone is richest—our readerswill remember the display of shooting-stars in 1 866and Sir J. Herschel surmises that i t may have beendestroyed in the encounter. Possibly this may be thetrue solution of the difficulty ; or, it may be that thecomet was merely dispersed for a while during thepassage of the meteor-zone, and may yet gather itselftogether and become visible to astronomers.‘

We pass over three or four comets belonging to thisclas s which present no special features of interest, to

The returnof thi s comet in 1 872 was eag erly looked for by as tronomers . But the comet was not s een. OnNovember 27, 1872 , therewas a fine di splay ofmeteors , as the earth pas s ed through the comet

’

s

track, and afterwards a cometic obj ectwas s eeninthe directiontowardswhich the meteors had beentravelling . But thi s was not B iela

’

s

'

comet.

which, indeed, must have pas s ed that place nearly twelve week s earlier.

Indeed, s ome doubt exi s ts whether the object was travelling in the

track either of the comet or of the meteors .

COME TS OF SHORT PERIOD. 1 93

come to anobject which has recently beenrediscovered,and will continue visible (in good telescopes) forseveral weeks . On February 26 , 1 846 , M . Brors en

discovered a telescopic comet, whose motions soonshowed it to belong to the class of obj ects we arenowdealing with . It was found to have anorbit of moderate eccentricity, extending just beyond Jupiter

’s orbit,and a period of about five and a half years . It was

not seen at its next returnto perihelion; but was rediscovered by M . Bruhns on March 18 , 1 8 5 7. In

1862, it againescaped undetected ; but at its presentreturn, it has been rediscovered (by three observerssimultaneously), and it is now being carefully trackedacross the northernskies .Inall, there have beenrec ognised thirteencomets ofshort period—that is, having periods of less thanseven

y ears . Amongst these are included several which haveonly beenseenonce

,and some which are knownto have

been subj ected to such disturbance as no longer totravel in orbits of sho rt period . Of these thirteencomets, no less than ten have the aphelia of theirorbits just beyond the orbit of the planet Jupiter ; twohave their aphelia just within Jupiter’s orbit ; and

Encke’s comet alone has its aphelionat a safe distancefrom that orbit. It appears to us that the peculiarity

not without meaning. Rememberingxell

’s comet, we seem to find a satisf the peculiarity. We have seenfirst introduced into the systemby the giant planet Jupiter

, and

0

1 94 LI GH T SCIENCE FOR LEISURE H OURS.

then summarily dismissed. So long as the comet remained within that system, the aphelion of its orbitlay just beyond the orbit of Jupiter, and thi s would

be the cas e with any comet introduced in. a s imi lar

manner . But for the coincidence which led to itsexpul sion, Lexell

’s comet would have continued torevolve as a short-period comet. It seems also clear,that in the course of many ages, its period and orbitwould have grown gradually smaller, through theoperation of the same cause (whateve r that may be)which is now reducing

,the period and orbit ofEncke’s

comet. At length it must have attained a path safewithin the orbit of the great disturbing planet. In

the list of short-period comets, then, we seem to seeillustrations of the successive stages through whichLexell ’ s comet would have passed inattaining the sortof orbit inwhich Encke’s comet is now moving. And

it s eems permissible to assume that all the short-periodcomets have beenintroduced to their present positionwithin the solar system by the same cause which ledto the temporary appearance of Lexell’s comet as a

comet of short period—that is , by the attractiveenergy of the planet Jupiter.

Chambers’

s Journal,July 1868 .

1 96 LI GH T SCIENCE F OR LEISURE H OURS .

course and habitudes of this current, and thento inquirea little into the vexed questionof its cause .Maj or Rennell traced the Gulf Stream from a suppo sed source in the Indian and Southern Oceans .M oderngeographers and physicists prefer to look forthe rise of the current somewhere near the Cape ofG ood Hope . ‘ The commencement and first impulseof the mi ghty Gulf Stream is to be sought,’ writesHumboldt

,

‘southward of the Cape of Good Hope .’

It appears to me, however, that the true source of thegreat stream is to be looked for in the equatorial zoneof the Atlantic. When we come to inquire into thecaus e or causes which give birth to the Gulf Stream ,

we are led, as I imagine, to this regionrather thantol

any other (though , perhaps, in a stream which formspart of a continuous system of circulation, we can

hardly speak ofany one portionas the source) I shalltherefore trace the stream, and the system to which itbelongs

,from the great equatorial waters which move

,

as Columbus was the first to discover, with the heavens

(las aguas can con lOs ci elos ), that is, from east towes t, following inthis the apparent motions of the sun,moon, and star s .

’

The map of the Atlantic Ocean on p . 2 17 , i s constructed upon one of those forms of isographic pro

j ecti ondescribed in my Essays on Astronomy. It i simportant, indealing with the subj ect of currents, thatthe question of area should be considered, and, therefore, that our illustrative charts should represent suchareas correctly. This Merc‘

ator's charts are far from

THE GULF STREAM. I97

doing. The portion of the Atlantic Ocean betweenEngland and the United States of America is undulymagnified, and still more i s this the case with theportionbetweenSwedenand Greenland . Onthe otherhand the portionbetweenAfricaand the Gulf ofMexicois unduly diminished . Thus it is scarcely possible toform from such charts just notions of the actual character ofthe oceanic circulationwhereof the Gulf Streamforms a part . (Compare the charts illustrating theEssay onthe Climate

’

ofGreat Britain, pp . 264 ,We see

,in our map (p. 2 17 ), l that there is a great

equatorial stream extending inits easternportionfar tothe south of the equator , but passing to the north als oevenhere, and still further to the north betweenthecoasts ofAfricaand South America. Near here the greatequatorial current divides into two portions . One passessouthward and thenreturns towards the east, accordingto some authorities, but, according to others, continuesits course southward until it is lost in the AntarcticOcean. We shall follow the northern bifurcation,however. The course of this portion of the Atlanticcurrent system has been far more exactly traced out.

Taking anorth-westerly course, the great current pour sitself against the barrier formed by the Leeward andWindward Islands . Passing between these i slands, itsweeps around the shores of the Gulf ofMexico , a por

For the sake of completenes s , and als o that the pres ent es say mayfai rly repres entmy views wheni t was written, I leave the account of the

map and of the cours e of the Gulf Stream unchanged here . By com

paring this es say w ith the follow ing , it will be noticed that only very

few pas sages are repeated insubstance.

r98 LIGHT SCIENCE FOR LEISURE H o URS.

tion, however, of its volume pass ing probably outsidethe West IndianIslands, to rej ointhe other outside thepromontory of Florida. At this point the stream hasbecome, probably, somewhat dimini shed in volume ,but being still more diminished inbreadth

,it flows as

a deep, strong, and swift stream, knownamong sailorsFrom hence the stream,as The Narrows ofBemini .

now become the true Gulf Stream , grows graduallywider, less deep, and less swift. Off Hatteras it i salready twice as broad as in the Florida Straits, andas it stretches with a wide easterly sweep across theAtlantic towards the shores ofIreland and the Hebrides

,

the current not only reassumes something of its o ri

g imal extent of surface, but againbifurcates ; a widebut somewhat sluggish stream is sent southwardtowards the shores of north-western Africa, to rej ointhe equatorial stream . The main portion of thecurrent, however, passes with a north-easterly courseup the Atlantic valley, between Iceland and Swedento the Polar seas . It seems uncertainwhether Rennell’scurrent, which passes around the Bay of Biscay, andthe current which streams southward past the shores ofSpain, are forks of the Gulf Stream. They are usuallyrepresented in maps as independent currents, and inCaptain Maury

’

s large map of the Gulf Stream thegreat southernbifurcationalready mentioned i s represented as a current impinging upon the flank of thestream which flows past Spain and north-westernAfrica. Yet, if these streams have not their source inthe G ulf Stream, it will be found no easy problem to

2 0 0 LIGH T SCIENCE F OR LEISURE H OURS .

namely, which extends between the most westerly partofAfrica and the West Indies— there is a wide expanseof waters unmoved by the flux or reflux of currents .Surrounded on every side by the circulating waters ofthe Central Atlantic current—system, this region re

mains undisturbed save by winds and the tidal wave .Accordingly its surface is covered with different formsof marine vegetation. My readers will doubtlessremember the interest which the Great Sargasso Seaexcited inthe mind of Christopher Columbus . Oviedotermed this regionthe seaweed meadow.

’ A host ofsmall marine animals,

’ says Humboldt,

‘ inhabit thisever-verdant mass of Fucus natans , one of the mostwidely-diffused of the social plants of the ocean

,con

stantly drifted hither and thither by the tepid windsthat blow across its sur face .’

In the South Atlantic there is a smaller and somewhat more sharply-defined Sargasso , covered chieflywith rockweed and drift. A weedy space occur s alsoabout the Falkland Islands , but i s probably not a trueSargasso . Maury considers that the seaweed reportedthere probably comes from the Straits of Magellan,where it grows so thickly that steamers find greatdifficulty in making their way through it ; for it socumbers their paddles as to make frequent stoppagesnecessary.Such is the distributionof the surface of the AtlanticOcean. But now the question will at once suggestits elf : Is the complete system of oceanic circulationexhibited onthe surface ? It s eems now quite certain

THE GULF STREAM .

that this questionmust be answered in the negative .We might, indeed , at once point to the existence of theimportant current which laves the shores of the UnitedStates as ananswer to the question for where canallthis water find anoutlet ? It does not pass the Peninsula ofFlorida as a current it does not cross the GulfStream where , then, can it go but underneath theocean’s surface But we have positive evidence of theexis tence of under ‘ currents .

In the first place it is found that indeep-sea soundings inmany parts of the ocean, far more line may bepaid out without any signof the bottom being reachedthan the depth of the ocean in those parts wouldaccount for. In places where it has been proved byother methods thanordinary sounding that the depthis not more thanthree miles

, no less thantenmiles ofline have been paid out, being carried out so stronglythat the slightest check in the paying-out apparatushas sufficed to break the sounding-line.Inthe second place, it has been found possible todetermine the depth at which a submarine current isflowing, and the direction. in which it flows . ThusLi euts . Walsh and Lee , inthe Americanservice , havingloaded a block ofwood to sinking, and let it downtodifferent depths, had repeatedly the satisfaction of

seeing the work of under-currents . It was wonderful,indeed

,

’ they write , to see the barrega (afloat attachedto the upper end of the line) moving off, against wind,sea, and surface current, at the rate of over one knotanhour, as was generally the case, and onone occasion,

2 0-2 LIGH T SCIENCE F OR LEISURE H O URS.

as much as one and three-quarter knots . The men inthe boat could not repress exclamations of surprise , forit really appeared as if some monster of the deep hadhold of the weight below, and was walking off with it .’

Lastly, we may mentionthat CaptainWilkes, of theUnited States Exploring Expedition

,established the

existence of a cold under-current no less thantwo hundred miles broad at the equator.We may assume , then, that acomplete system of c ir

culation, vertical as well as horizontal , exists throughoutthe whole of the waters contained within the greatAtlantic valley.Where are we to look for the originof this vast series

ofmovements The actual work done inthe AtlanticOcean i s so enormous—inother words, the transfer ofsuch large volumes of water represents so enormous a

force, that we might well expect to be able at once toassignthe motive-power of this great machinery. For

it would seem that the giant which works such wonderscould not readily hide himself from our recognition.

It has not beenfound , however, that the solutionof

the problem has been so simple as was to have beenanticipated.Passing over the earlier guesses which marked theGulf Stream as the oflspring of the M ississippi River,of the sun’s motion inthe ecliptic (amysterious inter

pretati onof the phenomena), and of the tidal wave , wemay remark that but two explanations of the Atlanticcurrents seem to merit discussion.

Sir JohnHerschel is the principal exponent of the

2 0 4 LIGHT SCIENCE F OR LEISURE H OURS .

great currents are set inmotion. The agents whichderange equilibrium inthe waters of the sea, by alteringspecific gravity, reach from the equator to the poles,and intheir operations they are as ceaseless as heat andcold consequently, they call for a system of perpetualcurrents to undo their perpetual work .

’ Other causeshelp to cause currents,

’ he says,but the currents created

by them are ep hemeral.’

Here we have what i s a very pretty quarrel as itstands .’ Each of the disputants points to causes ofacknowledged importance, and also (whether efficiento r not in the particular matter under question) of

acknowledged general efficiency. Each has much tosay infavour of his ownview, and each considers hisantagonist’s agent as utterly insufli cient for the workascribed to it. Each has heard his opponent’s arguments, and-reiterates his ownstatement . Nor canit besaid that the opponents are unequally matched for, ifwe must place Sir JohnHerschel far before Maury as amathematicianand physicist

,and if wemust undoubt

edly look upon the former as the more practisedreasoner , yet we must remember, in turn, the specialattentionwhich CaptainMaury has givento the subj ectunder discussion, and the practical acquaintance with itwhich his experience as a seamanhas givento him .

Let us briefly state the arguments adduced by ‘

Herschel against Maury’s View, and by Maury against

Herschel’s .Sir JohnHerschel asserts that, inasmuch as the sun

’sheat warms the surface of the oceanmost intensely, so

THE,G ULF STREAM . 2 0 5

that the water of least specific gravity is already uppermost

,there can be no tendency to motion. For the

heated water cannot des cend, being buoyant ; noras cend, being uppermost ; nor move laterally, havingno impulse to motionof that sort, and being only ableto move laterally by reason of a general declivity ofsur face , the dilated portionoccupying a higher level .

’

He then applies to this declivity the test of quantitative analysis . Taking a column of water at theequator having at the base a temperature of 39° (atwhich temperature fresh water attains i ts greatestdensity, and which is also the temperature of waterfeet beneath the surface at the equator), while its

top has a temperature of 84°

(the warmth of equatorialsurface-water), he finds that such a column i s 10 feethigher than a similar columninlatitude where3 9

° is the sur face temperature . And s ince “ fromthe equator to latitude 5 6° the distance isgeographical miles, we have a declivity of barely onetwenty-eighth of an inch per geographical, or one

thirty-second of an inch per statute mile . Such a

declivity is utterly insufficient to account for theexi stence ofa strong current from the equator towardsthe tropics ; while , so far from giving any account ofthe motionof the equatorial current from east to west

,

it would tend to form anorth-easterly cur rent .This seems strongly opposed to Maury

’s View, and I

do not find that he does much to get over the force ofHerschel’s reasoning. He points out, indeed, that seawater does not attainits greatest dens ity at a tempera


ture of but some 12° or 14° lower. This, however,does not affect Hersch ‘el’s argument. If he had takena columnwhose base had a temperature of 25 ° insteadof he would have had to extend, also, the range ofthe water-slope inlatitude ; and, infact, he would haveobtained a yet smaller declivity in thi s way than thatactually deduced by him . Maury does not seem to

,

have noticed the really weak point inHe'

rschel’ s argument. I shall presently show where this seems to meto lie .But if Maury fails in efficiently defending his ownviews, he certainly is sufficiently effective inhis attack

I

uponSir JohnHerschel’s .He asks, inthe first place, the pertinent questionHow can the north-easterly trade-winds, which blowonly 240 days out of 3 65 , cause the [equatorial currentto flow all through the year towards the north-westwithout varying its velocity either to the force or to theprevalence of the trade-winds ‘ That the winds domake currents inthe s ea, no one,

’ he says, will havethe hardihood to deny ; but currents that are born ofthe winds are as unstable as the winds ; uncertainas to time , place , and direction, they are sporadic and.

ephemeral.’

He thenpoints to a fact which militates stronglyagainst the vast current-begetting power that has beengiven by theory to the gentle trade-winds . In bothoceans , the Sargas so seas lie partly within the tradewind region; but inneither do these winds give riseto any current. The weeds are partly out ofwater, and'

2 0 8 LIGH T ! SCIENCE F OR LEISURE H O URS.

illustration, the Gulf Stream may be likened to the jet,and the Atlantic to the pool. We remember to haveobserved, as children, how soon the mill-tail loses itscurrent inthe pool below or we may now see at anytime , and ona larger scale, how soonthe Niagara, current and all, i s swallowed up inthe lake below.

’

Franklin, who was the originator of the theory supported by Herschel, had unnecessarily introduced thesupposition that the trade—winds maintain a ‘ head of

water inthe Gulf ofMexico, and that the Gulf Streamflows downwards like a river from this ‘ head,’ as a

fountainor source. Maury rightly attacks this view,which is undoubtedly amistakenone ; but indoing so,he falls into an error which exhibits his weakness inthe treatment ofhydrodynamical problems . He pointsout that, inasmuch as the Gulf Stream grows wider asit crosses the Atlantic, it necessarily grows shallower,so that the water-bed inwhich the stream flows has ahigher level under the shallow than under the deeppart of the current, and therefore, says Maury, ‘ the

current runs up hi ll.’ Herschel terms this a strange

perversionof language , but perhaps it would be morecorrect to speak of it as a strange blunder . The streamcould, of course, only be said to runup hill if i ts surfacewere seeking a higher level, which does not and cannothappen. That the spreading out of the water of thecurrent, so as to form a wider and shallower stream

,

does not correspond to anupward flow, is evident fromthis, that it happens oftenwith rivers, which no one

will suspect of running up hi ll.

THE GULF STREAM 2 09

Herschel does not find ananswer to the mainobjec

tions urged by Maury against the trade-wind theory.

Content with urging anapparently unanswerable obj ectionagainst his opponent’s view, he leaves his own totake care of itself.Informing an opinion respecting the two theories

,

one i s struck with the immense superiority inthep owerofMaury

’

s agent. For, if we consider, we shall see thatalmost the whole of the sun

’

s acti on up on the ocean

goes to produce those variations in temperature andsaltness inwhich Maury sees the originof the currentsystem but a very moderate portionof the sun’s actioni s called into play inthe productionof the trade-winds.Now it is very doubtful whether any large proportionevenof the force expended inproducing the trade-winds ,ever acts onthe water. For we know that the northeasterly and south-easterly ai r-currents of the northernand southern hemispheres, do not wholly merge intonorthern and southern currents meeting point-blanknear the equator, as Her schel

’s theory seems to imply.Onthe contrary, there is a wide zone of calms at theequator, and the two systems of trade-winds appear topass upwards above the calm ai r, without parting withthe whole of their easterly motion. When once theybeginto travel polewards, they lose their easterly motioninthe same way that they acquired it—that is, throughthe effects of the earth’s rotation. And whatever portionis lost inthis way—which , for aught we know, may bea very considerable portion— cannot be taken into

P

2 1 0 LIGH T SCIENCE F OR LEISURE H OURS .

account as available to generate the easterly equatorialcurrent .And now let us consider for a moment the relationwhich holds betweencause and effect in the case supposed by Herschel . W

'

e have more thana fourth partof the Atlantic Ocean in a state of perpetual motion

,

and it is assumed that the ai r immediately above theoceanis responsible for this circulation. Now even i fwe suppose that the whole of the via r

vi va inthe aerialcirculation is imparted to the waters , and neglect allconsiderationof the fact that for a large portionof the

year the winds do not act inthe manner available forthe production of the currents we are considering, yeteventhen, I apprehend that we shall find the viavivaof the aerial very far below that of the aqueous circulation. The volume of moving water is , of course, farless thanthat of the moving ai r , and the meanvelocityof the water-currents is less than that of the ai r-currents ; but, on the other hand, the specific gravity ofwater i s some 8 30 or 840 tifnes greater than that of

ai r, and this difference far more thancounterbalancesthe others .But now, whenwe come to consider the forces calledinto actioninproducing changes of temperature, etc . ,

we no longer find such a disproportion between causeand effect. The sun’s action on the equatorial andtropical regions of the Atlantic not only produces agreat change in the density of the water , but alsoraises immense masses by evaporation. Now the buoyancy caused by increase of temperature is partly


stream which i s the necessary consequence of the continual flow of the great western equatorial currentagainst the barrier formed by the Americancontinent.It would require much more space thanI have at mydisposal to deal at length with the subject ofmy paper.I therefore conclude by referring my readers to Maury

’

s

interesting work on the Physical G eography of theSea,

’ with the remark that his views seem to me onlyto require the mainspring or starting force towards thewest which I have ventured to suggest, to supply acomplete, efficient, and natural explanationof the wholes eries of phenomena presented by the great oceancurrents .

The Student for July 1868 .

OCEANI C CIR CULA TI ON.

THERE are some questions,seemingly innocent enough ,

which yet appear fated to rouse to unusual warmth allwho take part in their discus sion. One cannot, forinstance, find anything obviously tending to warmth of

tainobservations ofMars not perfectly according withhi s own; and Sir W. Herschel, usually so phi lOSOphi c,

was roused by SchrOter’s recognition of mountains inVenus to deliver himself ofa criticism justly described

OCEANIC CIRCULATI ON. 2 I 3

by Arag o as ‘ fort vive, et, en apparence du moins,9quelque peu passionnée. The question, again, whether

the EozoonCanadense is a true Rhizopod,’ though

not altogether removed from the regionofhard words,might appear to be unlikely to excite warlike emotions ;yet there has been some very pretty fighting over it.The solar corona has inlike manner givenoccasionforrather strong writing ; and if, on the one hand, thesupporters of a late ly-abandoned theory said of theiropponents that they made themse lves ridiculous ,’

these , in their turn, at times used a tone remindingone of the scholar who said of a rival

,‘May God con

found him for his theory of the Irregular Verbs yetthe corona seems at a firs t view rather calculated toproduce a sedative effect than to excite unphilosophicwrath. The subject of oceanic circulationwould appearto belong to the clas s of questions here cons idered.The very name of the Gulf Stream is to s ome phy

s i cal geographers as a red cloth i s to a bull. EvenSirJohnHer schel, usually placidity itself, was moved whenhe spoke on this point. But though he and Maury

g rew warm enough in its dis cussion, their warmth wasi ce-cold compared with the fire of more recent dis

putants . Wehave be fore us the latest contributionto thes ubject, a rather ponderous essay inone of our leading

q uarterlie s ; and hereinwe find pleasing references tothe stupidities ’ of one set of Opponents, the shallownonsense ’ of a s econd, ‘ the wrong-headednes s ’ of a

third, with other s imilar amenities. More than once


during the progress of this controversy the gentle publi chas beenreminded of Bret Harte’s remarks

about the row

That broke up the Society uponthe Stani s low

and has been inclined to urge, with Truthful James,’

that theyHold it i s not decent for a s cientific gentTo say another i s anas s , —at leas t to all intent ;Nor should the individual who happens to be meant,Reply by heaving rocks at him to any great extent.

The controversy has not, indeed, reached thi s laststage of development, and we trust it never will butit has gone so near to i t as to sugges t that the dis

putant s have wished to demonstrate, by example, thejustice ofDarwin’s theory about the human snarlingmuscles ."

I propose to inquire into the subject which has beenthus warmly discussed, trusting not to be myself inveigled by it into any warmth of express ion. Indeed,

‘ He who rejects with s corn the beli ef that the shape of hi s own

canine teeth, and their occas ional great development in o ther men, aredue to our early progenitor s having beenprov ided with thes e formidablew eapons , will probably reveal, by sneer ing , the line ofhis owndes cent.For though he no longer intends , nor has the power , to us e thes e teeth

as weapons , he w ill uncons ciously retract hi s snarling mus cles (thusnamed by Sir Charles Bell), s o as to expo s e them ready for action, likea dog prepared to fight.

’—Darwin’ s Des cent of Man,’

vo l. i . p . 176 .

We may mention, by the way , that an ins tance has recently occurred,inwhich the humanteeth were us ed to s ome purpos e against one of therecogni s ed mas ters inthe art of bi ting . A man

, proceeding incompanywith s everal others through awood, was attacked by a hyena (usuallyone of the mos t cowardly of beas ts ). H is companions fled, and havingno weaponhe was reduced to the neces s ity of showing tooth for tooth,and taking a good gr ip of the hyena

’

s no s e, he compelled that gentleman to howl w ith angui sh. Onthis , the man

’

s companions returnedand pres ently beat the hy

‘ena to death.

2 r6 LIGH T SCIENCE F OR LEISURE H O URS .

The accompanying map exhibits the nature of thesurface circulation of the North Atlantic. It is constructed onone of the forms of equal-surface proj ectiondescribed inmy ‘ Essays on Astronomy,’ and has theadvantage. over the ordinary Mercator’s charts of exhibiting the true dimensions of the various currents ;I would, however , invi te the student who wishes to fami liari se himself with the true nature of the Atlanticcurrents to construct other maps for instance , apolarmap on the ” first method of equal-surface projectiondescribed inthat essay (see pp . 2 64 , and amap ofthe whole Atlantic on the second plan, taking themeridian40° west of Greenwich as the central one .

Of the water carried westwards by the great equatorial movement, the most important portion afterreaching Brazil i s carried northwards towards the WestIndies . The reason of this is obviously to be foundin the fact that Cape SanRoque, forming the juttingangle of Brazil, lies several degrees south of the equator . The portioncarried southward forms the BrazilCurrent, and after travelling along the shores of SouthAmerica almost as far as the mouth of the La Plata

,

acquires gradually aneastwardlymotionwhich eventually carries i t back across the Atlantic towards the CapeofGood Hope , there to pass northwards, and so againto traverse the Bight of Biafra. The surface-circulation in the South Atlantic is thus seen to be comparatively simple .The larger portionofthe equatorial current is carriedless quickly northward , becaus e the northernshore-line

2 1 8 LIGH T SCIENCE F OR LEISURE H O URS.

ofBrazil and G uiana is inclined at a much smallerangle thanthe south-eastern to the westwardly courseof the great equatorial currents . Thus the water whichi s carried towards the West Indies has time to acquireunder the tropical suna much higher temperature thanit had possessed when traversing the Gulf of Guinea.

It i s divided into two parts by the quas i—barrier whichthe West IndianIslands (o r rather the semi-submergedmountains ofwhich they form the crests) oppose to itsprogress. A comparatively small portionfinds its wayinto the CaribbeanSea

,and making the circuit of the

Gulf of Mexico , passes out eastwards round the peninsula ofFlorida. We may fairly assume that this portion i s comparatively small ; simply because this truegulf stream, passing between Cuba and Florida onan

easterncourse, would continue so to move forat leastsome considerable di stance

,were it not in some way

deflected. But it actually turns almost due northwardsafter passing through the

'

Bahama Sea, traversing theBemini Narrows onthis course , and so onwards towardsHatteras . This would seem to imply that the true GulfStream is pressed northwards by the arrival of amuchlarger body of water which has travelled outside theWest Indies . It i s true that the diversionof the GulfStream northwards may be really caused by the greatBahama Bank . But this would equally establish ourposition; for if the Bahama Bank is thus effective indiverting the whole of this now swiftly moving current,the Windward Isles may be assumed to be corre sponding ly effective in diverting the greater portion of the


Thirdly,its course having carried it into narrow chan

nels,it has required a relatively rapid rate of outflow,

insomuch that'the surface flow of the current on itsoutward passage throug h the Narrows ofBemini , takesplace at the rate of from 21

5to 4 m iles per hour . Its

width here is at the surface not more thanabout 25miles, its maximum depth rather more thana quarterofamile (about two-fifths of the channel’s maximumdepth), and its mean rate of flow probably about 5 0miles per day.

I shall not follow the Edinburgh Reviewer incon

s i dering the details of the progress of the Gulf Streamfrom the Narrows ofBemini to Cape Hatteras, because ,though inthemselves of the utmost interest and importance, these details throw no special light onthe generalsubject of oceanic circulation. Suffice it that as far asHatteras the Gulf Stream remains distinctly recogni sable, and that even off Sandy Hook (New York ) itssurface temperature is little reduced

, and its velocitystill amounts to about one mile per hour. Off Nan

brought downby that r iver, the coars er having beendepos ited near its(the r iver

’s) mouth ; just as the intens e bluenes s of the waters of Lake

G eneva depends on its retention of the fines t s edimentary particles

brought downby the Rhone inthe upper part of its cours e, while that

of the waters of the Mediterranean i s due to its pervas ionby the likeparticles brought downby the r iver Rhone and other river s , which di s

charge thems elves into i ts wes ternbas in, and by the Nile into its eas tern.

’

It will be remembered that Prof. Tyndall , by res earches carr ied onduring the returnof the Urgent from the eclips e expedition of 1870,was enabled to throw cons iderable light onthe caus e of the colour andshades of colour inwater of g reater or les s depth. See als o Dr . Car

penter’

s Report ofRes earches inthe Mediterranean,’inthe Proceed

ing s of the Royal Society,’

vol. xix . p. 200.

oCEANIC CIR cULATION.

tucket the breadth of the current is about 410 miles,

its winter surface temperature only 10° below thatwhich it had in the Florida Channel , and its rate offlow still nearly one mile per hour. It has at this partof its course acquired a good deal of easting, a circumstance which must (unquestionably, we conceive ) beascribed to the fact that it brings from low latitudesthe more rapid easterly rotationmovement of the earth .The same would, of course , apply to the less characteri sti c but larger current which has arrived at the samelatitudes w i thout circuiting the G ulf ofM exico .Now here we approach a critical part of our subject.

It i s admitted by all that off Newfoundland the GulfStream loses its special characteristics . As Dr. Hayesremarks, ‘ its strength diminishes ; the air ofa higherlatitude brings its temperature down to that of theNorth Atlantic generally (not, however,

“

withoutraising the temperature of the North Atlantic to someextent) ; the water loses all its Gulf Stream characteras to course, warmth , and flow ’

(and as to colour also)and it dies away into the sluggish Atlantic drift whichsets from a westerly to aneasterly direction.

’It i s not

so generally noticed, but will scarcely, I suppose, bedisputed, that the Gulf Stream water strengthens, andthat appreciably, this sluggish Atlantic drift. Theniti s reinforced by the portionwhich has travelled outsidethe West IndianIslands and we may assume (without

g i vmg r i se to objections) that the general prevalence ofsouth-westerly Winds will further strengthen the eastward motion of the combined mas s . At any rate, let

2 2 2 LIGH T SCIENCE F OR LEIS URE H OURS.

the causes be what they may (and presently we shallhave a further cause to take into account), it is admi tted by all physical geographers that agreat, thoughslow current, or drift, does pass eastwards from theneighbourhood of Newfoundland. Moreover , it i s admi tted by all that the southern part of thi s current

(which the Edinburgh Reviewer actually regards asidentifiable with the Gulf Stream‘

) traverses the Atlantic until, nearing the Azores, it j oins the southwardlyGuinea current ; while the northern part pas ses onanorth-easterly course , which carrie s it betweenBritainand Iceland, betweenSwedenand Spitzbergen, onwards,evenas far as the very neighbourhood ofNova Z embla.

Lastly, it is admitted by all that,directly or indi

rectly, this great north-easterly current causes theclimate of Great Britain, and of the north-westernparts of Europe generally, to be milder than that ofNorth Americanregions incorresponding latitudes .It might appear, then, that all these things being

admitted, no question of any importance remains, sofar as the actual facts of the oceanic surface-circulationare inquestion. We shall presently see that aquestionhas arisenas to the caus e of the observed facts ; but asto their nature everything that seems worth di scus smgat all appears to be satisfactorily disposed of.Let those readers who in their simplicity have

adopted this notionhasten to dispossess themselves ofit by reading some remarks by Dr . Hayes, the American

1 He says that the great equatorial current is partly supplied‘ by the

returnofa portionof the Gulf Stream.

’

2 24 LIGH T SCIENCE FOR LEISURE H OURS .

ofBuchan, the meteorologist, that the north-easterlycurrent above r eferred to produces anafflux ofwarmthbrought to the British Isles by the water that laves ourwesterncoasts .’ He proceeds : There is ample evidencethat the cold of some parts of the north polar area i sgreatly mitigated by anafflux of water bringing withit the comparative warmth of temperate seas . It has

long beenknownthat cocoa-nuts, tropical seeds , trunksof tropical trees, timbers and spars of ships wrecked farto the south

,and sometimes portions of their cargo,

are found onthe shores of the WesternHebrides , theOrkney

,Shetland, and Faroe Islands , the north of

Norway, and even Spitzbergen and since their transsport has taken place just in the cours e of the GulfStream if prolonged to the north-east, their arrival hasbeenaccepted almost without question as evidence ofits agency. The evidence furnished by the surface tem

perature of that north-eastern portion of the AtlanticOceanwhich intervenes betweenIceland and theNorthCape , and thenstretches away to the eastward betweenSpitzbergenandNova Z embla, seems at first sight conclus ive to the like effect. A large amount ofadditionalthermometric evidence has beencollected oflate years ;and this has beenmost ab ly digested by t he eminentGermangeographer

,Dr. Petermann, who has recently

put forward aseries ofmaps for different peri ods of theyear, inwhich these observations are embodied, andtheir results made obvious to the eye by the course ofthe ‘ lines of equal temperature ,’ which inthe summerpass betweenIceland and the Shetland Islands, a little

OCEANI C CIR CULATI ON. 2 2 5

to the east of north towards Spitzbergen, and thencewith more ofaneasterly bend evenbeyond the seventyfifth degree of north latitude . The existence ofa warmstream inthis direction has been confirmed still morerecently by two adventurous officers—Lieutenant JuliusPayer, of the Austrian army

,and Lieutenant Wey

precht, of the German army—who followed i ts pathlast summer in a small sailing vessel hired by themselves, and state that they found open water fromeast longitude 42° to east longitude even beyondthe seventy-eighth parallel of north latitude, thehighest point they reached being north latitude in

east longitude A Russianexpeditionunder PrinceAlexl s Alexandrovitch, of which the distinguishedsavant, VonM i ldendorf, had the scientific charge, wasabout

'

the same time exploring the Polar Sea betweenNova Z embla and Iceland ; and VonM i ldendorf hasstated to the Imperial Academy of St . Petersburgthat ‘ the corvette Wajay has proved the extensionofthe Gulf Stream to the west coast of Nova Z embla, andthat we find it onthe meridian of BaninNoss (ineastlongitude still ofa width equal to two degrees oflatitude, and of a temperature of fifty

~four degreesFahrenheit, cooling down only four or six degrees atdepths of thirty and fifty fathoms. ’

As if to remove all question as to his real opinionthe reviewer immediately adds that be fully accepts,not only the great body of facts so industrious ly correlated by Dr . Petermann, but the inference Dr. Petermanndraws from them that anattempt to penetrate

Q

2 2 6 LIGH T SCIENCE F OR LEISURE IIo URS.

the polar i ce-wall to the north-east of Spitzbergeni smore likely to be successful than the search for a passage inany other direction.

’

So that ( 1 ) Dr . Petermann, regarded by our revieweras aneminent geographer ; (2 ) VonM ildendorf, whomhe regards as a distinguished savant ; and (3 ) the reviewer himself, who no doubt does not regard himselfas either shallow or stupid , seem all agreed as to thevery points which the reviewer has spoken of as in

volving stupidities and shallow nonsense . Certainlythey all agree as to the only points which seem intheleast worthy of discussion.

What, then, the reader will ask , i s the matter indispute ? Over what momentous question have theangry words quoted above beenbandiedAfter diligent search for the apple of discord, thestudent of the review will be led to the conclusionthati t is neither morenor less thanthe name Gulf Stream.

’

We have seen that Von M i ldendorf calls the warmcurrent which passes by Nova Z embla the Gulf Stream.

Inthis , it appears, he has shown shallowness and stu

pidity. Dr . Petermannhas equally committed himself,o r rather has committed a more serious offence . ForVonM i ldendorfmight have used the offensive epithetonly through inadvertence ; but Dr . Petermann notonly uses it, but has the hardihood (we might almostsay the cruelty ) to maintain that it is a matter of noconsequence.’ M oreover , as our reviewer sadly admits,other physical geographers agree with Dr . Petermann.

The reviewer is so grieved by the defection of the

2 2 8 L IGH T SCIENCE F OR LEISURE E C URS.

names at every stage where some new influx changesthe size and character of either. And the title GulfStream ’

has , in like manner, advantages in point ofconvenience

,which are likely to prevent geographers

from rejecting it yet awhile . It may mislead some fewinto supposing that the whole of the great northeasterly current has passed through the Gulf ofMexico,just as we canconceive that some few students of geography might imagine all the water which flows pas tCologne or Coblentz to have come from the Grisons, orall that flows past Nikopoli s to have come from Baden.

Almost every convenient name , however, is open to

some such disadvantage ; and the student of oceaniccirculationwho finds he has beento some degree misledby aname must not mistake the detectionof his errorfor a great geographical discovery.Maj ora cauamus .

We have hitherto considered surface-currents only.We have not, indeed, considered all the surface currentswhich traverse the North Atlantic ; but the principals treams have beenindicated . We must now direct ourattentionto submarine currents .It is impossible to consider carefully the nature anddistribution of the surface circulationwithout recogni s ing the fact that there must be currents beneaththe surface . It is true that one can conceive theexistence of a complete system of oceanic circulationwithout any movement in the depths of the s ea ; butwhenwe examine the actual surface currents we findthat either the commencement or the prolongationof

OCEANI C CIR CULATI ON. 2 29

some currents must necessarily be submarine . For

instance, the quantity of water carried by the greatnorth-eas terly drift into the Arctic Oceanis very muchgreater thanthat which flows out of the Arctic Ocean,by the s o-called Arctic current

,past Greenland. Ex

amining , indeed, the ordinary current charts, alwaysdrawnonM ercator’s proj ection(seemingly because thisprojection i s the very worst that could be devised forthe purpose), we might suppose that this arctic streamwas much more extensive thanit really is . But whatcan be expected of a proj ectionwhich makes Greenland (whose real area i s not much greater thanthat ofthe Scandinavianpeninsula) actually as large as SouthAmerica. The Arctic current, however, affords yetbetter evidence of the occurrence of submarine streams ,for the extensionwhich passes betweenthe Gulf Streamand the Uni ted States, is in places completely losts ight of (the Gulf Stream touching the Americanshores), and reappears farther on. It i s clear thatit mus t have passed under the Gulf Stream in suchcases .Now,the study of the submarine currents has of late

years thrown considerable light onthe whole questionof oceanic circulation, and has supplied the solutionof

some problems which had formerly appeared altogetherperplexing.We owe to Drs. Carpenter and Wyville Thomsonsome of the most important facts recently ascertained.Others ,

'

however, have shared in the work. I would,indeed, particularlyinvite attentionto the fact that I

30 LIGH T. SCIENCE F OR LEISURE H OUR S.

do not here pretend to give anything like a completehi story of recent investigations into the subj ect. I

select only those facts which bear most significantly onthe wider relations— the more marked features—of

oceanic circulation.

Inthe first place, a result which had long perplexedphysical geographers has been shownto be erroneous .

It had beensupposed that the temperature of sea-waterbelow a certaindepth i s in all latitudes constant, andabout seven degrees above the temperature at whichfresh water freezes. Sir JohnHerschel, in his ‘ Phys ical Geography,

’

adopted this supposed discovery aswell established , Now, let one consequence of such arelationbe carefully noted. The surface water inthetropics i s warmer than this supposed constant bottomtemperature the surface water in arctic regions i scooler ; at some intermediate latitude the surface waterhas the same temperature as the water at the bottom .

Hence inthis intermediate latitude the water i s uniformly warm (according to the supposed relation) fromthe surface to the bottom . We may therefore regardthe water inthis latitude as constituting, in effect, aconstant barrier between the tropical waters and thearctic waters . Without regarding it as absolutely immovable we should yet be compelled to regard it as sofar steadfast as to negative the theory of the existenceof submarine currents of an importance correspondingto that of the surface currents . Accordingly, the theoryput forward by Humboldt and Pouillet to the effectthat there i s an interchange of waters betweenpolar

2 3 2 LIGH T S CIENCE F OR LEISURE H OURS .

meters not protected against pressure , set this uniformtemperature too high . In the western basin of theM editerranean, as shownby the P or cup ine observationsof 1 870, the uniform temperature is 5 4 or 5 5 degreesbeing, 111 fact, the winter temperature of the entirecontents of the basin, from the surface downwards and

being also, it would appear, the mean temperature ofthe crust of the earth inthat region.

’ We learn, then,two things— viz . , first, that where extensive submarinemotions are impossible

,a constant submarine tempera

ture may be expected to prevail inthe same latitudesand, secondly, that inthe latitude ofthe Mediterraneanthe submarine temperature is about 5 4 or 5 5 degreesFahr . Thus, it i s clear, in the first place , that thevarieties of temperature observed in the depths of theAtlantic must be due to the continual arrival of waterof the observed temperatures

,at a rate great enough to

prevent the deep water from acquiring a constant tem

perature and inthe second place it becomes possibleto tell whence the submarine currents are flowing. If

they are cooler than they should be supposing latitudealone inquestion, then they are travelling from arctictowards tropical regions, and vi ce versa. Onthis lastpoint no doubt remains . In a latitude correspondingto that of the Mediterranean basin, the depths of theAtlantic are far colder , even in their warmest portions, than they would be if latitude alone were inquesti on. In regard to surface-temperature,’ saysDr . Carpenter , ‘ there is no indication of any essential difference between the M editerranean and the

OCEANIC CIR CULATI ON. 2 33

Eastern Atlantic ’

in the same latitudes ; ‘ and thethickness of the stratum that undergoes superheatingduring the summer is about the same . Atthe depth ofa hundred fathoms , in the Atlantic as inthe M editerranean, the effect of the superheating seemsextinct, the thermometer standing at about 5 3 degrees ;and beneath this (inthe Atlantic only), ‘ there is a slowand tolerably uniform reduction at the rate of abouttwo-thirds of a degree for every fathom ,

down to700, at which depth the thermometer registers 49degrees. But the rate of reduction then suddenlychanges inthe most marked manner ; the thermometershowing a fall ofno les s thannine degrees inthe next200 fathoms, so that at 900 fathoms it stands at 40degrees . Beneath this depth the reductionagainbecomes very gradual the temperatures shownat

and fathoms (the last being the deepestreliable temperature sounding yet obtained) being,respectively, 3 8 , 37, and degrees .’

Thus, there can be no possible question that thedepths of the Atlantic are occupied by a vast currentmuch colder thanthe deep sea temperature due to thelatitude, and, therefore necessarily flowing from thearctic towards the tropical seas .Such are the broad facts of the Atlantic circulation.

We have a surface circulation whose general featuresare such as have been described, and are generally admitted, though a dispute has arisenas to a questionofnomenclature and thenwe have a general submarine‘ set ’ of water from the arctic regions towards the

2 34 LIGH T SCIENCE FOR LEIS URE H 0 UR s .

tropics, the existence.

of which is also generally admitted.

Butnow we againapproach a subj ect of controversy,and one which is certainly better worthy of discussionthanthat which we considered above . It relates, infact, to the question how this wonderful system of

oceanic circulationis brought about.Passing over several crude theories which have longsince beendisposed of, we come first to the theory thatthe system of oceanic circulationis due to the actionof

the trade-winds . This theory has beenmaintained byFranklin and others in former times, by Sir JohnHerschel inour own, and i s warmly advocated inthepresent day, 'by many whose Opinions are not to berashly contradicted .Against this theory it has been urged by Captain

Maury with sing ular wrongheadedness accordingto the Edinburgh Reviewer

,but as it seems to me with

no small degree of reason— that the trade-winds areneither powerful enough nor persistent enough to aocount for the great equatorial currents, or therefore forthe Gulf Stream . Maury says, with the view ofasoertaining the average number of days during the yearthat the north-east trade-winds of the Atlantic Operateuponthe water between the equator and 25 degreesnorth latitude, log-books containing no less than

observations onthe force and directionof theWlnd in that oceanwere examined. The data thusafforded were carefully compared and discussed. Theresults show that withinthese latitudes—and on the

2 36 LI GE T SCIENCE FOR LEIS URE H O URS.

(that i s from arctic or from antarctic regions) wouldacquire a strong westerly motion (just as the tradewinds do). Thus they would generate from below thegreat equatorial westerly current. Inthis upflow of coolcurrents having a strong westerly motion, we find themainspring of the seri es of motions . The water thuspouring in towards the equator is withdrawn frombeneath the temperate and arctic zones

,so that room i s

continually being made for that north-easterly surfacestream which is the necessary consequence of the continual flow of the great westerly equatorial currentagainst the barrier formed by the Americancontinent.

CaptainMaury’

s Views seem only to requirethe mainspring or starting-force towards the westwhich has been here suggested, to supply a complete,efli cient, and natural explanationof the whole series ofphenomena presented by the great oceancurrents .’

Four or five months later, and while the results onwhich Dr. Carpenter subsequently based his theory ofthe oceanic circulationwere as yet unpublished, I drewattenti on in the columns of the Dai ly News to thecomparatively limited extent of the influences due topolar cold. This cause

, I pointed out, scarcely hasany influence inlatitudes lower thanthe parallel of 50degrees .’

Inthe year 1869 Dr. Carpenter was first led to advocate the theory that the continual descent of cold waterinthe Arctic Seas i s the mainspring of the system of

oceanic circulation. He reasoned that the Arctic Seasbeing exposed to great cold

,the surface water descends

OCEANI C CIR CULATI ON. 2 37

invirtue of its reduction in temperature and increaseof density, its place being taken, not by the rising upofwater from beneath, but by aninflow ofwater fromthe neighbouring area ; and since sea-water becomescontinually heavi er in proportion to its reduction of

temperature , this cooling actionwill go onwithout thecheck which is interposed in the case of fresh water."

Thus the water becoming denser and heavier willdescend , and ‘ there will be a continual tendency tothe flowing off of its deepest portioninto the warmerarea by which the polar basinis surrounded ; producinga reduction in the level of the polar area, which mustcreate afresh indraught ofsurface-water from the warmerarea around to supply its place. This, in its turn,being subjected to the same cooling action, will descendand flow offat the bottom, producing a fresh reductionof level and a renewed indraught at the surface .’

Dr. Carpenter illustrated this theory, or rather thecombined actionof polar cold and equatorial heat, byanexperiment , the planofwhich had occurred also tomyself

, and been described by me in conversationsomewhat earlier. A long narrow trough having glasss ides was filled with water, and a piece of ice waswedged in at one end between its side plates justbeneath the top , whilst the surface of the water at theother end was warmed by a piece of metal, of which apart pr0j ected beyond the trough , and was heated by a

Fresh water expands with r eduction of temperature, near thefreezing po int, and hence, becoming lighter , the des cending motionabovedes cribed i s interfered with inthe cas e of fresh water .

2 38 LI GIIT SCIENCE F OR LEISURE H 0 URS.

spirit lamp placed beneath it : thus representing therelative thermal conditions of the polar and equatorialbasins . A colouring liquid viscid enough to holdtogether inthe water, while mi xmg with it sufficientlyto move as its moves, being thenintroduced, the liquidas it impinged on the ice was seen to sink rapidly tothe bottom

,then

,to flow slowly along the floor of the

trough towards the opposite extremity, thengraduallyto rise beneath the heated plate , and thent o flow slowlyalong the surface towards the glacial end, repeatingthe same movement until the ice had melted .’

It will be observed that inthis experiment the effectof cold is not exerted alone , so that it by no meansproves that the arctic cold is the chief agent in producing the system of oceanic circulation. Moreover,the conditions of the polar and equatorial basins are inone respect not accurately (or evennearly) reproduced,for the real arctic area is very much smaller , comparedwith the real equatorial area, than in the case of theexperiment. Indeed it appears to me that Dr. Carpenterpaid far too little attentionto the relative smallness ofthe arctic area. This may have beenpartly due to theerroneous ideas suggested by the ordinary maps on

Mercator’s Projection, in which, as I have alreadymentioned, the arctic regions are enormously exaggerated . It i s almost impossible to study such amap asthat which illustrates this paper ( see page 2 17) withoutfeeling that the theory presented by Dr. Carpenter willscarcely hold water , or rather—if thisway of presentingthe argument be permitted—that the arctic area does

240 LIGH T SCIENCE F OR LEISURE H OURS.

more intens e action, as its maximum effect is limitedto a much smaller area thanthat of the maximum of

equatorial heat. The actionof the trade and countertrade winds, in like manner, cannot be ignored ; andhenceforward the question of ocean-currents will haveto be considered under a twofold point of view.

’

It appears to me that not only is the equatorial o rrather tropical actionmuch wider

'

in range , but it i salso more intense than the polar action. For, let usconsider what happens during the heat of the day overthe tropical Atlantic . Here , over anarea enormouslyexceeding the whole arctic basin(we are considering,be it understood, only the northernpart of the systemof circulation) a proces s of evaporationis taking placeat so rapid a rate as to furnish almost the whole ofthat rain-supply whence the rivers ofEurope andNorthAmerica (east of the Rocky M ountains) take theirorigin. There i s thus produced a continual defect of

pres sure, .not merely along an equatorial strip, but sofar as 20 or even30 degrees of north latitude , whilethe downfall of rainwhich , taking one part with another of the temperate and sub-arctic Atlantic, may beregarded as incessant, continually adds to the pressurein these last-mentioned regions . That on the wholethere must result a most effective excess of pressureover the temperate zone of the Atlantic, as comparedwi th the tropical and equatorial portion, seems to meindisputable . Now, if we compare this with the effectsof refrigerationover the relatively insignificant arcticarea, which as I have said has to supply the North

OCEANI C CIR CULA TI ON. 24 I

Pacific submarine circulation(if Dr . Carpenter’s theorybe true), as well as that of the North Atlantic, we canscarcely doubt, as it seems to me , which cause is themore effective . I would venture to predict that ifDr. Carpenter’s experiment were tried first with theice alone to produce circulation

,and secondly with the

heat alone , the superior efficiency of the latter causewould be at once recognised but I much more confidently prediét that if, as in the experiment I myselfproposed, the relative areas of the equatorial and arcticbasins were represented , there would be found to bescarcely any comparison between the effects of arcticcold and equatorial heat

,so largely would the latter

predominate .It i s necessary to mention, however, that the prin

ciple itse lf of the experiment has beenobj ected to , onthe ground that the gradation of temperature mustalways /

be much more rapid in such an experimentthan in the actual case of the Atlantic Ocean. Thisobjection

,however

,is , in reality, based ona misappre

hens ion. It i s sufficient that the difference of tem

perature at the two ends of the trough should correspond to the difference betweenthe temperature of thearctic and equatorial seas ; and it is amatter of no importance whatever that the real rate of gradationshouldbe represented . The case may be compared to theillustrationof the descent of water to form springs o rthe like . Here anexperiment would be val i d inwhichthe outflow of the illustrative spring was obtained bycausing the vent to be so much below the level of the

R

2 4 2 LI GH T SCIENCE F OR LEIS URE H OURS .

reservoir,though the slope from the reservoir to the

vent were very much greater than in the case of anynatural spring. Just as in the case of a spring it isthe difference Of level, and not the rate Of s lop e, whichis effective in causing the rate of outflow, so in thecase of the oceanic vertical circulation, it is the actualdifference of temperature , and not the rate of gradation, which produces the activity of the circulation.

Another obj ectionhas been urged against the ‘ heatand cold theory ’ by a very skilful mathematician, Mr.

Croll . He reasons onthis wise : Since the water whichis carried from the equator to the latitude ofEngland 1

(say) must have partaken, whenat the equator, of theearth's rotation there, which has a velocity of morethan miles per hour eastwards , whereas , whenitreaches our latitudes, it partakes of a rotation-movement reduced to about 620 miles per hour , it followsthat, neglecting the drift motions as relatively quiteinsignificant, frictionhas depr ived the water which hasthus travelled from the equator to our latitudes of avelocity amounting to no less than3 80 miles per hour.If friction is thus effective , how utterly inconceivableis it, says Mr . Croll, that the descending currents ofDr . Carpenter’s theory (or the ascending currents ofthe evaporationtheory ) should maintain their motion.

Hence, Mr . Croll i s an earnest advocate of the tradewind theory .The worst of this reasoning is that it proves too

I pres ent the general nature of Mr. Croll’

s reasoning , without fol;

lowing him indetails .


it breeds the fogs infesting the path of the great streamwhich flows from Vicksburg to Placquemines .

’ Allthis i s utter nonsense . The Missis sippi has no moreto do with the great stream flowing through Louisianathanwith the Thames at London. The realM ississippii s a stream of singular purity, and presents other characteri sti cs clearly recognisable as far as its junctionwiththe M is souri ; but in the stream which runs past St.Louis none of the characteristics of the M ississippi canbe traced. Here, to all intents and purposes , the M i ss i s s ippi comes to an end. As for the cause of themotion of the great stream itself there can be littlequestion. Some have urged that it is due to the gradual slope of the land but in all the experimentalillustrations of the effects of such slope which we haveyet seen, the inclination has beenmonstrously exag

gerated. If slope were the cause of the river’s flow,then unquestionably the effective part of the actionmust reside in the Rocky Mountains, and not in thegreat reaches of the river. we admit that the chiefbulk of the river lies in the great reaches ; but thisfact has no bearing, we assert, onthe questionat issue .However, it i s demonstrable that no cause of this sortcan be in question. For let the fo llowing reas oningbe carefully marked. InWisconsin, in 40

°

northlatitude, the river partakes of the earth’s rotationmotion, there equal in rate to about 800 miles perhour ; in Louisiana, in 30

°

north latitude, the riverstill partakes of the earth’s rotation movement, hereequal to about 900 miles per hour .

.Hence

,were it

OCEANI C CIR CULATI ON. 245

not for the friction exerted by the banks,the water of

the river inLoui siana would be flowing at the rate of100 miles per hour westwards . If, then, friction deprives the river of this enormous velocity—as it obviously does —how much more must it deprive the riverof the minute velocity of four or five miles per hourdue to slope or inclination. It is certain, therefore,that the flow of the stream is due to the prevalentnortherly winds of the so-called M ississippi valley.There are not wanting those, indeed, who assert thatthis cannot be the case

,because northerly winds are

not prevalent inthis region. But the singular wrongheadedness of this reasoning renders reply unnecessary.That the flow of the great stream i s caused by thesewinds is as certainas the rotundity of the earth .

FromEng lish M echanic for July and Augus t 1872 .

ADDEND Hi ll . I

IT is impossible but that on a subject so difficult andcomplicated as that of oceanic circulation, differentviews should be entertained by students of science .And it i s clear that inthe present stage of the inquiryno useful purpose could be fulfilled by making theproblem amatter for controversy. Dr. Carpenter himself has shown that much more is to be gained by

'

This paper was writteninreply to comments by Dr. Carpenter on

the former paper . The nature of thes e comments will be inferred frommy reply ; infact I quote the mos t important pas sages .

246 LI GH T SCIENCE FOR LEISURE H O URS.

observationthanby reasoning onimperfect knowledge.

If I venture to remark that his deep-sea researcheshave led to the most important contributionwhich hasbeenadded for many years to our informationrespecting oceanic circulation, he will not, I trust, considerthat I am passing beyond the bounds of controversialcourtesy. But I am,

indeed, not anxious to treat thematter as one for controversy in any sense . It will beperceived by those who have read my remarks onthesubject, that I have rather put them forward as sug

gestions than as indicating theories which can bemaintained with any degree of assurance, far less withconviction. Nor does it seem to me likely that oneexplanation can suffice to account for all the phenomena recognised inoceanic circulation. This is a case,i f ever such case were, in which more causes are inoperation than one ; so that it may very well happenthat excellent arguments can be adduced in maintenance of different views . If, therefore , I enter onthe defence of what I have already written on thissubject, it i s not with the wish to show that one particular explanationof oceanic circulationi s correct, andall others erroneous . If I am desirous of dealing withthe considerations urged by Dr . Carpenter, it i s notbecause they seem to him to militate against the viewsI have to some extent advocated. What I wi sh toshow is that I have not addressed your readers onthe subject of oceanic circulation without makingmyself familiar with the facts which bear upon thatsubject, and at the very least, with those compara

2 48 LI GH T SCIENCE F OR LEISURE H O URS.

equatorial water is lower than that of tropical water.Now,

i t i s unque stionablyt rue that the effect of evaporationi s to increase the specific gravity of s ea waterbut it is equally true that the effect of the heat whichcauses the evaporation is to diminish the specificgravity. The point is considered inmy essay entitled‘ Is the G ulf Stream a Myth ? ’

in the first series ofLight Science for Leisure Hours .’ We recogni se ,

’I

there say, two contrary effects as the immediate resultsof the sun’s action. Inthe first place, by warming theequatorial waters it tends to make them lighter ; inthe second place, by causing evaporationit renders themsalter, and so tends to make them heavier.

’And I

proc eed to inquire which caus e is likely to be the moreeffective , arriving at the conclusion that the water i smade lighter . The case, indeed, appears to me to bealtogether different from that of the MediterraneanSea cited by Dr . Carpenter. Inthe Medi terraneanwehave the same heating actionas on the Atlantic inthesame latitudes , but not the same relatively enormousquantity of water freely commumcatingwith the regionso heated . We have

,then

,in the M editerranean

evaporationas everywhere else , and evaporationto thesame degree, appreciably, as elsewhere in similar latitudes but evaporationnot compensated as inthe openAtlantic by the effects of free communication withsurrounding water . Hence we have in the Mediter

raneananincrease of saltness ; in other words, an increase of specific gravity. And precisely because this.increase takes place in the Mediterranean, whereas the

OCEANI C CIR CULA TION. 49

water of the Atlantic inthe same latitudes,exposed to

the same average degree ofheat, i snot rendered heavier,it may be maintained not unreasonably that thewater of the equatorial Atlantic being unconfined

,

will in like manner not be rendered heavier byevaporation. It seems to me that we have here apositive argument of great weight infavour of myviews . But independently of thi s I would ask whetherit can be questioned that enormous evaporationdoestake place over the equatorial area. This is what Icontend for, and I should have imagined that few wouldundertake to deny the proposition.

In passing, I must remark that I do not adopt thedistinctionbetweenequatorial and tropical water whichDr. Carpenter appears to recognise . I have in viewthe evaporationover anenormously larger area thanheconsiders— no less an area, in fact, than the wholeocean between latitudes 40° north and south of theequator (at the equinoxes, and varying according tothe season) . It by no means follows that because theequatorial cur rent does not cover this enormous area,therefore the relation which I have suggested asthe mainspring of oceanic circulation has not thatextent. Onthe contrary, while it is on the one handcertain that there is an excess of heat over thisenormous area, it i s on the other almost a necessityofmy theory that the resulting current should be foundrunning along the middle only of the great region of

evaporation.

This brings me to Dr. Carpenter’s second objection,

0

2 50 LI GH T SCIENCE F OR LEISURE H OURS

that if the removal of equatorial water draws in polarwater from the bottom,

the whole intermediate stratumshould first rise towards the surface . I do not holdthe view thus demolished, but simply that the inflowis from below. The ques tionwhether the inflow wouldbe from above or below was dealt with by me in a

paper on Oceanic Circulation’in the Student for

July 18 68 . I do not urge this as a proof that Dr.Carpenter’ s objection i s invalid . My reasoning mayadmit of being refuted. But I wish to show that theobjectionis not anew one to me. The inflow may befrom below without being from the bottom. If it werefrom the bottom it would not have the effects I haveascribed to it, that is, it would not result in a westwardly

-flowing current . What I conceive is that sincethe whole tropical and e quatorial area is a region of

excessive evaporation(as surelyno physicist will deny),there is over the whole regionadepressionof the oceanlevel . This depression may be , or rather must be,exceedingly minute ; but the total quantity of waterthus , as it were , wanting, must be enormous . Thedifference must by the laws of fluid equilibrium besupplied, and though the immediate supply in equatorial regions may come from tropical regions, theactual source of the total supply must be sought for inhigher latitudes . That the water drawninunder thesecircumstances would traverse the surface of the Atlantic, is byno means proved by the fact that the eminentmathematicians cited by Dr. Carpenter consider thatan in-draught to replace water swept off from the


the slow removal, the creep ing away, as it were, of thatwhich it replaces . That this cause, p er s e, can everbecome one of sufficient activity1 to generate a completesystem of vertical oceanic circulationseems at the leastopen to grave question. It appears to me also thatwhen applied to the North Pacific this theory fails .Very little water canpass through Behring’ s Straits , andbeyond Behring’s Straits there i s anisland-locked andshallow sea of enormous area, altogether unlike the deepNorth Atlantic .

I would further point out that the interesting factabove mentioned,namely that the equatorial heat exertsno perceptible effect at adepth exceeding 200 fathoms,i s inreality almost anecessity for my theory. For ifthe whole of the equatorial ocean were heated, and,therefore, of reduced specific gravity, the water arrivingfrom higher latitudes would flow to the bottom,

and so

have to force up the intervening strata, inorder to produce the observed effects ; and this may be regarded asimpossible . As it i s , such colder and heavier waterwould be indynamical equilibrium withina very shortdistance of the surface .Next, as to the question of rainfall. Dr. Carpenter

considers that I have ‘ overlooked the considerations (1 )that the rainfall ofEurope and North Americamay beaccounted for by the evaporationinthe M id-Atlantic,

Inpas s ing I may notice that I did not suppo s e Sir J. Hers chel to

be humorous inreference to the intens ity of the polar action, but inhi sus e of the word emphas i s .

’

I should not have touched onthe point,did Inot thoroughly sympathi s ewith the emphatic utterance of speculative or theoretical Opinions .

OCEANI C CIR CULATI ON. 2 5 3

beyond the regionof the trade-winds, say between20°

and 40°

north latitude and (2 ) that there is anenormous rainfall inthe regionof equatorial calms

,which

Sir JohnHerschel attributes to the deposit of waterstakenup by the N.E. and S E . trades . To this I mustreply that inmy essay on Rain in the ‘ IntellectualObserver ’ for December 1 8 67 , I have weighed thewhole questionof rainfall at least with great care, andwith constant reference to the best sources of information. One circumstance I there note which seems at afirst view (or rather viewed as Dr. Carpenter appears toconsider the matter) much more fatal as anobj ectionto my theory thaneither of those noted by Dr. Car

penter ; viz. , that according to the observations ofHumboldt and others, the annual rainfall is at amaximum at the equator, and diminishes with increase oflatitude . But the whole question is, where does allthis raincome from ? If it comes from tropical andequatorial evaporationit will surely not be argued thatwhat falls in or near the place of evaporationitself,represents the total amount of such evaporation. It i sunquestionable , I conceive , that the rainfall is only theexcess of the aqueous vapour poured so copiously intothe ai r from the whole of this region. It is the quantity which the air, as it were, rejects . It is amatter oflittle importance where the rainfall of higher latitudescomes from

,though it should be noticed that the views

of Dove, Kaemtz, and other leading meteorologists respecting the winds and rains of high and low latitudes,support my remark about the great rivers.


Now we have inthe phenomena of the zone of calmsa crucial test of Sir J. Herschel’s theory as to the originof the equatorial rains. It appears to me that this testaltogether negatives Herschel’s theory. If the moistureto which these equatorial rains are due came from thetrade-wind regions, we should certainly not expect thefall of these rains to be associated inany marked degreewith the progress of the equatorial day ; or, if at all,then the cooler parts of the day, when the point ofsaturationi s lower, would be the time of precipitation.

With the mid-day heat would come a cessationof precipitati on. As a matter of fact the contrary is thecase . The sun(we are told by Dove, Kaemtz , Humboldt

,Maury, Buchan, and many more) rises commonly

ina clear sky inequatorial regions . As the day proceedsclouds form

,and towards mid-day they grow dense. It

is at noonthat heavy showers fall, and towards eveningthe skies again become clear. Now, any one who hasnoticed what happens oncalm summer days inany wellwatered regioncansee that the equatorial phenomenarepresent the same processes ona greatly enlarged scale.Ona summer’s day insuch regions we see how scatteredcumulus clouds beginto form inearly morning, becomelarger and more numerous as the day proceeds ,.and inthe afternoon begin to be transformed into cumulostratus. The explanation i s simple . The sun’s heathas caused aqueous vapour to rise into the air, unti lthere is so much that not very far above the earth’slevel the saturationpoint is reached . The further riseof the vapour is followed by

'

the process of condensation

2 5 6 LI GH T SCIENCE F OR LEISURE H OURS.

I should not be greatly concerned if the resul t of theexperiments I spoke of shouldnot accord with my prediction. But merely to put ice inwater capable ofmeltingit,is not inany sense to represent the conditions of the

actual case . The addition of water from the ice as itmelts i s not in accordance wi th these conditions . It

cannot surely be maintained that the oceanic circulationdepends on the addition of water from the melting ofice ; and yet I apprehend that the melting of i ce i s nounimportant feature of Dr. Carpenter’s experiment. Atany rate, the i ce does melt, and the movement comes toanend whenall the ice has melted away. Let the i cebe packed outside the arctic end of the canal

,so as

merely to produce a refrigerationcorresponding to whatactually takes place with water carried into arctic latitudes, and I conceive that a very feeble circulationwould result. Under the actual circumstances

,the

melting of the ice produces effects much more nearlycorresponding to those due to rainfall thanto the mereeffects of arctic cold. The very activity of the c ircu

lation shows that the water which moves towards theice does not undergo refrigeration. Water does notcool quite so quickly. It i s the melted ice-water whichdescends ; and nothing takes place inthe arctic regionswhich corresponds to this continued addition of waterto that already circulating. Otherwise

,the arctic i ce

would be continually diminishing,which

,of course, i s

not the case.It will be gathered that I agree entirely with theopinion which Sir W. Thomson expressed

,as to the

OCEANIC CIR CULA TI ON.

reasonwhy heat is necessary for Dr. Carpenter’s experiment. Heat i s necessary, because the i ce mus t bemelted to make the experiment succeed. But comparingthe effects of heat and refrigeration (not of heat andthe continual inflow of ice-cold water), I conceive thatheat would be found altogether the more effective.Lastly

,as to the wind theory of the Gulf Stream

,Dr.

Carpenter remarks that,so far as he knows

, I am theonly manof science inthis country agreeing with Capt.Maury inattributing the Gulf Stream to some othercause thanthe impelling force of the trade winds .’ Hemust be aware that there are not half a dozenstudentsof science in this country who have expressed definiteopinions onthe subject after athorough and independentinquiry into the evidence . Amongst those who maintainthe wind theory there is not one, so far as I know,with whom Dr.Carpenter is inagreement. Mr .Laughtondisputes the very principle of Dr. Carpenter’s reasoning,holding that the change oftemperature from equator topoles proceeds too slowly mile for mile to produce theeffects which Dr . Carpenter indicates. Mr . Croll, inlike manner, has expressed hi s complete dissent fromDr . Carpente r’s reasoning . So also has M r . Findlay. I

believe these gentlemento be mistaken, and I conceivethat I have beenable to put my finger on the preci sepoint where their respective lines of reasoning fail.But

,if Dr. Carpenter i s to take general consent as an

argument,and to maintainthat I am wrong because he

knows ofno one who agrees with me , I may as wellpoint out that he is entering into a very questionable

S

2 58 LI GH T SCIENCE FOR LEIS URE H O URS .

alliance , so far as hi s special views are concerned . So

far as I know, all the continental students of sciencewho share our commonviews as to vertical circulation,reject the wind theory as s olely sufficing to account forthe Gulf Stream . Again

,he sets Si r J. Herschel’s

opinion (thirty years ago ) that ‘ the Gulf Stream i s

enti rely due to the trade winds as almost conclusi veagainst me. It i s

,at least

,

'not new to me , since iti s cited inevery paper I have wr itten on the subject .But i s there no evidence to show that Sir J . Herschelabandoned the View he formerly entertained ? I wouldask what Sir JohnHerschel implies when, inhis letterto Dr . Carpenter

,he writes

,

‘ The actionof the tradeand counter-trade winds

,in like manner , cannot be

ignored ; and henceforward the questionof oceancur

rents will have to be considered under a twofold point7

of view. The word ‘ henceforward ’ implies very distinctly that Sir J. Herschel was entertaining a newopinion—that is

,anopinionnew to him ; and I think

Dr. Carpenter would find it difficult to demonstrate thatthis new Opinionwould not have enforced the omissionof the word enti rely from the sentence quoted by Dr.Carpenter.I need hardly say that I do not agr ee with Captain

Maury, whose theory of oceanic circulation appears tome to be wholly untenable . Nor do I for a momentassert that the winds play no part inproducing oceaniccirculation. I may have beenmistaken inattachingso much weight as I have to Maury

’

s evidence as to thetrade wind zones, though it is knownthat science owesmore to him thanto anymanfor our present knowledge

2 60 LIGH T SCIENCE F OR LEIS URE H O URS.

former would produce a current flowing as the GulfStream actually flows ; the latter would produce a cur

rent flowing precisely inthe opposite direction. Thisbe ing the case

,I do not find the evidence for the trade

winds as the sole or even the main cause of the GulfStream altogether convincing. The case does not, forinstance

,seem quite ‘

as clear as the rotation of the

earth .’ It seems,also

,not undesirable to mentionthat

the equato rial current and the Gulf Stream are not

mere drift-currents,and that ona careful estimationof

the frictional actionof such wmds as the trades onthesurface of the ocean

,the action will be found quite

unequal to the propulsionof s o vast abody ofwater as i sactually carried westwards (not, by the way, before thesewinds). Until difficulties such as these have beenremoved from the trade wind theory as s olely sufilci ent

to account for the Gul f Stream,I think I would rather

be the only student of science opposing that theory,thanone of a phalanx

,however large

,maintaining it.

There is, however, no such phalanx ; the subject beingregarded by nearly all s tudents of science as a veryopenone .

Eng lish M echanic,Aug . 30

,1872.

THE CLIMA TE OF GREA T BRI TAIN.

IF there is one feature in the material relations ofacountry which may be considered as characteristic—asof itself sufli cient to define the qualities of the inhabitants , and the position theyare fitted to occupy inthe

,

THE CLIMA TE OF GREAT BRITAIN. 2 6 1

world’s history— it i s climate. ‘ It includes ,’ says Hum

boldt, all those modifications of the atmosphere bywhich our organs are affected— such as temperature

,

humidity, variations of barometric pressure, its tranquillity or subjection to foreign winds, its purity or

admixtur e with gaseous exhalations,and its ordinary

transparency— that clearness of sky so important throughits influence

,not only onthe radiationof heat from the

soil,the development of organic tissue and the ripening

of fruits, but also onthe outflow Ofmoral s entiments in

the difierent races .

’I do not propose, however , to deal

with the constitution of the climate of Great Britainunder this g eneral View. To do so , indeed, would requiresomewhat more space thancan in this volume be con

veniently allotted to a single subj ect. I wish chieflyto consider the subj ect of temperature (meanannualand extreme winter or summer temperature) ; thoughI shall have a few words to say respecting that featureof our climate which most foreigners consider to be itschief defect— the want of transparency or clearnes s in

our skies as comparedwith those ofsome other European

countries .The mean annual temperature of a country is lessimportant to the welfare of the inhabitants than the

of the year. Of two countries which have the samemean annual temperature , one may have a climatemost admirably adapted to the welfare of its inhabit

ants,while the other may have a climate offering such

fierce and violent extremes of heat and cold that its

2 6 2 LIGH T SCIENCE FOR LEISURE H OURS .

inhabitants resemble the unfortunates described byDante

,doomed

a s offr ir tormenti e caldi e geli .’

However, I shall deal first with this feature—mean

annual temperature—as affording a starting-point fromwhich to proceed to other considerations.If the surface of the earth were perfectly uniform, orsymmetrically distributed into districts of land and

water arranged in zones along latitude-parallels, and ifthe strata of the soil were throughout of like density,radiating power

,and elevation

,the lines of equal mean

temperatur e would be parallels of latitude . This hypothetical condition of things i s, we know, very far fromrepresenting the true condition of the earth’s surface .Land and water are distributed in a manner whichhardly presents the semblance of law ; elevations anddepressions , not merely ofareas of limited extent, butofwhole countries

, are exhibited in each hemisphere ;and endless diversities of soil

,contour

,and distribution,

disturb that mathematical uniformity and exactness,which could alone produce the co-ordinationof climatesunder latitude-parallels .It i s to Humboldt that we owe the valuable propo

s itionthat maps of the world should exhibit p arallelsof heat, as well as latitude-parallels ; and no atlas i snow considered complete without maps inwhich i s otherms , or lines of equal mean annual temperature,i s ochimenals or lines of equal winter heat, and i s otherals

‘

or lines of equal heat insummer, are exhibited .

2 64 LI GH T SCIENCE F OR LEI SURE H O URS.

to possess when we come to consider the extremerange of temperature during the year . Infact

,Eng

FIG . 1 .

Northernhemisphere onanequal-surface projection, show ing curves of

meanannual andmidw inter temperature through London.

land is thus brought to the centre of the true temperatezone of the northern hemisphere ; and the consideration of Fig s . 1 and 2 will show that the isotherm ofLondon approaches as near to the tropic of Cancerin one part of its cour se

,as to the Arctic circle in

another.Before leaving this part of the subject

,let me note

a circumstance, not immediately connected with the

THE CLIMA TE OF GREAT BRITAIN. 2 6 5

climate ofGreat Britain,but geographically interesting.

In examimng the polar presentation of the London

FIG . 2 .

Northernhemisphere onanequal-surface pro jection, showing curves ofmeanannual and midsummer temperature through London.

i sotherm, we s ee that in two parts of its course itexhibits a tendency to travel northwards , and becomes,in fact

, convex towards the pole . Ifwe laid downisotherms of greater mean temperatur e— that is , nearerthe equator—we should find this peculiarity graduallydiminishing. But if we laid down isotherms of lowermean temperature

,we should find the convexities

gradually becoming sharper and more defined, approach

2 66 LIGH T SCIENCE F OR LEIS URE H O URS.

ing each other more and more nearly, until finally theywould meet

, and the isothermal curve be divided intotwo irregular ovals . Proceeding to trace out curves ofstill lower temperature, we should find the two ovalsclosing in towards two poles of cold . These are indicated in Figs . 1 and 2 by two black spots, one northof the American, the other north of the Asiatic continent. It is to be noted

,however, that at the American

pole the mean annual temperatur e is not quite so lowas at the Asiatic pole, the former temperature beingthe latter 1° Fahrenheit.

Returning to our subj ect,let us consider the all

important question of range of climate . The effectsof climate

,unimportant to the stronger inhabitants of

a country,but largely influenc ing the health and com

fort of the maj ority,are chiefly felt through the changes

that occur during the year. Now,we have seen that

the line ofmeanannual temperature ofEngland departsin a very marked manner from coincidence with a

latitude-parallel ; but we shall find the lines indicatingthe extreme temperatures of the year much moreirregular ; and the peculiarity of climate, which theirconformationillustrates

,much more important.

InFig. 1 the i s ochimenal,or the line of equal winter

heat, through London, is indicated by a strongly markedclosed curve . Its form i s remarkable. It passes nearlyin a north and south direction

,along the leng th of

England and Scotland,approaches singularly near

to Iceland , but turns sharply southwards and travelsacros s the Atlantic ina directionwhich brings it to

~

2 68 LI GH T SCIENCE F OR LEIS URE H O URS.

pole of the earth .’ I cannot but think that this i s amistake . I believe that if the isotherms traced, inpart, in Fig. 1 could be completed , they would befound to form two ovals . ‘ The American oval wouldenclose the American pole of mean temperature, butvery eccentrically, showing that the pole of extremewinter temperature lay westwards and s outhwards , probably near Victoria Land . The Asiatic oval wouldnot probably enclose the Asiatic pole of meantemperature ; and the position indicated for the Asiatic poleof extreme winter cold lies onor near the Arctic circle

,

where it i s crossed by the river Lena. At the truepole of the earth the extreme winter cold i s probablynot nearly so intense as the cold at either of the pointshere indicated .From the direction of the isochimenal through Lon

don,it is evident that the Eastern Counties and Kent

experience the coldest winters of all places in theBritish Isles , while Cornwall and the south-westerlyparts of Ireland enj oy the mildest winter climates .Infact

,winter inCornwall is not more severe than in

Constantinople ; and in south-west Ireland the winteris still milder

,approaching

,in this respect, to the

winter climate of Teheran.

The contrast, whenwe turnto the i s othefral of London, i s remarkable . Instead of travelling nearly northwards, this curve passes ina south-westerly direction,reaching its greatest southerly range in the centralpart of the Atlantic Ocean; thence it travels with anortherly sweep through Nova Scotiaand Canada, till

it reaches i ts greatest northerly range near the RockyM ountains .‘ Thence it turns sharply southwards

,

crosses Vancouver’s Island,sweeps nearly to latitude

45°

inthe central part of the Pacific,whence passing

slightly northwards it crosses the southern part ofSaghalien Island . Here it turns sharply northwards

,

crosses that very district of Siberia which,inFig. 1 ,

i s occupied by the isochimenal of intensest winter cold,

traverses Siberia,and passes near St. Petersburg, through

Berlinand Amsterdam to London.

The relations thus presented by the isotheral ofLondon are precisely the reverse of those exhibitedby the i sochimenal. The isotheral forms a closedirregular oval

,whose greatest length lies on the two

oceans : here it falls outside the line of meanannualheat, while on the continent it falls far within this

In another respect the isotheral presents a noteworthy contrast to the isochimenal. While the latterencloses anarea largely exceeding the area enclosed bythe mean annual line, the isotheral encloses anarea

noticeably smaller.2

A tendency to break up into two curves is exhibitedinthe isotheral, evenmore markedly than in the twoother curves . But singularly enough , here , where one

It i s noteworthy that the minimum di s tance of the i sotheral fromthe North Pole here attained i s exactly equal to the minimumdi s tance

of the i s ochimenal from the equator .

2 Here an important advantag e of the i s ographic projection i s ex

hibited. The relationpo inted out i s altogether obliterated inMercator’

s

projection, and could only be roughly inferred fromany but anis ographic

projection.


would expect more certaininformationof the existenceof poles of cold, since so much more of the northernhemisphere canbe traversed insummer thaninwinter,we have no satisfactory evidence . Infact, the irregularcurve marked innear the pole inFig. 2 i s the mostnortherly isotheral yet determined . The temperaturecorresponding to this i sotheral is 3 6° Fahrenheit, orfour degrees above freezing. From a considerationofthe form-variations of the isotherals as they travelnorthwards, I have beenled to the opinionthat thereexist three poles of summer cold, and that thes e fallnot very far from the positions indicated by the smalldark circles in Fig . 2 .

From the direction of the isotheral line throughLondon, it i s evident that along the south-easterncoastofEngland the heat of summer is greater thaninany

other part of the British Isles . Onthe other hand,the northern parts of Scotland

,which we have seen

enj oy awinter climate fully as warm as that of London,have a much cooler summer climate . The s outhwestern parts of Ireland exhibit a change evenmoreremarkable For whereas the winter climate in theseparts i s the same as that of Persia, the summerclimate i s the same as that of the very portion of

Siberia in which (most probably) the greatest coldever observable in our northern hemisphere is ex

per i enced in winter. The summer of the OrkneyIslands , again, i s no warmer thanthat of the southernparts of Iceland .


winter,while those which have a contrary effect operate

insummer.Humboldt enumerates among the causes tending toexalt temperature the following non-variables —Thevicinity ofa west coast inthe northerntemperate zonethe configurationof a country cut up bynumerous deepbays and far-penetrating arms of the sea ; the rightpositionofa portionof the dry land, its relationtoanoceanfree of i ce

,extending beyond the polar circle

or to a continent of considerable extent which liesbeyond the same meridional lines under the equator

, or

at least inpart withinthe tropics ; the rarity of swampswhich continue covered with i ce thr ough the spring

,or

even into summer ; the absence of forests on a dry,

sandy s oil ; and the neighbourhood ofanocean-currentof a higher temperature thanthat of the surroundingsea.

All these causes,it will be observed—except the

neighbourhood of a tropical continent on the samemeridian—tend to increase the meanheat of the climatein England . The great Gulf Stream probably exercises amore important influence thanany of the others .

Its positioni s indicated in Figs . 1 and 2 . Humboldtattaches a high importance to the presence ofa tropicalcontinent onthe same meridian; and he considers thatthe climate of Europe i s warmer thanthat of Asia,because Africa, with its extensive heat-radiating deserts,lies to the south of Europe

,while the Indian Ocean

lies to the south of Asia. There are objections, however

,to the reasoning he adopts . In the fir st place , if

THE CLIMA TE OF GREAT BRITAIN. 2 73

the heat-radiating power ofacontinent really influencedthe country lying to the north, it should tend to lowerrather than raise the temperature

,for the ascending

currents of ai r would strengthenthe currents of colderai r pouring in from the north

, and these currentsonHumboldt’s assumptionthat the country directly tothe north is that aflected—would lower the meanannual temperature. It would only be exceptionallythat the warmer returning currents would descend

,and

thus exalt the temperature . It seems clear , however,that Asia is the continent chiefly affected by the heatradiating power ofAfrica; since the cold currents fromthe north travel eastwards, while the warm returncurrent has a westerly motion. We should thus attribute the milder climate of Eur ope rather to theinfluence of the tropical parts of the Atlanti c

'

Ocean,

than to the cause assigned by Humboldt, and weshould invert the effects he attributes to oceans andcontinents respectively. With this change—somewhata bold one , I confes s l—we may say that all the nonvariable causes tending to exalt temperature operate inEngland’s favour .The constant causes tending to lower temperature

are simply the converse of those above considered.

1 Not unsupported, however , by good author ity. Thus Profes sorNichol, speaking of the climate of Europe, wri tes : The air that ri s es

inAfri ca blow s rather over As ia thanEurope. The cradl e of our

winds i s not inSahara but inAmerica.

’Again, Kaemtz notices , that

if the eflfects of oceans and continents were tho s e as s igned by Humboldt,we should find inthe wes tern parts of America a colder climate than

inthe easternparts the rever s e, however, i s the cas e.


Of variable caus essky in summer, and a cloudy

doxi cal to as sign opposite effects to a cloudy sky. It

must be remembered , however, that clouds consideredwi th reference to temperature, have two fimcti ons :

they partially prevent the ac cess of heat to the ear th ,and they partially prevent the escape of heat from theearth. Now,

in summer the fir st-named influence ismore important than the second : the days are longerthan the ni ghts ; that i s, the earth i s rece iving heatduring the greater part of the time in summer. Acaus e, therefore, whi ch affects the receipt of heat is

affecting -the es cape ofheat. On the other hand, inwinte r the ni ghts areshorte r than the days , and the influence of clouds inpreventing the es cape of heat becomes more importantthan their effect on the receipt of heat . 1 In fact, wemay compare the influence of clouds to the effects ofcertain kinds of clothing ; flannel, for ins tance, is assui table an article of dres s for the cricketer as for the

Now the climate of England i s remarkably humidboth in winte r and summer. And this humidi ty i sshown, not so much by the quantity of rainwhi ch falls ,

G ilbert Whi te noticed long ago—apparently without understanding-the influence of a clouded sky onthe temperature. We have o ften

observed,’

he says , that cold s eems to des cend from above ; for , whenathermometer hang s abroad ona fro sty night, the interventionofa cl oud

shall immediaw rai s e the mercury tendegrees ; and a clear sky shall

againcompel it to des cend to i ts former gaug e.

’

2 76 LI GH T SCIENCE F OR LEIS URE H 0 URS.

In Fig . 3 the annual variations of mean diurnaltemperature are represented graphically. The figurewas formed in the following manner —A rectanglehaving been drawn

,each of the longer sides was

divided into 3 65 parts, and a series of parallel linesj oining every tenth of these divis ions was pencilled in.

The spaces separating these lines represented successiveintervals of tendays throughout the year . The shorters ides were divided into thirty-three parts and parallellines drawn

,j oining the points of divis ion. Of these

longer parallels the lowest was taken to represent atemperature of 3 2° Fahrenheit the freezing point)and the others, inorder, succes sive degrees of heat upto Then, from the Greenwich tables , which havebeenformed from the obs ervations of forty-three years

,

the temperature of each day was marked in,at its

proper level and at its proper distance from either end

of the rectangle . Thus 3 65 points were marked in, andthese being j oined by a connected line, presented thecurve exhibited in Fig . 3 . The lines bounding themonths

,and the lines indicating &c .

,Fahren

heit,were theninked inand the figure completed.The resulting curve i s remarkable inmany respects .

Inthe first place,it was to have been expected that a

curve representing the average of so many years ofobservation would be uniform ; that is , would onlyexhibit variations in its rate o f rise and fall , not suchamultiplicity of alternations as are observed inFig . 3 .

And this irregularity will appear the more remarkablewhen it is remembered that the temperatures used as .

2 78 LI GH T SCIENCE FOR LEIS URE H O URS.

the Greenwich means are not the true average tem

peratures . They were obtained by constructing a curvefrom the true averages

,and taking a curved line (the

curve of Fig. 3 , in fact) in such a way as to take offthe most marked irregularities of the true curve of

averages or to use the words of the meteorologist whoconstructed the Greenwich table of means

,Mr . Glaisher

,

a curved line was drawnwhich passed through or nearall the points determining the true curve of averages

,

and in such away that the area of the space abovethe adopted line of meantemperature was equal to that

2below the line . Despite this process, the curve exhibitsno less than fourteen distinctly marked maxima of

elevation, and a much larger number of variations offlexure . The sudden variations of temperature at thebeginning of February

,early in April, and early in

May are very remarkable ; they have their counterpartsinthe three variations which take place between thelatter part of November and the end of the year, onlythese occur in much more rapid succession. Thenature of the curve between June and August is alsoremarkable , as are the three convexities which are exhibited in the September

,October

,and November

portions of the cur ve .Ifwe follow our leading meteorologists intaking the

curve of Fig. 3 as representing the true annual climateofLondon

,how are we to assignphysical causes for the

remarkable variations above indicated ? Not easily, Itake it. It were

,indeed

, as easy as inviting to speenlate on cosmical causes ; to follow Ertel, for instance,

2 8c LIGH T SCIENCE FOR LEISURE H'

o URS.

tions of the variation of mean diurnal temperature,since it is clear that a cause of variationdue to objectsexternal to the earth could affect only the temperatureof certainhours of one day or ofseveral days . A clusterof meteors between the earth and the sun mightdiminish the mid-day heat one external to the earth’sorbit might increase the nocturnal temperature ; andthough in either case the mean diurnal temperaturewould be affected

,yet it is obvious that the effect

would be masked intaking the mean, or eventhat twoo r more opposing influences might cancel each other.If it could be shownthat the curve for mid-day, or formidnight heat corresponded to the curve ofmeanheat,Ertel

’

s theory would be overthrownat once ; since, fori ts support it i s necessary to show that depressions inthe mean curve are due to mid-day loss of heat, andelevations to midnight gainofheat.There are , however, terrestrial causes to which theirregularities of our curve (which irregularities, be itremembered

,represent regular ly recurring i rr egulari

ti es of heat) may be ascribed. For instance,there can

be no doubt that our climate i s considerably affected bythe changes which take place inthe Polar seas and itmay not unfairly be assumed that the processes bywhich different regions of Polar i ce are successivelyset adrift (to be carried southward by the strong undercurrent knownto exist inthenorthernAtlantic Ocean),take place at epochs which recur with tolerable regularity. And it may be that the irregularity of theris ing as compared with the falling of the heat-curve

THE CLIMATE OF GREA T BRITAIN. 2 8 1

i s due to this cause ; since the breaking-up of ice-fieldsand their rapid transport southwards would clearlyproduce sudden changes

,having no counterpart inthe

effects due to the gradual process of freezing.‘

It may well be, however, that the observations offorty-three years are not sufficient to afford the truemeandiurnal temperature for a climate so variable asours . Indeed, if the curves given by Kaemtz forcontinental climates be as accurately indicative of

observed changes as that of Fig. 3 , we must eitheraccept such an hypothesis, or else assume that theEnglish climate is marked by regularly recurringvariations altogether wanting in continental climates ;and it is to be noted that the regular recurrence ofchanges is a peculiarity wholly distinct from variabilityof climate

,properly s o termed, and seems evenincon

sistent with such a characteristic. It may happen,therefore

,that the observations of the next thirty or

forty years will afford a curve of different figur e and

that by comparing the observations of the eighty orninety years , which would thenbe available, many, or

all,of the irregularities exhibited in Fig. 3 might be

removed . In this case we might expect our climatecurve to assume the form indicated by the light linetakenthrough the irregular ities of Fig. 3 . It will beobserved that this modified curve exhibits but one

maximum and one minimum. It i s not wholly free,

1 Iceberg s have been s een travelling s outhwards against a s trop g

northward surface-current, and evenforcing their way through field-i ceinso travelling .

2 8 2 LI GH T SCIENCE FOR LEISURE H o URS.

however, from variations of flexure . It presents, indeed,six well-marked convexities , and as many concavitiesinother words

,no less thantwelve points of inflexi on.

The most remarkable irregularity of this sort is thatexhibited near the end of November ; and it is noteworthy that this irregularity i s presented by continentalclimate-curves also . It has beenascribed by Ertel tothe effect of the meteor-zone which causes theNovember shower. But as it is exhibited by the curvesof horary as well as of diurnal means , while themeteor-zone cannot by any possibility affect the tem

perature of the earth’s following hemi sp here, and as,

further, it does not correspond to the true date of theshower , this view may be looked upon as doubtful .The August curve occurring near the maximumelevation—where slow change was to be expected, i salso well worthy of notice ; as are the January andMay flexures .

It will be noticed that nothing has been said of

extreme heat o r cold occasionally experienced inEngland. As such visits generally last but for a shorttime, their effects are not very injurious , save onthevery weak, the aged, or the invalid. Corresponding tothe passage of an immense heat-wave or cold-wave,there invariably occurs a suddenrise inthe mortalityreturns ; but almost as invariably the rise is followedby a nearly equivalent

,but les s sudden fall ; showing

conclusively that many of the deaths which marked theepoch of severest weather occurred a few Weeks onlybefore their natural time .

2 84 LIGH T SCIENCE F OR LEISURE H o URS.

traordinary appearance, unlike anything knownwithinthe memory ofman. By my j ournal, I find that I hadnoticed this strange occurrence from June 23 to July20, inclusive, during which period the wind varied toevery quarter

, wi thout making any alterationinthe air.

The sunat noonlook ed as blank , and ferruginous as aclouded moon

,and shed a rust-coloured ferruginous

light on the ground and floor s of rooms , but was parti cularly lur id and blood-coloured at rising and setting .

All the time the heat was so intense that butchers’ meatcould hardly be eatenthe day after it was killed ; andthe flies swarmed so inthe lanes and hedges, that theyrendered the horses half frantic, and riding irksome .The country people began to look with a superstitiousawe at the red, louring aspect of the sun. M ilton’snoble simile, in his first book of Paradise Lost,” frequently occurred to my mind ; and it is, indeed, parti cularly applicable, because, towards the end, it alludesto a superstitious kind of dread, with which the mindsofmenare always impressed by such strange and un

us ual phenomena

As whenthe sunnew r i s en,

Looks through the hor izontal mi s ty air ,Shornofhi s beams o r , frombehind the moon,Indim eclips e, di sas trous twi light sheds

Onhalf the nations , and wi th fear of changePerplexes monarchs .

’

Intellectual Observer,March 1867.

LOW BAROME TER OF ANTAR CTI C zoNE . 2 85

THE LOW BAR OME TER OF THE ANTAR CTI C

THE great difficulty presented by the science ofmeteorology lies inthe intricate combinationof causesproducing atmospheric variations, and the impos s ibi lity of determining by experiment the relativeefli ciency evenof the most important agents of change .As Sir W. Herschel well observed, we are in thepositionof a man who hears at intervals a few fragments of a long history narrated in a prosy, unmethodical manner. A host of circumstances omittedor forgotten

, and the want of connection between theparts

,prevent the bearer from obtaining possessionof

the entire history. Were he allowed to interrupt thenarrator

,and ask him to explainthe apparent contra

dictions, or to clear up doubts at obscure points, he

might hope to arrive at ageneral view. The questionsthat we would address to Nature, are the very experiments of which we are deprived in the science of

meteorology.’ 1

It is, therefore, but seldom in the study of thisscience that we meet with phenomena to which we can

assign a definite cause, or which we can explain on

simple princ1ples . Even those marked phenomena,which appear most easily referable to Simple agencies,

1 Kaemtz’s Meteorology.

2 86 LIGH T SCIENCE F OR LEIS URE H o URS.

present difficulties ona close investigationwhich compel us at once to recognise the efficiency ofmore causesthanone . For instance, the phenomenonof the tradewinds, as explained by Halley, appears at first sighteasily intelligible ; but whenwe look onthis phenomenonas a part merely—as indeed it i s—of the marvellously complex circulationof the earth’s atmospherewhen we come to inqui re why these winds blow s o

many days in one latitude, and so many inanother, orwhy they do not blow continually in any latitudewhen we consider the character of these winds asrespects moisture and temperature, the variationof thevelocity with which they blow, and of the quantity ofair they transfer from latitude to latitude—weencounter difficultie s which require for their elucidation the comparison of thousands of obs ervations, orwhich baffle all attempts at elucidation.

There is,however, one atmospheric phenomenon

that which I have selected for the subj ect of this paper—which presents a grand simplicity, rendering theattempt at a s imple solution somewhat more hopefulthani s usually the case with meteorological phenomena.

The discovery of this phenomenon formed one of themost interesting results of Captain Sir J. C. Ross ’scelebrated expedition to the Antarctic Ocean. Hefound, as the result of observations conducted duringthree years, that the mean barometric pressure variedin the following manner at the latitudes and placesspecified

2 88 LIGH T SCIENCE FOR LEISURE H o URS.

about 40° south latitude we find the same pressure asat the equator, and thence a more rapid diminution.

The rate of change is illustrated graphically inFig. 1 ,which represents the height of the barometer above28% inches for different southern latitudes . In thenorthern hemisphere there i s a similar increase of

pressure as we leave the equator, a maximum is therealso attained in about latitude but from thispoint towards the poles there is amarked difference inthe rate of diminution of pressure in the two hemispheres . The following table by Schow is sufficient toindicate this

North latitude

00

1O

20

30

40

45

5 0

5 5

60

6 5

70

75

There are minor irregularities in this table, due,doubtless , to local pecul iarities, the arrangement ofland and water being s o much more complicated inthenorthernthaninthe southernhemisphere . Neglectingthese (as inFig . 2 , which represents for the northernhemisphere the relations corresponding to those exhibitedfor the southern hemisphere in Fig. we see thatthere is a much greater resemblance betweenthe rise

Barometric pressure

298 5 3 in.

300 02

300 04

300 69

29 943

299 60

298 35

297 22

298 63

L OW BAR OMETER OF ANTAR CTI O ZONE. 2 89

and fall of barometric pressure as we proceed northwards than as we proceed southwards. In fact

,the

curve is almost exactly symmetrical on either side of30

° north latitude to the equator on one side, and to

latitude 60° onthe other. From 60° the pressure continue s to diminish for awhile

,but appears to attaina

minimum inabout latitude and thence to increase.Inthe southernhemisphere, if there is any corresponding minimum, it must lie ina latitude nearer the southpole thanany yet attained.The most marked feature in the comparison of the

two hemispheres is the difference of pressure over thesouthernand northernzones

,betweenlatitudes 45 ° and

This is a pecul iarity so remarkable,that for a

long time many meteorologists considered that theobservations ofCaptainRoss were insufli cient to warrantour concluding that so important a difference reallyexists betweenthe two hemispheres. But not only hasCaptainMaury —from acomparisonof observations—confirmed the results obtained by Ross, but, inmeteorological tables published by the Board of Trade, thesame conclusions are drawn from observations,takenduring a period of no less than days . In

fact, it is now shownthat the difference is yet greaterthanit had beensupposed to be from the observationsof CaptainRoss . From a comparison of observations

Antarctic Seas with those of CaptainSirlintock , i t appears that the average dif

ght inthe northernand southernis about one inch.


Figs . 1 and 2 exhibit a relationmidway between theselater results and those tabulated above .Assuming anaverage difference of only three-quarters

ofaninch inthe northernand southernzones,between

latitudes 40° and let us consider what is the difference of pressure on these two zones of the earth’ssurface. The area of either zone is squaremiles

,and the pressure on a square mile due to a

barometric height of three-quarters of aninch is abouttons

,therefore the pressure on the northern

zone,betweenthe latitudes named

,exceeds the pressure

onthe southernzone byno less thantons . Including all latitudes withinwhich there hasbeenascertained to be adifference of barometric pressureinthe two hemispheres , we shall probably be withinthemark if we say, that the atmospherical pressur e on thenorthernhemisphere i s tons greaterthanthe atmospherical pressure on the southernhemisphere .Such a peculiarity as this may almost deserve to be

spoken of in the terms applied by Sir J. Herschel tothe distributionof land and water uponour earth

,it is

mas s ive enough to call for mentionas anas tr onomi cal

feature.

’I propose to examine two theories which have

been suggested in explanation of this feature of theearth’s envelope . These theories are founded onlocalpeculiarities, and the feature considered appear s as a

dynami cal one—that is , as a peculiarity resulting fromstates ofmotioninthe aerial envelope . I shall endeavour to establish a theory formded on a consideration

2 9 2 LI GH T SCIENCE FOR LEISURE Ho URS.

To speak of the confined ai r of the North AtlanticOcean i s surely unreasonable . An ocean milesacros s , swept by more frequent storms thanare experi

enced in any other part of the globe , cannot be veryaptly compared to the bottom ofamine .’ Aninelasticfluid flowing steadily over a rugged surface shows notrace , or but the s lightest trace , of the nature of thatsurface , by any variations of its own level. But it isstill less conceivable that an elastic fluid should be influenced in the manner suggested. In fact

,if this

happened, we should no longer be enabled to determinethe heights of mountains by barometric observations ;for according to the theory the ai r should extend to agreater height above mountains thanabove plains ; andas regards comparis on betweena barometer at the footofamountainand one at the summit, we might arguethat the barometer in the valley

,compared with a

barometer at the same level in a plane district,

‘ ispres sed upon, not only by the air clear of the mountaintops, but also by the air confined withinthe valley,

’ sothat the altitude over the valley is greater by somehundr eds of feet than the altitude over a plainat thesame level as the valley ; and thus, before we coulddetermine the height of the mountainabove the levelof the plain

,we should have to determine the exact

effects due to the confinement of the air inthe valley.We know that, onthe contrary, the average barometricpressure in the most confined valley does not differappreciably from the average pressure over the mostwidely extended plainat the same level.

LOW BAR OME TER OF ANTARCTIC ZONE . 2 93

We may, however, reasonably inquire whether thepresence of continents in the northern hemispheremight not operate in another manner. If we placeany mass within a vessel containing fluid

,it is clear

that we increase the fluid pressure over every pointof the vessel’s bottom, because this pressure dependswholly onthe depth of the bottom below the level ofthe fluid, and the level rises when any solid substancei s placed withinthe vessel. Now if we suppose aglobecovered all over by water to be surrounded by a

perfectly uniform atmospheric envelope, the meanpressure of this envelope at the water-level wouldcertainly be increas ed if continents were supposed to beraised in any manner above the surface of the water ;and if the atmosphere over one half of such a globewere supposed to be prevented in’

any way from mixingfreely with the atmosphere over the other half, theniti s clear that the meanpressure at the water-level wouldbe greatest on that

“half-globe over which the mostextensive and highest continents had beenraised. On

the assumption, then, of some such arrangement overour ownearth—anarrangement, that is, which should

of the northern hemisphere .We have, however, not only no evidence that such

an arrangement exists , but very strong evidence of an

atmospheric circulationwhich carries air from hemi

Sphere to hemisphere, and mixes in the most intimate


manner the whole mass of gases which form the earth’satmospheric envelope . The whole question of thecirculation of the ai r is investigated inMaury

’

s in

teresting work onthe Phys i cal Geograp hy of the Sea,and he appears to establish in the most convincingmanner the interchange of ai r between the northernand southernhemispheres .And evenif we could assume that the atmospheric

covering of any portion of the earth’s surface was inany way prevented from passing freely to otherregions

,yet the cause assigned would be inadequate

to account for the difference of barometric pressureactually existing between the two hemispheres . Allthe land above the sea-level inthenorthernhemisphere,if uniformly distributed over the surface of thathemisphere

,would be raised to a height of less than

200 feet above the present sea-level, and the actualdifference of level corresponding to the observed difference of barometric pressur e is more thanfour timesas great.Passing over this theory as neither consistent withthe knownlaws regulating the motions of elastic fluids,nor sufficient even if the consideration of those lawswere neglected, we come to the theory suggested byCaptain Maury- a theory deserving of much moreattentive consideration. I shall quote his own words ,as the fairest method of presenting his theory ; afterstating the observed difference of barometric pressureinthe two hemispheres, andmentioning the expulsionofair from the northern hemisphere as the cause of this


undoubtedly great, but it cannot be more than theequivalent of the amount of heat rendered latent as thevapours are formed, and therefore the expansive effectsdue to the liberationofheat cannot be greater thanthecontrary effects due to the prior imprisonment of heat .It i s quite true , and has beenaccepted as the undoubted!

explanationof many climatic effects, that if vapour beraised inone place and condensed over another, thenthe temperature of the air over the latter place is raised.But whenwe have to consider aphenomenonextendingover a zone twenty or thirty degrees inwidth

,we can

not argue inthis manner. Nay, it is neces sary to theforce of Maury

’

s second cause that the condensationof

vapour should take place over the very zone inwhichthe vaporisationi s proceeding. To assignsimilar effectsto both processes

,is to require that the winding up and

the loosening of the spring should take place in the

same direction.

Whatever effects, then, are due to the constantevaporationgoing oninthe southernhemisphere, mustnot be derived from changes of temperature. So faras these are effective at all, they must depend on theexcess of evaporation over condensation (since theexcess cannot possibly lie the other way), and thereforerepresent diminutionofheat or increase ofpres sure, thecontrary effect to that we have to account for. Wehave , therefore , only to consider the first cause men

ti oned by Maury ; that i s the expulsive effects due tothe formationofaqueous vapour.At first sight, this process of expulsionappears s imple

LOW BAROMETER OF ANTAR CTI C ZONE . 2 97

enough , and seems further to coincide with many wellknown phenomena. The theory supposes that over awi de zone of the southern hemisphere aqueous vapouri s continually rising ; that as it rises it displaces inpart the heavier ai r over these regions ; and thatequilibrium being thus disturbed, the excess of ai r flowsoff continually towards the equator. Now we knowthat the prevailing surface-winds over that zone of thesouthernhemisphere in which the barometer exhibitsthe peculiarity we are considering, blow from theequator ; that is, they tend to sweep the lower strata of

the atmosphere towards the south pole . They the reforetend to increase the quantity of humid an in highsouthernlatitudes . We know also that the prevailingupper currents over the southernzone we are cons ideringblow towards the equator. They tend, therefore, tocarry the drier portion of the air towards the equator".All this seems inaccordance wi th Maury

’

s theory, and

indeed if the prevailing upper and lower currents flowedin directions contrary to those indicated, the theorywould fall at once .Again

,although we find no evidence in barometric

pressure over the south tropical zone of that increasewhich Maury

’s theory would lead us to expect (since

the surplus air i s carried first to this zone), yet it mightbe argued that the surplus is so distributed as to appearinanother way. It is evident that if the atmospheric

envelope normally appertaining to the southernhemisphere were

,through

'

the effects of the causes assignedby Maury, increased inextent, this increase might show

2 98 LIGH T SCIENCE F OR LEISURE H O URS.

itself,not in an increase of pressure over the south

tropical zone—that is,not inanincrease of height there

—but inthe extens 1onof the surplus atmosphere intothe northernhemisphere . This would be shownby theextensionof the southern trade-winds to or beyond theequator , so that the (s o-called) equatorial zone of calmsshould lie north of the equator . As this i s really thepositionoccupied by the belt of calms , Maury

’

s theoryappear s to gainadditional force by the coincidence .Another argument may be drawnfrom the analogy

of the low barometer inmoist weather. In fact, it i swell knownthat Deluc explained this phenomenoninamanner precisely accordant with the views exp ressedby Maury.Despite the apparent force of these arguments, andothers that might be adduced , it will not be difficult,I think, to show that neither is Maurv

’

s theory consistent with knownphysical laws ,nor (passing over thisobjection) is the theory safii ci ent to account for thegrand phenomenonunder consideration.

It i s quite true that a volume of aqueous vapourweighs less thananequal volume of ai r ; it is equally truethat a volume of moist air weighs less thanan equalvolume of dry air at the same tens ion: But water

,

quietly evaporating in the openai r , does not displacethe air, but penetrates into its interstices, accordingto the well-established law regulating the mixtureof vapours . The aqueous vapour which thus intimatelymixes itself with the ai r produces no effect whatever,either by its weight or by its elasticity

,on the move


a proces s to account for the great phenomenon we aredealing with.It must be remembered, in the first place

,that the

theory requires that there should be a greater volumeof mixed air and vapour over the southerntemperatezone thanthere i s m the corresponding northernzone

,

otherwise there would not be that continual overflowtowards the equator which i s required by the theory.So far as it goes, this increment of volume implies anincrement ofweight. The increase of volume is morethancompensated (intheory) by diminutionof specificgravity, but it must be held inmind that the increaseof volume has to be accounted for by the theory as wellas the difference inbarometric pressure .Again

,the theory requires that the upper regions of

air should be dry, for it is-the upper air that i s carriedtowards the equato r ; and if this air were moist, weshould no long er have the different proportions ofmoistand dry air which are required by the theory. Wemus thave anaggregationof moist air inhigh southernlatitudes

,and of dry air towards the equator.

Again, we must call to mind that one-half of thenorthernhemisphere i s covered by water

,and a part of

the southern hemisphere i s not so covered,s o that the

effects suggested by Maury are (1) not peculiar to the

with thos e metereologi sts who cons ider that the notionof any appreci

able up lifting of the air by the ri s ing vapour ofwater i s amis takenone.

But whether it be so or not, it i s evident that Hers chel’

s view would re

quire a. regular increas e of pres sure from the equator to the antarctic

pole, and therefore is oppos ed to Maury ’

s explanation.

LOW BAROMETER OF ANTARCTI C ZONE . 30 1

s outhernhemisphere, nor (2) do they prevail over thewhole of that hemisphere .Lastly, we must remember that the process conceivedby Maury must be wholly or principally a diurnal process , and so can only take place (onanaverage) overone half of the southernzone at any one time.All these considerations tend to diminish very im

portantly the efficiency of the cause assigned byMaury.Let us, however, cons ider what is the maximum valuethat efficiency could have if all these circumstances wereneglected. We shall see that eveninthis case

,which

assigns anefficiency at least three or four times as greatas would be consistent with actual facts, we shall stillfind the cause as signed by Maury inadequate to theproductionof the phenomenonunder consideration.

The greatest weight ofaqueous vapour which is everpresent ina givenvolume ofair is equivalent to aboutone-sixtieth part of the weight of the air. Now,

if wesuppose the barometer at thirty inches, and the wholecolumnofair above the barometer to be impregnatedwith air in the above-named proportion—a View veryfavourable to the theory, since the cold of the upperregions of ai r largely diminishes the proportionateweight ofaqueous vapour— it is clear that one-sixtiethpart—or half aninch—of the barometer’s height is dueto the presence of aqueous vapour. Now

,at mean

tensions the specific gravity of aqueous vapour is aboutthree-fifths of the specific gravity of ai r , so that theproportion of one-sixtieth part of weight correspondsto a proportionof one-thirty-sixth part of volume in


"

other wo rds, our column of air owes one-thirty-sixthpart of its height to the presence of aqueous vapour.If we suppose this thirty-sixth part to flow off—not

from the upper regions only, but insuch amanner thatone complete thirty-sixth part of the volume of thecolumn should pass off— then, instead of standing at aheight of thirty inches, the barometer would stand at aheight of 29g inches, less by only one-third of an inchthanthe height of 294

,L inches due to the dry ai r alone .

Now we cannot, in accordance with Maury’

s theory,legitimately add the five-sixths of aninch ofbarometricpressure to the height of the barometer under aneighbouring column. For we have no evidence to showthat the ai r assumed to be expelled from the southerntemperate zone is heaped over the southern tropicalzone ; on the contrary

,we have a barometer in the

latter zone not quite so high evenas the barometer inthe corresponding northern zone. Therefore if ai r i sexpelled inthe manner supposed by Maury, it must bedistributed over a very much greater portion of the

globe’s surface thanit had beenexpelled from. Hence ,

returning to our imaginary columnof ai r , but a smallfraction of the five-s ixths of an inch due to overflowmust be added to the barometer under aneighbouringai r-column. The latter barometer originally at 29;may be fairly assumed to rise at most to about 29%inches . We have

,then, a difference of 29g~ 29é

inches, or two-thirds of aninch ; so that despite all theOpposing considerations we have neglected, we stillhave a difference less by one-third thanthat for which

30 4 LI GH T SCIENCE FOR LEISURE H OURS.

pos ing these arrangements to exist, it i s evident thatthey form mere local peculiarities . The general tendency ofwater towards the southernhemisphere is veryobvious , and, s o far as I am aware ,no other explanationofthe peculiarity has ever beenoffered thanthat foundedon a slight displacement southwards of the earth’scentre of gravity . If, then, C is the centre of the blackcircle in Fig. 3 , representing the s olid part ' of theearth , the centre of gravity of this part lies (inthe Fig.)s lightly below C—betweenC and C’ , let us suppose .Now we see that, owing to this slight displacement,the watery envelope of the earth tends southward . If

the earth were a perfectly uniform spheroid,it i s clear

that there would be a tendency to some such arrangement as is represented (ona greatly exaggerated scale )inFig. 3 , inwhich the shaded part represents the sea—that is

,a shell of water, thicker towards the south ,

probable one. The theory of anantarctic continent i s hardly inthe

same pos ition, s ince antarctic explorations have g ivenus but faint indications , here and there , of the hab itudes of the s outh-polar regions .

But I may note, inpas s ing , a very s ingular argument us ed by Captain

Maury infavour of the exi s tence of such a continent. He states it as

a phys ical law that land i s s carcely ever antipodal to land ; therefore,’he says , s ince the north-polar reg ions are probably occupi ed by avas t

ocean, the s outh-polar regions are probably occupied by a vast con

tinent.

’He s eems to forget that it by no means follows that becaus e

land i s s eldom antipodal to land, water should s eldom be antipodal to

water . Since the extent of water i s nearly three times that of land, it

i s abs olutely neces sary that nearly two-thirds of the water should be

antipodal to water . The suppo s ed peculiar ity that nearly all the land

i s antipodal to water (one twenty-s eventh only being antipodal to land),i s inreality no peculiarity at all. It would have beenfar more s ingularifany large proportionof the land (which occupies little more thanone

fourth ofthe globe) had beenantipodal to land.

ment, as minor inequalities of the earth’s surfacecontour have clearly nothing whatever to do with thephenomenon’ we are considering.Let C

’ be the centre of the spheroid which boundsthe earth’s fluid envelope. Then it is very clear thatif this envelope were of the same specific gravity as thesolid portionof the earth, the centre of gravity of theentire mass would lie very near C’

, but slightly southof that point, on account of the slight southerly di s

x

30 6 LI GH T SCIENCE F OR 'LE ISURE H 0 URS.

placement of the centre of gravity of the solid portion.

But whenwe consider that the specific gravity of thefluid envelope i s less thanone-fifth of that of the solidglobe, it i s perfectly clear that the centre of gravity ofthe entire mass will not be so far south as C’

. For, ofthe entire mass. the northern half i s the heavier, andtherefore the centre of gravity must lie north of the .

centre of the entire mass— that is , north of C’. In'

fact, it must lie much nearer to C thanto C’

Thus, the centre of gravity of the solid globe, andthat of the entire mass, solid and fluid , both lie betweenC and C

’. Now it i s evident that the central

point, about which the earth’s atmospheric envelope '

tends to form itselfas a spherical or spheroidal shell , i sthe centre of gravity of the entire solid and fluidterrestrial globe—that i s , is a pointnorth ofC’

. Therefore, preci sely as the effect of the fluid envelope collecting itself centrally about a point s outh of C i s to causethe meandepth ofwater to be greatest inthe s outhernhemisphere

, s o the fact that the atmospheric envelope ,collects itself centrally about a point north ofC

’ shouldresult in gi ving a greater meandepth of air (referredto

‘

the s ea-level) over the ,northern hemisphere. Thisarrangement is represented in Fig. 3 , inwhich the unshaded part is supposed to represent the atmosphere .I have endeavoured to make the above explanation

of my theory inexplanationof the low antarctic barometer as complete and exact as possible ; but there i sanother way of presenting the theory, which, thoughless complete, may appear clearer to some minds

30 8 LI GH T SCIENCE F OR LEISURE H O URS.

centres of gravity li e considerably Wi thin400 feet ofC,and C’ lies considerably within8 25 feet of C. Thusthe centres of figur e and the centres of gravity of theearth’s solid mass, and of the entire fluid and solid massare collected within a space less thanone-eighth of

mile inlength—a distance almost evanescent in com

parisonwith the dimens ions of the earth’s globe .

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