.LIGHT SCIENCE* FOR 
 LEISURE HOURS. 
 
 FAMILIAR ESSAYS ON 
 SCIENTIFIC SUBJECTS, NATURAL PHENOMENA, &c. 
 
 WITH A 
 
 SKETCH of the LIFE of MAEY SOMERVILLE. 
 
 BY 
 
 RICHARD A. PROCTOR, B.A. CAME., 
 
 n 
 
 HONORARY SECRETARY OP THE ROYAL ASTRONOMICAL SOCIETY; 
 
 AUTHOR OP 
 
 THE SUX OTHER WORLDS SATURN ESSAYS ON ASTRONOMY THE ORBS AROUND US 
 
 ETC. 
 
 Truths of Science waiting to be caught. TENXYSON. 
 
 LONDON : 
 LONGMANS, GREEN, AND CO. 
 
 1873. 
 
 All rights reserved. 
 
0/71 
 
 T43 
 I 873 
 
PBEFACE. 
 
 THE First Series of Light Science Essays met with a 
 success 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 have 
 also felt a wish to publish these essays because they con 
 tain facts collected at the cost of much labour and care 
 fully discussed, useful, therefore, I trust, to others as 
 well as to myself, when thus gathered into a volume. 
 
 Those who have read my former series of essays, viz., 
 Light Science, Series I., The Orbs around Us, and 
 4 Essays on Astronomy, will perceive that even when 
 I treat here of subjects already dealt with by me else 
 where, I have been careful to avoid the repetition of 
 any statements, except those few without which a sub 
 ject would be incomplete. For instance, it will not 
 be easy to find in my two papers on comets in The 
 Orbs around Us, statements or reasoning repeated in 
 the two papers on comets in the present volume. 
 
 However, for the most part, the papers in this series 
 
VI PREFACE. 
 
 are distinct in subject as well as in treatment from any 
 of my essays which have formerly appeared. 
 
 I invite special attention to the second essay on the 
 Transit of Venus. The time is drawing near when it 
 will be too late to take action to extend and render 
 complete and satisfactory our preparations for this im 
 portant phenomenon the most important, I venture 
 to assert, of all the astronomical phenomena of the 
 present century. Without imputing blame to any per 
 son, I must dwell strongly on the fact that the share 
 proposed to be taken by Great Britain in the observa 
 tions of the transit, ia unworthy of her position in the 
 scientific world, and as a nation. There is great risk 
 that, for want of an adequate number of southern sta 
 tions, the whole series of observations by all countries 
 engaged in the work urill result in failure; and it 
 appears to me nay, more positively it certainly is 
 a deplorable circumstance, that while Russia and 
 America are providing for more than thirty northern 
 stations, whereof sixteen are Halleyan, Great Britain 
 will supply but three southern stations, of which only 
 one chances to be Halleyan as well as Delislean ; while 
 even as respects this one station, Mr. Goschen has told 
 the country, speaking in his place in Parliament, that 
 either Halley s method 6 will not be applied at all, 
 or at least very little reliance will be placed upon it. 
 Yet at sixteen northern stations, some of them most 
 
PREFACE. VI I 
 
 difficult of occupation, Russia and America will apply 
 this very method ; while even the criterion devised to 
 minimise the value of the method, leaves it superior, 
 when applied at the stations I have indicated, to De- 
 lisle s, as applied at selected stations. 
 
 I appeal to all who have influence in these matters to 
 examine the evidence for themselves (whether as pre 
 sented here, or with charts in my Essays in Astronomy, 
 or in recent numbers of the Monthly Notices of the 
 Astronomical Society), and to form their own judgment 
 as to the position of affairs. That is all I ask, since 
 I am satisfied that that will be altogether sufficient to 
 suggest the promptest and most energetic action.* 
 
 EICHD. A. PROCTOR, 
 
 LONDON: May 1873. 
 
 * Since this was written I have received letters from the greatest master 
 of mathematical astronomy this country has produced since Newton s day, 
 strongly confirming my views as to the extreme importance of providing 
 many southern stations for applying Hall ey s method in 1874, and urging 
 me, moreover, to appeal to America to take part in this special work, for 
 which sJie is peculiarly fitted, because of the bravery and enterprise of her 
 seamen, the skill and ingenuity of her astronomers and physicists, and 
 her singular liberality as a nation in all scientific matters. 
 
CONTENTS, 
 
 PAGE 
 
 LIFE OF MRS. SOMERVILLE 1 
 
 THE COMING TRANSIT OF VENUS, AND BRITISH PREPARATIONS 
 
 FOR OBSERVING IT 15 
 
 THE EVER- WIDENING WORLD OF STARS ..... 40 
 
 MOVEMENTS IN THE STAR-DEPTHS .55 
 
 THE GREAT NEBULA IN ORION 78 
 
 THE SUN S TRUE ATMOSPHERE ....... 95 
 
 SOMETHING WRONG WITH THE SUN . . . . . .118 
 
 NEWS FROM HERSCHEL S PLANET 122 
 
 THE Two COMETS OF THE YEAR 1 868 : 
 
 PART I. BRORSEN S COMET 147 
 
 PART II. WINNECKE S COMET .163 
 
 COMETS OF SHORT PERIOD . . . . . . .181 
 
 THE GULF STREAM 195 
 
 OCEANIC CIRCULATION . . . .. . . . .212 
 
 ADDENDUM IN REPLY TO DR. CARPENTER ..... 245 
 
 THE CLIMATE OF GREAT BRITAIN 260 
 
 THE Low BAROMETER OF THE ANTARCTIC TEMPERATE ZONE , 285 
 
LIST OF ILLUSTRATIONS. 
 
 PAGE 
 
 CHART OF THE NORTH ATLANTIC ON AN EQUAL- SURFACE PRO 
 JECTION, SHOWING THE GrULF STREAM, &C 217 
 
 CHART OF THE NORTHERN HEMISPHERE, SHOWING THE CURVES 
 OF EQUAL MEAN ANNUAL TEMPERATURE AND EQUAL MID 
 WINTER TEMPERATURE FOR LONDON 264 
 
 THE SAME, SHOWING THK CUP.VES OF EQUAL MEAN ANNUAL 
 TEMPERATURE AND EQUAL MID-SUMMER TEMPERATURE FOR 
 LONDON ...... 265 
 
 DIAGRAM SHOWING THE ANNUAL VARIATION OF MEAN DIURNAL 
 TEMPERATURE AT GREENWICH . 277 
 
 DIAGRAM SHOWING THE BAROMETRIC PRESSURE OVER SOUTHERN 
 HEMISPHERE 287 
 
 DIAGRAM SHOWING THE BAROMETRIC PRESSURE OVER NORTHERN 
 HEMISPHERE . ib. 
 
 FIGURE ILLUSTRATING A THEORY IN EXPLANATION OF THE Low 
 BAROMETER OF THE ANTARCTIC ZONE ..... 305 
 
LIGHT SCIENCE 
 FOR LEISURE HOURS. 
 
 SECOND SERIES. 
 
 MRS, SOMERVILLE. 
 
 MARY SOMERVILLE (nee FAIRFAX) was born at Jedburgh 
 on December 26, 1780, and died on November 30, 1872, 
 at Naples, aged nearly ninety-two years. In consider 
 ing her education, we have not to mention important 
 seminaries, where skilled teachers make it their chief 
 business to impart to others the knowledge for which 
 they are themselves eminent, but to speak only of 
 studies pursued in the calm of a quiet home. This, 
 rightly understood, is perhaps the most remarkable 
 feature of her career. There are few mathematicians 
 so eminent as she deservedly was, in whose fame great 
 public schools and universities do not in some degree 
 partake. But we owe almost to accident the discovery 
 of the powers of Mary Fairfax s mind, while the gradual 
 development of those powers proceeded under the 
 guidance of tutors unknown to fame, and with access 
 only to such assistance as could be given by the friends 
 of her own family. 
 
 B 
 
2 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 Mrs. Somerville has herself described how it chanced 
 that the peculiar powers of her mind came first to be 
 recognised. She was in the habit of working at her 
 needle in the window-seat, while her brother took his 
 lessons in geometry and arithmetic. Fortunately (in 
 her case) the work which is regarded as most suitable 
 to the capacity of women leaves the mind unoccupied ; 
 and consequently there was nothing to prevent Mary 
 Fairfax from attending to the lessons intended for her 
 brother. She gradually became interested in the subject 
 of these lessons, and took care not only to be present 
 regularly, but to study her brother s books in her own 
 room. It happened that, on one occasion, young Fair 
 fax failed to answer a question addressed to him, and 
 his sister involuntarily prompted him. The tutor was 
 naturally surprised that the quiet Mary Fairfax should 
 have any ideas beyond the needlework which had ap 
 parently engaged her attention ; but, being a sensible 
 man, he was at the pains to ascertain the degree arid 
 soundness of her knowledge, and, finding that she had 
 really grasped the first principles of mathematics, he 
 took care that she should have liberty to go on in 
 her own way. If a boy had shown similar fitness for 
 mathematical research, anxious attention would have 
 been devoted to the choice of books and teachers, 
 school and university ; but the case of a girl showing 
 such tastes seemed to be adequately met by according 
 to her the privilege of following her own devices. We 
 shall never know certainly, though it may be that 
 hereafter we shall be able to guess, what science lost 
 through the all but utter neglect of the unusual powers 
 
MRS. SOMERVILLE. 3 
 
 of Mary Fairfax s mind. We may rejoice that, through 
 an accident, she was permitted to reach the position 
 she actually attained ; but there is scarcely a line of 
 her writings which does not, while showing what she 
 was, suggest thoughts of what she might have been. 
 
 While studying mathematics in her own way, she 
 found a difficulty which for a time threatened to inter 
 fere with her progress. She was unable to read the 
 Principia, because she could not understand Latin. 
 In this strait, she applied, after much hesitation, to 
 Prof. Playfair. She asked if a woman might, without 
 impropriety, learn Latin. After ascertaining the pur 
 pose which the young lady had in view possibly in 
 doubt lest she might follow in the steps of Anne Dacier 
 Prof. Playfair told her that it would not, in his 
 opinion, do her any harm to learn Latin in order to 
 read the Principia. It is noteworthy, as having pro 
 bably a bearing on the course which Mrs. Somerville s 
 reading subsequently took, that Playfair was one of 
 the few in this country who at that time appreciated 
 the methods of the higher mathematical analysis, and 
 had formed a just opinion of their power a power, 
 however, as Sir John Herschel well remarks, which 
 he was content to admire and applaud rather than 
 ready to wield. His excellent review of the Mecanique 
 Celeste probably gave (as Herschel suggests) a stronger 
 impulse to the public mind in the direction of the 
 higher analysis than he could have communicated by 
 any researches of his own. 
 
 It was not, however, as a mathematician that Mrs. 
 
 B 2 
 
4 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 Somerville first became known to the world. A subject 
 of research, exceedingly difficult and only to be pursued 
 successfully under very favourable conditions, was 
 undertaken by her during the life of her first husband, 
 Captain Greig, son of High-Admiral Greig of the Eus- 
 sian Navy. She sought to determine by experiment 
 the magnetising influence of the violet rays of the 
 solar spectrum. It is not surprising, says Sir John 
 Herschel on this subject, that the feeble though 
 unequivocal indications of magnetism which she 
 undoubtedly obtained should have been regarded by 
 many as insufficient to decide the question at issue. 
 Nevertheless it was justly regarded as a noteworthy 
 achievement that, in a climate so unsuitable as ours, 
 any success should have been attained in a research of 
 such extreme difficulty. That she achieved, and, what 
 is more, deserved success, will be inferred from the 
 words in which Sir John Herschel indicates his own 
 opinion 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 the 
 simple and rational manner in which her experiments 
 were conducted, the absence of needless complication 
 and refinement in their plan, and of unnecessary or 
 costly apparatus in their execution, and the perfect 
 freedom from all pretension or affected embarrassment 
 in their statement. 
 
 In 1832 Mrs. Somerville published the work on 
 which, in our opinion, her fame in future years will be 
 held mainly to depend. The Mechanism of the Heavens 
 
MRS. SOMERVILLE. 5 
 
 was originally intended to form one of the works 
 published by the Society for the Diffusion of Useful 
 Knowledge, though it soon outgrew the dimensions 
 suited for such a purpose. Indeed, it is remarkable 
 that either Mrs. Somerville herself or Lord Brougham, 
 at whose suggestion the work was undertaken, should 
 suppose it possible to epitomise Laplace s magnum 
 opus, or so to popularise it as to bring it within the 
 scope of the Society s publications. 
 
 It will be well, in weighing the value of the book, 
 to consider it first with reference to the purpose of its 
 author, though a judgment based on that consideration 
 alone would not be a fair one. These, then, are the 
 words in which Mrs. Somerville presents the scope and 
 purpose of her work : 
 
 A complete acquaintance with physical astronomy 
 can only be attained by those who are well versed in 
 the highest branches of mathematical and mechanical 
 science : such alone can appreciate the extreme beauty 
 of the results, and the means by which these results 
 are obtained. Nevertheless, a sufficient skill in ana 
 lysis to follow the general outline, to see the mutual 
 dependence of the several parts of the system, and to 
 comprehend by what means some of the most extraor 
 dinary conclusions have been arrived at, is within the 
 reach of many who shrink from the task, appalled by 
 difficulties which perhaps are not more formidable than 
 those incident to the study of the elements of every 
 branch of knowledge, and possibly overrating them by 
 not making a sufficient distinction between the degree 
 
6 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 of mathematical acquirement necessary for making 
 discoveries and that which is requisite for understand 
 ing what others have done. That the study of mathe 
 matics, and their application to astronomy, are full of 
 interest, will be allowed by all who have devoted their 
 time and attention to these pursuits ; and they only 
 can estimate the delight of arriving at truth, whether 
 it be the discovery of a world or of a new property of 
 numbers. 
 
 It cannot be doubted that Mrs. Somerville here indi 
 cates her belief in the possibility of presenting her 
 subject in a form suited to the capacities of a large 
 number of readers, and to some extent advocates this 
 as her object. Whether she succeeded or failed in this 
 purpose must therefore be the first question to engage 
 our attention. Sir John Herschel considers that she 
 succeeded, < for all those parts of her subject, at least, 
 which the work < professes to embrace, that is to say, 
 the general exposition of the mechanical principles 
 employed, the planetary and lunar theories, and those 
 of Jupiter s satellites, with the incidental points natu 
 rally arising out of them. With the utmost respect for 
 the authority of one who was so thorough a master of 
 the subject which Mary Somerville endeavoured to 
 popularise, I venture to express a different opinion. 
 
 find it impossible to come to any other conclusion 
 than that, as respects the main purpose of her work, 
 Mrs. Somerville failed entirely; though I hasten to 
 qualify this statement by the remark that, in my 
 opinion, success was altogether impossible. I believe, 
 
MRS. SOMERVILLE. 7 
 
 in fact, that neither Mrs. Somerville nor Sir John 
 Herschel thoroughly apprehended the difficulty of con 
 veying to the general reader clear ideas respecting even 
 the elements of the subjects they severally endeavoured 
 to expound. But I feel bound to add that Mis. 
 Somerville s failure, inevitable from the very nature of 
 her task, would in any case have been brought about 
 by the manner in which the task was accomplished. 
 It will presently be seen that, in saying this, I am, in 
 fact, touching on the most remarkable and distin 
 guishing quality of Mrs. Somerville s mind. 
 
 There are two essential requisites in a treatise in 
 tended to introduce a difficult subject to general readers. 
 First, there must be a clear apprehension of the position 
 of such readers, of what they can and of what they 
 cannot understand, and of the form in which what is 
 written for them may most usefully be presented. It 
 is not too much to say that if just ideas had been en 
 tertained by Mrs. Somerville on this point, the attempt 
 to present the Mechanism of the Heavens in a popular 
 form would never have been made. But, secondly, it is 
 essential that in any work of the kind each statement 
 each sentence, in fact should be presented in terms 
 so precise as to be absolutely unmistakable. This is 
 not so necessary in advanced treatises indeed, it is 
 too well known how large a proportion of our works on 
 advanced science are wanting in strict precision of 
 expression. But it is absolutely necessary in works 
 intended to popularise science. It is a somewhat 
 remarkable circumstance that in the Mechanism of the 
 
8 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 Heavens the boldest attempt ever made, perhaps, in 
 this direction not only is precision of expression not 
 a notable feature, but, on the contrary, the most strik 
 ing fault in the work is the inexactness of the language. 
 Even Sir John Herschel, whose perfect familiarity with 
 the subject of the work would tend to render the fault 
 less obvious to him, was nevertheless struck by it : 
 The most considerable fault we have to find, he 
 wrote, with the work before us consists in an habitual 
 laxity of language, evidently originating in so complete 
 a familiarity with the quantities concerned as to induce 
 a 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, must 
 infallibly become a source of serious errors, and which, 
 at all events, renders it necessary for the reader to be 
 constantly on his guard. 
 
 These words form the penultimate sentence of Sir 
 John Herschel s critique. I have preferred to speak 
 first of the subject touched on, so as to pass without 
 reservation to a more pleasing topic the real and 
 unquestionable value of Mrs. Somerville s chief work. 
 And, after all, the good qualities of the work are intrin 
 sic, while its main fault relates to a purpose which the 
 work never could have fulfilled, no matter how care 
 fully the fault had been avoided. 
 
 It is in this sense regarding the work apart from 
 its special purpose, and judging of it only as a contri 
 bution to advanced scientific literature that we may 
 fairly say, with Sir John Herschel, that the work is 
 
MRS. SOMERVILLE. 9 
 
 one of which any geometer might be proud. There is, 
 indeed, ample evidence of the disadvantage under 
 which Mrs. Somerville laboured, in the want of tho 
 rough mathematical training ; but so much the more 
 wonderful is it that she should have completely mas 
 tered her subject. Every page indicates her apprecia 
 tion of the methods employed by Laplace and Lagrange. 
 Where she does not strictly follow the Mecanique 
 Celeste, she evidences a clear recognition of the pur 
 poses to be subserved by adopting a different course. 
 I would not be understood as commending all the 
 departures thus made ;-on the contrary, there are cases 
 where it appears to me that on the whole it would have 
 been preferable to have followed the processes of the 
 Mecanique Celeste more closely, while there are others 
 where certain more modern processes might perhaps 
 with advantage have been introduced. But even in 
 such instances we recognise in the course pursued by 
 Mrs. Somerville the decision of one perfectly familiar 
 with the subject in hand. And many of the changes 
 must undoubtedly be regarded either as improvements, 
 or else as altogether desirable when the scale of Mrs. 
 Somerville s treatise is taken into account. Amono-st 
 
 o 
 
 instances of the former kind must be classed the method 
 employed in the investigation of the equations of con 
 tinuity of a fluid ; amongst instances of the latter, I 
 would specially cite the treatment of the theory of 
 elliptic motion, in the opening chapters of the second 
 book. 
 
 If however I were asked to point out the feature of 
 
10 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 this work which, in my opinion, most strikingly indi 
 cated the powers of Mrs Somerville s mind, I should 
 unhesitatingly select the preliminary dissertation. In 
 this we have an abstract of the Newtonian philosophy 
 such as none but a master-mind could have produced. 
 Apart from its scientific value and it has great scien 
 tific value it is a work of great literary merit. If it 
 is not in plan and purpose altogether original, inas 
 much as it must be regarded as to some degree an 
 abstract of Laplace s Systeme du Monde, it is never 
 theless, as Herschel has well remarked, an abstract so 
 vivid and judicious as to have all the merit of origin 
 ality, and such as could have been produced only by 
 one accustomed to large and general views, as well as 
 perfectly familiar with the particulars of the subject. 
 
 Three years after the appearance of the Mechanism 
 of the Heavens, Mrs. Somerville published the work by 
 which she is probably best known to general readers. 
 The Connexion of the Physical Sciences was, I believe, 
 written at the suggestion of Lord Brougham, as an 
 expansion of the admirable introduction to the Celestial 
 Mechanism. It is a work full of interest, not only to 
 the student of advanced science, but to the general 
 reader. In saying this we indicate its chief merit and 
 its most marked defect. It is impossible to conceive 
 that any reader, no matter how advanced or how limited 
 his knowledge, could fail to find many most instructive 
 pages in this work; but it is equally impossible to 
 conceive that any one reader could find the whole work, 
 or even any considerable portion, instructive or useful. 
 
MRS. SOMERVILLE. II 
 
 The fact was that Mrs. Somerville recognised, or which 
 is practically the same thing, wrote as if she recognised, 
 no distinction between the recondite and the simple. 
 She makes no more attempt at explanation when 
 speaking of the perturbations of the planets or discuss 
 ing the most profound problems of motecular physics, 
 than when she is merely running over a series of state 
 ments respecting geographical or climatic relations. 
 It would almost seem as though her mind was so con 
 stituted that the difficulties which ordinary minds ex 
 perience in considering complex mathematical problems 
 had no existence for her. A writer, to whom we owe 
 one of the best obituary notices of Mrs. Somerville 
 which hitherto have appeared, tells us that the sort of 
 pressure Mrs. Somerville underwent from her publisher 
 as the earlier editions of the Connexion of the Physical 
 Sciences passed through the press convinced her of her 
 own unfitness for popularising science. When there was 
 already no time to lose in regard to her proof sheets, she 
 had hint upon hint from Mr. Murray that this and that 
 and the other paragraph required to be made plainer to 
 popular comprehension. She declared that she tried very 
 hard to please Mr. Murray and others who made the same 
 complaint, but that every departure from scientific 
 terms and formulas appeared to her a departure from 
 clearness and simplicity ; so that, by the time she had 
 explained and described to the extent required, her 
 statements seemed to her cumbrous and confused. In 
 other words, this was not her proper work. 
 
 Respecting her two other works, I shall merely 
 
12 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 remark that the Physical Geography appeared in 1848, 
 and the Molecular and Microscopic Science in 1869, 
 when she had reached the advanced age of eighty-eight 
 years. 
 
 I may be excused for regarding Mary Soinerville s life 
 with reference rather to her astronomical and mathemati 
 cal researches than to her proficiency in other branches 
 of science. In this aspect of her career it is difficult, 
 great as was the reputation she deservedly obtained, not 
 to contemplate with regret those circumstances, the 
 effects of unfortunate prejudices, whereby she was pre 
 vented from applying the full powers of her mind to 
 the advancement of science. It is certain that no 
 department of mathematical research was beyond her 
 powers, and that in any she could have done original 
 work. In mere mental grasp few men have probably 
 surpassed her ; but the thorough training, the scholarly 
 discipline, which can alone give to the mind the power 
 of advancing beyond the point up to which it has 
 followed the guidance of others, had unfortunately been 
 denied to her. Accordingly, while her writings show 
 her power and her thorough mastery of the instruments 
 of mathematical research, they are remarkable less for 
 their actual value, though their value is great, than as 
 indicating what, under happier auspices, she might 
 have accomplished. 
 
 I have mentioned that Mrs. Somerville was twice 
 married. By her first marriage she had one son, Mr. 
 Woronzow Grreig, since .deceased. A few years after 
 Captain Greig s death she married her cousin, Dr. 
 
MRS. SOMERVILLE. 13 
 
 Somerville, by which marriage she had three daughters, 
 two of whom survive her. The latter years of her life 
 (twenty-three years, we believe) were passed in Italy. 
 It has been said by one who was well acquainted with 
 the circumstances that the long exile which occupied 
 the latter portion of her life was a weary trial to her. 
 She carried a thoroughly Scotch heart in her breast ; and 
 the true mountaineer s longing for her native country 
 sickened many an hour of many a tedious year. She 
 liked London life, too, and the equal intercourses which 
 students like herself can there enjoy ; whereas, in Italy, 
 she was out of place. She seldom met any one with 
 whom she could converse on the subjects which in 
 terested her most ; and if she studied, it could be for no 
 further end than her own gratification. It was felt by 
 her friends to be a truly pathetic incident that, of all 
 people in the world, Mrs. Somerville should be debarred 
 the sight of the singular comet of 1843; and the cir 
 cumstance was symbolical of the whole case of her 
 exile. The only Italian observatory which afforded the 
 necessary implements was in a Jesuit establishment, 
 where no woman was allowed to pass the threshold. At 
 the same hour her heart yearned towards her native 
 Scotland, and her intellect hungered for the congenial 
 intercourse of London ; and she looked up at the sky 
 with the mortifying knowledge of what was to be seen 
 there but for the impediment which barred her access 
 to the great telescope at hand. With all her gentleness 
 of temper and her lifelong habit of acquiescence, she 
 
14 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 suffered deeply, while many of her friends were indig 
 nant at the sacrifice. 
 
 I shall venture to quote, in conclusion, some remarks 
 by Sir Henry Holland on features of Mrs. Somerville s 
 character and life which have been hidden from general 
 knowledge : She was a woman not of science only, 
 he tells us, but of refined and cultivated tastes. Her 
 paintings and musical talents might well have won 
 admiration, even had there been nothing else beyond 
 them. Her classical attainments were considerable, 
 derived probably from that early part of life when the 
 gentle Mary Fairfax gentle she must ever have been 
 was enriching her mind by quiet study in her Scotch 
 home. ... A few words more on the moral part 
 of Mrs. Somerville s character ; and here, too, I speak 
 from intimate knowledge. She was the gentlest and 
 kindest of human beings qualities well attested even 
 by her features and conversation, but expressed still 
 more in all the habits of her domestic and social life. 
 Her modesty and humility were as remarkable as those 
 talents which they concealed from common observation. 
 
 Scotland, he justly adds, is proud of 
 
 having produced a Crichton. She may be proud, also, 
 in having given birthplace to Mary Somerville. 
 
 (From Monthly Notices of the Royal Astronomical Society 
 for February, 1873.) 
 
THE COMING TRANSIT OF VENUS. 15 
 
 THE COMING TRANSIT OF VENUS, AND BRITISH 
 PREPARATIONS FOR OBSERVING IT. 
 
 BY far the most important of all the phenomena which 
 astronomers are now expecting is the transit of Venus, 
 which will take place on December 8, 1874. Even 
 the eclipses of the last few years, though they have 
 attracted so much attention, and have been observed so 
 carefully,have in reality been regarded as altogether less 
 important than the next transit of Venus. Total eclipses 
 are almost every-year phenomena, but transits of Venus 
 occur only at average intervals of more than half a 
 century. The last took place in 1769, and after the 
 transit of 1882 none will occur till 2004. Apart from 
 this circumstance, a transit of Venus is of extreme 
 importance in the science of astronomy. It admittedly 
 affords the most satisfactory means of determining the 
 distance of the sun in other words, the dimensions of 
 the solar system itself. And such determination of the 
 scale on which our system is constructed affords the 
 only means we possess of measuring the vast spaces 
 which separate us from the fixed stars. So that the 
 observations which are to be made in December, 1874, 
 and renewed (but under somewhat different conditions) 
 in December, 1882, bear directly on the fundamental 
 problem of astronomy, so far as astronomy relates to 
 
1 6 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 the determination of the distances and the magnitude of 
 the celestial bodies.* 
 
 I propose here, after inquiring briefly into the 
 general question of the determination of the sun s 
 distance, to describe the nature of the opportunities 
 which will be afforded during the transit of 1874, and 
 to discuss the preparations which are being made by 
 this country to take her part in the work of observation. 
 It will be seen, as I proceed, that this discussion of the 
 
 * I venture to quote here the appeal made by Halley (when Astro 
 nomer "RojAl") forty-five years before the transit of 1761, the earlier of 
 the pair of transits then looked forward to. It will show that, in dealing 
 with a transit 21 months before the date of its occurrence, I am not 
 looking forward so inordinately as might be supposed by those unfami 
 liar with the nature of these inquiries. I should remark, however, that 
 since Halley s day other methods for determining the sun s distance have 
 been devised and employed. Six methods are described in my treatise 
 on the Sun, and a seventh has, within the last few months, been 
 suggested by the great French astronomer Leverrier. Thus, then, wrote 
 Halley in 1716 : < I could wish, indeed, that observations of the transit 
 should be undertaken by many persons in different places : first, because 
 of the greater confidence which could be placed in well-according obser 
 vations ; and, secondly, lest a single observer should, by the interven 
 tion of clouds, be deprived of that spectacle which, so far as I know, 
 will not be visible again to the men of this and the next century, and 
 on which depends the certain and sufficient solution of a most noble and 
 otherwise intractable problem. I therefore again and again urge upon 
 those inquiring observers of the celestial bodies, who, when I have de 
 parted this life, will be reserved to observe these things, that, mindful 
 of my counsel, they should devote themselves strenuously and with all 
 r energies to conduct the observation ; I desire and pray that they 
 i favoured in every way, and especially that they may not be 
 deprived of that most desirable spectacle by the inopportune darkness 
 rf a clouded sky; and that, finally, the magnitudes of the celestial 
 ies, forced into narrower limits (of exactness), may, as it were make 
 Biibmission-to the glory and eternal fame of those observers. 
 
 These hopes were not fulfilled, so far as the transit of 1761 was con 
 ed; but the transit of 1769 was observed with great care at no less 
 lan seventy-four stations, fifty of which, however, were in Europe 
 
THE COMING TRANSIT OF VENUS. 17 
 
 subject does not labour under the fault of being prema 
 ture. On the contrary, the time is now at hand when 
 a final decision must be made as to the course which 
 this country is to pursue ; and inasmuch as my purpose 
 is not solely to describe what is being done, but to 
 point out what (in my opinion) should be done, the 
 present is the proper time to speak. 
 
 A surveyor, who wishes to determine the distance of 
 an inaccessible object, measures a convenient base-line 
 and observes the direction of the object as seen from 
 either end of the line. He thus has the base and 
 the two base-angles of a triangle ; and the simplest 
 geometrical considerations teach that the other two 
 sides of the triangle can thence be determined. These 
 sides are, of course, the distances of the inaccessible 
 object from the two ends of the base-line. Now this is 
 the fundamental method employed by astronomers to 
 determine the distances of the celestial bodies. It is 
 applied directly to the moon. An observer at Green 
 wich (let us say), notes the direction of the moon when 
 at her highest, or due south ; another at Cape Town 
 (let us say), does the like ; then a line joining Green 
 wich and Cape Town is a base-line of known length, 
 and the two directions give the base-angles. The 
 triangle is a very long one, its vertical angle (that is, 
 the angle opposite the base) being one of about a degree 
 and a half, or about the angle swept out by the hand 
 of a clock or watch during a quarter of a minute ; but 
 such a triangle is quite within the methods of treat 
 ment available to astronomers. 
 
 c 
 
1 8 LIGHT SCIENCE FOR LEISURE HOURS 
 
 In applying this method to the sun, a serious diffi 
 culty comes in. He is so far off that, instead of a 
 triangle with a respectable vertical angle, there is a 
 triangle having a vertical angle of about the 240th 
 part of a degree (under the most favourable conditions 
 which can be conveniently obtained). To know how 
 small such an angle is, let the reader note the minute 
 hand of a clock or watch, and observe how little it 
 shifts around its centre in a single second of time ; yet 
 this angular shift is twenty-four times as great as that 
 we have mentioned. 
 
 It must not be forgotten that, in all such cases, the 
 question is not whether the astronomer can recognise 
 such and such an effect, but whether he can measure 
 it. It is not the whole quantity about which astrono 
 mers are troubled. Unquestionably the observer at 
 Greenwich can recognise the depression of the mid-day 
 sun, 1 due to the fact that Greenwich lies above (or 
 north of) the earth s centre. For this depression is an 
 element which he has to take into account in his obser 
 vations. The corresponding depression, even in the 
 case of bodies far more distant than the sun, as the 
 planets Jupiter and Saturn, is announced systematically 
 in our national astronomical almanac. But the direct 
 measurement of the depression is altogether out of the 
 question. 
 
 If the stars which really bestrew the heavens beyond 
 
 1 Only observations of the mid-day sun would avail, because the only 
 instruments having the requisite delicacy of adjustment are meridional. 
 There is an instrument suitable for observing the moon when she is not 
 on the meridian ; but it is quite unfit for the purpose we are considering. 
 
THE COMING TRANSIT OF VENVS. 19 
 
 the sun could be seen, the case would be different, for 
 they would serve as index points, by means of which to 
 estimate the sun s displacement. But although stars 
 not lying near the sun s place on the heavens can be 
 seen by day with powerful telescopes, those close around 
 him are quite invisible. This method failing, the 
 astronomer has to look for other means of solving the 
 problem. The planet Venus, which comes at times 
 much nearer to the earth than the sun is, and in fact 
 nearer than any celestial body except the moon, na 
 turally claims attention as a suitable object for the 
 astronomer s purpose. For it is to be remembered that 
 the proportions of the solar system have long been 
 accurately determined ; so that as soon as the distance 
 of any one planet is ascertained, the scale of the whole 
 solar system becomes also known. 
 
 Venus, however, when at her nearest, is lost in the 
 sun s light, and, though discernible in powerful tele 
 scopes, is quite unsuitably placed for the delicate obser 
 vations which would alone avail to determine her 
 distance. 
 
 This brings us at once to the recognition of the 
 importance of a transit of Venus. When Venus passes 
 between the sun and earth, in such a way as not to 
 cross the sun s face, that is, when she passes above 
 or below the long and almost linear portion of space 
 lying actually between the earth and sun, she cannot 
 be well observed ; but when, in making the passage, 
 she comes so close to the line joining the earth and sun 
 as actually to be seen on the sun s face, she can be 
 
 c 2 
 
20 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 observed to great advantage. For she is then seen as 
 a round black spot on the sun s face ; this face is thus 
 as a sort of dial-plate on which the black disc of Venus is 
 as an index. The sharply-defined edge of this black 
 disc presents the same advantage which a neatly-cut 
 index possesses, enabling the observer to measure satis 
 factorily the place of the planet. All the circumstances 
 are favourable, except two: first, the index, that 
 is, the black disc, is not even for an instant at rest ; 
 and, secondly, the index-plate, that is, the sun s disc, 
 is itself displaced by any difference in the position of 
 the terrestrial observers. 
 
 Nothing can be done to remedy the latter circum 
 stance. Its effects are easily seen. Suppose an observer 
 at some northern station sees Venus in reality depressed 
 by a third of a minute of arc, which is about the hun 
 dredth part of the sun s apparent diameter. Then the 
 sun, being farther away in the proportion of about 
 ten to three, is depressed by about the tenth of a 
 minute. Accordingly, Venus only seems to be depressed 
 by the difference of these amounts, or by little more 
 than a quarter of a minute. Nevertheless it is far 
 easier to measure this reduced displacement on the 
 sun s face, than to measure the larger displacement 
 without his face as an index-plate. 
 
 The other circumstance has been dealt with in two 
 ways. 
 
 First, in accordance with a suggestion of Halley s, 
 instead of attempting to measure the position of Venus 
 on the sun s face, the astronomer may simply time her 
 
THE COMING TRANSIT OF VENUS. 21 
 
 as she crosses that face, and so judge how long the 
 chord is which she has traversed. This shows how 
 nearly the chord approaches the sun s centre, and thus 
 gives a determination as satisfactory as an actual mea 
 surement. Of course, there are many details to be 
 taken into account : for instance, the apparent path of 
 Venus is not a straight line in reality, because the 
 observer s station is not at rest, but carried round the 
 axis of the rotating earth. But the mathematician 
 finds no difficulty in taking such considerations fully 
 into account. 
 
 Secondly, Delisle proposed that astronomers should 
 note the actual moment (of absolute, not local time) 
 when Venus seems to enter or leave the sun s face, as 
 seen from different stations on the earth. It will be 
 manifest, on a moment s consideration of the actual 
 circumstances of the case, that the transit will not 
 seem to begin or end at the same instant, as seen from 
 different parts of the earth. There is the great globe 
 of the sun at one side, and the smaller globe of the 
 earth on the other ; and Venus passes between. Now, 
 in order to show more clearly what must happen, let 
 us take an illustrative case drawn from an event which 
 in a few weeks from the present time will interest a 
 large proportion of our population. Suppose that on 
 one side of the river Thames there is a long building 
 whose extremeties* we call A and B. Suppose that just 
 opposite there is a barge whose corresponding extreme- 
 ties we call a and b. Now suppose the winning boat to 
 be coming along so as to pass between the house and 
 
22 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 the barge (coming first between the ends A, a). And 
 for simplicity of description let us confine our remarks 
 to the little flag carried at the bow of the boat. It is 
 manifest that an observer at a will see the little flag 
 cross his line of vision towards A before an observer at 
 1) sees the like. And the observer at a will in like 
 manner see the light blue flag (I beg pardon, I should 
 say the blue flag simply) crossing his line of vision 
 towards B before an observer at b sees the like. The 
 flag will traverse the range A B as seen both from a 
 and from b, but both its ingress on this range and its 
 egress from it will be earlier as seen from a than as 
 seen from b. Now our earth may be compared to the 
 barge; the sun to the building A B.; and Venus to the 
 boat. There is one spot on the earth at which Venus 
 will seem to enter earliest on the sun s face, and an 
 other spot (on the opposite side, just as b is farthest 
 away from a) where Venus will seem to enter latest ; 
 and in like manner there is one spot at which Venus 
 will seem to leave the sun s face earliest, and another 
 (on the opposite side) at which Venus will seem to leave 
 the sun s face latest. 
 
 And as our illustrative case explains the nature of 
 Delisle s method, so also it illustrates the rationale of the 
 method. Of course, the two cases are not exactly similar ; 
 but they are sufficiently so to make the illustration in 
 structive. Suppose that the length of the barge a b is 
 known (as the dimensions of the earth are known) ; 
 thus, say that it is 24 yards in length. Now suppose 
 that the course of the boat is known to be in mid- 
 
THE COMING TRANSIT OF VENUS. 23 
 
 stream, or exactly midway between the house and the 
 barge. Then a moment s consideration will show that 
 the boat traverses 12 yards between the moments when 
 the spectators at a and b severally see it towards A. 
 Now suppose that the observer at a indicates by a call 
 or other signal the moment when the flag is thus seen 
 by him, and that the observer at 6, provided with a 
 stop-watch, notes that two seconds elapse before he sees 
 the flag towards A. This, then, is the time occupied 
 by the boat in traversing 1 2 yards ; so that she is mov 
 ing at the rate of six yards per second. Similar remarks 
 apply to the apparent transit of the flag past B as seen 
 from a and 6. * In like manner, the astronomer can 
 gather from observations by Delisle s method the rate 
 at which Venus is moving in her orbit, that is, the 
 exact number of miles over which she moves per minute. 
 So that, since he knows exactly how long she is in com 
 pleting the circuit of her orbit, he learns, in fact, the 
 exact circumference of her orbit in miles, whence its 
 radius (or her distance from the sun) follows at once. 
 
 It is manifest that Delisle s method can be applied 
 with equal advantage either to the ingress or to the 
 egress of Venus. The comparison of two observations 
 in one of which her ingress happens as early as possible, 
 while in the other it happens as late as possible is 
 quite sufficient to determine the sun s distance. So also 
 the comparison of two observations of egress (most 
 accelerated and most retarded) is separately sufficient 
 to determine the sun s distance. This is an important 
 advantage of the method. Because while, as in Halley s 
 
24 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 method, two stations are absolutely necessary, there is 
 but a single observation to be made at each, whereas in 
 Halley s the beginning and end of the transit must be 
 observed at both stations. This introduces a double 
 difficulty. For first, there is the necessity for a longer 
 continuance of clear sky, since the transit may last 
 several hours ; and, secondly, there is the difficulty of 
 securing a station where the sun is well placed on the 
 sky, both at the beginning and end of the transit. It 
 will not suffice, in applying Halley s method, to have 
 the sun well above the horizon at the moment of ingress 
 if he is low down at the moment of egress, or to have 
 the sun high at egress if he is low at ingress. Accord 
 ingly, the condition has to be secured that at stations 
 where the day is short (that is, in December, at north 
 erly stations) the middle of the transit shall occur 
 nearly at mid-day. This limits the choice for northern 
 stations considerably. 
 
 On the other hand, Delisle s method has this disad 
 vantage, that the exact moment at which ingress or 
 egress occurs must be known. A mistake, even of a 
 second or two, would be of serious moment. So that 
 the clocks made use of at each station where this 
 method is applied, must not only have good rates, but 
 must show absolutely true time at the moment of the 
 observed phenomenon. Moreover, the latitude and 
 longitude of the place of observation must be known, 
 the latter (the only difficult point) with especial 
 accuracy, since on its determination depends the change 
 of local time into (say) Greenwich time; and this 
 
THE COMING TEANSIT OF VENUS. 25 
 
 change must be accurately effected before two observa 
 tions made in different longitudes can be compared as 
 respects the absolute time of their occurrence. On the 
 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 on the duration 
 of the transit as seen at different stations. A clock 
 must be badly rated indeed utterly unfit, in fact, for 
 any astronomical use whatever which should lose a 
 single second in four or five hours. 
 
 But the most important point to be noticed is, that 
 both methods ought to be employed, if possible, apart 
 from all nice considerations of their relative value. It is 
 certain that astronomers will place much more confidence 
 in closely concordant results obtained by the application 
 of these two methods, differing wholly as they do in 
 principle, than in as many and equally concordant 
 results all obtained by one method. A third method is 
 indeed to be applied, viz., a method based on the 
 ingenious use of photography. But as yet too little is 
 known respecting the chances of success by this method 
 to warrant too implicit reliance upon it. 
 
 Let us inquire what preparations are being made by 
 astronomers, and especially by the astronomers of Eng 
 land, to make adequate use of the opportunities pre 
 sented by the coming transit. 
 
 It has first, unfortunately, to be noted, that, so far 
 as this country is concerned, no provision whatever has 
 been hitherto made for the employment of Halley s 
 
26 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 method. If this resulted from the simple preference of 
 Delisle s method, there would be little to say. Most 
 assuredly, speaking for myself, I should be very loth to 
 urge the advantages of Halley s method, if I found 
 against such a view the practical experience of those 
 astronomers who are continually testing the value of 
 various methods of observation. But the rejection of 
 Halley s method for the transit of 1874 was not origin 
 ally, and is not now, based on any objection to the 
 principle of the method, but on certain mathematical 
 considerations, which appeared to prove that the method 
 could not be advantageously applied in 1874, while it 
 could be applied successfully in 1882. It was accord 
 ingly reserved for the latter transit, and all the stations 
 for observing the transit of 1874 were selected with 
 special reference to the method of Delisle. 
 
 Now it happened that early in 1869 I was attracted 
 to the examination of the subject of the coming tran 
 sits, by the circumstance that the investigation applied 
 to the matter by the Astronomer Eoyal had struck me 
 as imperfect in method. I was interested, viewing the 
 matter merely as a mathematical problem, to inquire 
 what corrections might occur if all the niceties of re 
 search of which the question admitted were applied 
 throughout the investigation. Working with this sole 
 object in view, I analysed the whole matter in two 
 independent ways, viz., first as a problem of calculation, 
 and secondly as a geometrical problem. The results, 
 perfectly concordant, differed so remarkably from those 
 published by the Astronomer Royal, that I was con- 
 
THE COMING TRANSIT OF VENUS. 27 
 
 strained (in mere fealty to the cause of science) to sub 
 mit them to the examination of the scientific world. 
 
 To begin with : Halley s method, of which, in 1857, 
 and again in 1864, and yet again in 1868, the Astro 
 nomer Eoyal had said that it is totally inapplicable in 
 1874, was found to be applicable under circumstances 
 altogether more favourable than those which will exist 
 in 1882. 1 It was found not only to be applicable with 
 
 1 The origin of this mistake on the Astronomer Eoyal s part is 
 thus explained in an article in the Spectator for March 1, 1873: 
 Everyone is asking whether it is possible that an astronomer so emi 
 nent and so skilful as Sir George Airy for the time is past when 
 names need be concealed can have made any serious mistake in a matter 
 of this importance. And again, everyone is anxious to know precisely 
 what mistake is imputed, and how it arose (granting that a mistake has 
 been made). 
 
 To this last question the reply is easy. It chanced unfortunately 
 that in 1857 the Astronomer Royal delivered a lecture on the subject of 
 the now approaching Transits. In that lecture his great mistake had 
 its origin. Intent on presenting the more striking and popular features 
 of his subject, and in a way which would be clear and convincing to 
 everyone, he was led to adopt a method of reasoning which on the face 
 of it seems convincing enough (and which, indeed, is sound in itself) ; 
 but the conclusions derived from which may be, and in the actual case 
 are, dependent on certain details into which the Astronomer Eoyal 
 neither then entered nor has ever entered since. It is the palpably con 
 vincing nature of the evidence at a first view which led to all the 
 mischief. We will endeavour to give a brief but sufficient sketch of 
 the line of argument. 
 
 Let it be premised that, for applying Halley s method or the 
 English method, as it is often called with advantage, what is wanted 
 is that at some station the transit shall last as long as possible, while 
 at another it shall last as short a time as possible. It matters nothing 
 whether the increase or reduction of the time be obtained by a seeming 
 change in the length of the line traversed by Venus, or by a change in 
 the rate at which she seems to move during transit. So much premised, 
 let it be noted that in 1874 Venus will cross the sun s face on a line 
 placed somewhat as a line from the figure X to the figure I on a 
 clock-face. As seen from northern stations, the line of transit will be 
 
28 LIGHT SCIENCE FOR LEISURE HOURS. 
 advantage, but even more advantageously than De- 
 
 lowered, and therefore manifestly will be lengthened. From southern 
 stations, the line will be raised, and therefore shortened. We therefore 
 set an observer at as northerly a station as we can, to get as great a 
 lengthening as we can, and that is one point gained. We set an observer 
 at as southerly a station as we can, and so get as great a shortening as 
 possible, and that is a second point gained. But it is easily shown (we 
 do not trouble our readers with the proof) that our northerly observer 
 is so shifted by the earth s rotation while the transit is in progress that 
 Venus is seemingly hastened on her course in transit. This shortens 
 the time of transit at the northern station, and is discordant with the 
 lengthening obtained by setting an observer as far north as possible. 
 Here, then, is one point against us. Lastly, the southern station can 
 be taken so as to give either a hastening or a retarding of Venus s 
 motion, simply because the transit occurs in the southern summer, when 
 places far south have no night, so that we can set the observer either 
 where he will have the sun moving from east to west during the transit, 
 or where he will have the sun moving from west to east. We set our 
 observer so that Venus is hastened (which is secured by taking a station 
 where, during the transit, the sun moves from east to west). This 
 hastening is manifestly accordant with the shortening of her path at 
 southern stations, and thus we get a third point in our favour. We 
 have, then, three points in our favour and one against us, or * balance 
 of only two favourable points. 
 
 Now, in 1882, Venus crosses the lower part of the sun s face, or some 
 what as from figure VII to figure IV on a clock-face. In this case, the 
 northern station gives the lowest or shortest course, while the southern 
 gives the highest or longest course. As before, we get two points in our 
 favour by setting an observer far to the north and another far to the 
 south. As before, the northern observer sees Venus hastened on her 
 course ; but now this is a favourable point, since it manifestly accords 
 with the shortening of the northern line of transit. This makes point 
 three in our favour. And again, as before, we can set our southern 
 observer where the motion of Venus can be hastened or retarded as we 
 please. We assign him a station where she will be retarded (which is 
 secured when the sun moves from west to east during the transit) : this 
 manifestly accords with the lengthening of her path. Thus we have 
 four favourable points in all in 1882; whereas in 1874 we can secure 
 only three or (one being unfavourable), a majority of only two favour 
 able points. 
 
 It seems manifest, then, that the transit of 1882 is twice as favour- 
 
THE COMING TRANSIT OF VENUS. 29 
 
 On this point all doubts should have been very 
 quickly removed. For, almost simultaneously with 
 
 able for applying Halley s method as the transit of 1874. So the 
 Astronomer Royal concluded. He did not enter into details, but after 
 summing up the evidence much as we have done above, he said " the 
 observable difference of duration in 187 4 will probably tiotbe half of that 
 in 1882." It was in 1857 that he thus spoke; and he has never said a 
 word or written a line since that time implying that he had gone into 
 the details of the matter. When he next touched on the subject (in 1864) 
 he referred to the lecture of 1857 as showing the suitability of Halley s 
 method in 1882, and he left the transit of 1874 wholly unnoticed. 
 Again, in December 1868, he touched on the matter, simply saying that 
 Halley s method fails totally in 1874. That fatal lecture, or rather the 
 error suggested in the process of popularising the subjectforthatoccasion, 
 led to so established a conviction as to the uselessness of Halley s 
 method in 1874, that it had never seemed worth while to re-examine the 
 matter. But now let us consider details a little, and see how the matter 
 will then appear. 
 
 In the first place, the transit of 1882 at once loses its apparent 
 superiority. The southern observer must have the sun moving from west 
 to east during the transit, or in other words, he must have the sun on 
 the night side (so to speak) of the sky. There is, of course, no night 
 near the Antarctic Pole on December 6, but at nominal midnight the 
 sun is at its lowest ; and the sun must be towards this part of his diurnal 
 course, if the observer is to get the advantage we are considering. There 
 is no known Antarctic station where this can be, the sun being also 
 fairly high at the beginning and end of the transit. This at once dis 
 poses of the superiority of the transit of 1882. If an Antarctic station 
 is sought at all, there will be a hastening instead of a retarding of the 
 planet s transit, or an unfavourable point, as in the case of the earlier 
 transit. In reality, the loss thus accruing is found to be much more 
 serious in 1882 than the corresponding loss in 1874, when we inquire 
 into actual details. 
 
 But in 1874, as we have seen, there must be an unfavourable hastening 
 of Venus s motion as seen from a northern station ; and this hastening 
 seems to cancel the effect due to the lengthened transit-path. When we 
 inquire, however, to what extent this cancelling takes place, we at once 
 see that the Astronomer Royal was frightened away from Halley s 
 method without sufficient reason. He manifestly (see the italicised re 
 mark quoted above) supposed that the duration would scarcely be in 
 creased at all at the northern station. Let us see, however, whether 
 Mr. Proctor has been right or not in saying that the duration is con- 
 
30 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 the announcement of my result, the news arrived that 
 the French astronomer, Puiseux, had obtained almost 
 
 siderably increased at a suitable northern station, notwithstanding the 
 undoubted partial cancelling which takes place from the cause indicated. 
 Not to favour one side or the other, we go direct to the Nautical Alma 
 nack for 1874. We take Nertchinsk, the place pointed to by Mr. Proc 
 tor so far back as March 1869 ; and we note that he then assigned to 
 this station a lengthening of the duration of transit by 15^ minutes (a 
 very considerable amount, much more in fact than at the most favourable 
 station in 1882). Now, what says the Nautical Almanack for 1874 ? At 
 page 434, it states that the mean duration of transit is 3 hours 42 min. 2 
 sec. At page 20 of the appendix, it states that at Nertchinsk the duration 
 is 3 hours 57 min. 6 sec., exceeding the former duration by 15 min. 
 4 sec. This is very close indeed to Mr. Proctor s result, and shows 
 how nearly the values obtained by his graphic constructions accord 
 with those deduced by rigid calculation. (Moreover, a part even of the 
 slight difference is due to a difference in the adopted value of Venus s 
 diameter.) Here, then, instead of that complete cancelling of the value 
 of the northern station which Sir G. Airy too hastily assumed, we have 
 a lengthening of the transit period by more than 15 minutes, which in 
 this problt-m is an unusually large amount. To show that this is so, 
 and how slightly the northern station is affected by the peculiarity which 
 Sir G. Airy had hastily regarded as introducing a fatal objection, we 
 have only to remark that at Possession Island, the most favourable 
 southern station (where the two conditions conspire, instead of opposing 
 each other], the shortening of the transit amounts only to 17| minutes. 
 Combining this shortening with the lengthening at Nertchinsk, we have 
 a difference of duration of fully 32J minutes. And now observe how 
 greatly this result differs from Sir G. Airy s anticipation ! He thought 
 the difference of duration would probably " not be half of that in 1882 " ; 
 but his own estimate of the greatest difference of duration in 1882 (to 
 be obtained only by seeking an inaccessible station, where the sun will 
 be but four degrees high at egress) amounts only to 28 minutes. In 
 stead of being less than half, the difference of duration in 1874 is 
 greater in the proportion of about 7 to 6. Add to this that in 1874 the 
 solar elevation, both at ingress and egress will exceed twenty degrees, 
 and the importance of having a station at Possession Island becomes 
 manifest. Eussia has occupied Nertchinsk, and it is Great Britain s 
 duty (and that of no other country) to occupy Possession Island. If 
 she shrinks from this duty, it will be no answer to the reproach which 
 she wdl hereafter incur, that she occupied stations in other respects ad 
 vantageous. Other countries are occupying these stations, the Papelotte 
 
THE COMING TRANSIT OF VENUS. ^l 
 
 *J 
 
 exactly the same conclusion. The sole difference be 
 tween his result and mine was, that he simply an 
 nounced that Halley s method was advantageously 
 applicable, whereas I showed that it was more advan 
 tageously applicable than Delisle s. Even this differ- 
 ance, however, is readily accounted for, since, in 
 Puiseux s investigation, several of the niceties to which 
 I had attended were neglected as unimportant. 1 
 
 To show how completely the application of Halley s 
 method has been neglected in the choice of stations 
 for English observing parties, let the following con 
 siderations be noticed : 
 
 At northern stations Venus will be seen lower down 
 that at southern stations, so that as she transits the 
 upper part of the sun s disc, her chord of transit is 
 necessarily longer at northern than at southern stations. 
 Now Eussia occupies the best northern stations, as is 
 her due, since they fall in Russian territory. At 
 Nertchinsk, near Lake Eaikal, Russia will have an 
 observing party ; and here the transit will last longer 
 than as supposed to be seen from the earth s centre, by 
 fully 15 J minutes. For at this place the transit will 
 begin nearly 6 minutes early , and end nearly 10 
 minutes late. Now, if we had only a southern station 
 
 and La Haye Sainteof the scientific "Waterloo ; this country s duty calls 
 her to a post so important and so difficult of tenure, that it may fairly 
 be described as the Hougoumount of the position. 
 
 1 For example, Puiseux left out of consideration the dimensions of 
 Venus s disc, regarding her transit as that of her centre. He omitted 
 also, as unimportant, the fact that mean time and apparent time are not 
 coincident on December 8. The correction due to this cause is consi 
 derable. 
 
32 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 where the transit began several minutes late, and ended 
 several minutes early, we should have a transit lasting 
 for a shorter time than as seen from the earth s centre : 
 and then, comparing what was observed at such a 
 station with what was observed at Nertchinsk, we should 
 have Halley s method applied under effective and 
 favourable conditions. But the southern stations to 
 which England sends observing parties are Rodriguez and 
 Christ Church (New Zealand) ; l and at the former station 
 the transit begins late and ends late, while at the latter it 
 begins early and ends early ; so that at neither is there 
 the combination of a late beginning and an early end 
 ing, required for the effective application of Halley s 
 method. 
 
 Now there is a station a station which this country 
 ought unquestionably to occupy where the transit 
 would be even more shortened than it is lengthened at 
 Nertchinsk. This station is an Antarctic island on 
 which Sir James Eoss landed a party in 1846, and to 
 which he gave the name of Possession Island. It lies 
 due south of the southernmost extremity of New Zea 
 land, close by the rugged shore-line of Victoria Land, 
 and within 18 degrees of the south pole. At this 
 station the transit will begin 6 minutes late and end 
 1 1J minutes early, or be shortened altogether no less 
 than 171 minutes. Adding to this the lengthening of 
 the transit by 151 minutes at Nertchinsk, we obtain a 
 
 There has been a change as to the station selected in New Zealand, 
 from Auckland to Christ Church. The change is in accordance with 
 my own suggestions, so far as the application of Delisle s method is 
 concerned. 
 
THE COMING TRANSIT OF VENUS. 33 
 
 difference of duration of fully 33 minutes. Nothing 
 like this difference was available in the transit of 
 1769; nothing like it will be available in 1882. I 
 do not know the circumstances of the transits of 
 2004 and 2012, but it is altogether unlikely that the 
 opportunity of applying Halley s method will be so 
 favourable during either of these transits as in 1874. Be 
 that as it may, however, it is absolutely certain that 
 no opportunity equal to that which will be afforded 
 during the transit of 1874 will recur for one hundred 
 and thirty-two years, nor has such an opportunity been 
 ever before offered to astronomers. Absolutely the 
 best opportunity of applying Halley s ingenious method 
 which has ever been afforded, or will be afforded for 
 more than a century and a quarter, is available to astro 
 nomers during the approaching transit. The duty of 
 seizing this opportunity belongs assuredly to our 
 country, which alone has colonial possessions close to 
 the station in question, and which alone also has sea 
 men still living who have actually set their foot on 
 Possession Island. 
 
 I must confess that when, four years ago, I indicated 
 this opportunity, I thought that it would have been 
 seized at once. I thought that reconnoitring expedi 
 tions would quickly have been prepared, and that by 
 the present time complete arrangements would have 
 been made for landing an observing party on Posses 
 sion Island in due season for the required observations. 
 It would have been a matter of complete indifference 
 to me whether this had been done with or without 
 
 D 
 
34 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 acknowledgment of the source whence the suggestion 
 had come. But assuredly I hoped that some steps 
 would have been taken without delay to seize an 
 opportunity so important, the loss of which could not 
 but reflect some degree of discredit upon the science 
 of this country. 
 
 For up to that very time the spring of 1869 the 
 importance of an Antarctic expedition for observing 
 the transit of 1882 by Halley s method had been in 
 sisted upon over and over again by leading astrono 
 mical and geographical authorities. Nay, this very 
 station, Possession Island, had been selected as the 
 most suitable. The feasibility of reaching it and land 
 ing on it had been insisted upon. The superior 
 meteorological chances presented by the station, as 
 compared with other southern stations, had been dwelt 
 on strongly. Everything promised that before long an 
 Antarctic reconnoitring expedition would set forth to 
 prepare the way. It was in the full height of these 
 anticipatory inquiries that I pointed out the inex 
 pediency of any attempts to apply Halley s method at 
 an Antarctic station in 1882, dwelling earnestly on the 
 fact that when the transit began at Possession Island, 
 in 1882, the sun would be barely five degrees a,bove 
 the horizon, an elevation utterly unfit for exact obser 
 vations. Upon this all the plans for an Antarctic 
 expedition in 1882 were abandoned. But although 
 this was as it should be (for the lives of our seamen 
 are not to be endangered without the prospect of 
 valuable results), there was no necessity for abandon- 
 
THE COMING TRANSIT OF VENUS. 35 
 
 ing all idea of an Antarctic expedition. The schemes 
 set afoot for observing the transit of 1882 should 
 have been transferred to the transit of 1874. Not a 
 single argument which had been argued in their favour 
 was wanting in the case of the latter transit. The 
 main argument was greatly strengthened ; for the 
 difference of duration in 1882 would only be twenty- 
 four minutes, if Possession Island were the selected 
 station; whereas we have seen that in 1874, the corre 
 sponding difference will be fully thirty-three minutes. 
 And the fatal objection to Possession Island as a sta 
 tion in 1882, has no existence in the case of the transit 
 of 1874. Instead of the utterly insufficient solar ele 
 vation of five degrees just mentioned, there will be, in 
 1874, a solar elevation of thirty-eight and a half 
 degrees when the transit begins, and of twenty-five 
 degrees when the transit ends. And necessarily all 
 the considerations which had been urged as to the 
 importance of Antarctic expeditions, per se, and 
 especially of the interest which would attach to the 
 experiences of a wintering party near the south pole of 
 the. earth, remain unchanged. 
 
 While there is still a possibility of retrieving matters, 
 I would earnestly appeal to all who can assist in bring 
 ing about such a result to spare no pains in the endea 
 vour. I believe the scientific credit of this couutry to 
 be seriously imperilled* Hereafter, the very arguments 
 used in favour of the now abandoned scheme for ob 
 serving the transit of 1882 from Possession Island, will 
 be urged, even as now (for a better purpose) I am 
 
 2 
 
36 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 urging them, to show that the importance of such 
 observations (if feasible) had not been overlooked. It 
 has been shown, and is now admitted, that they are 
 feasible in 1874. What, then, I ask, will be thought 
 of this country if the task which is her duty shall be 
 neglected? It was sufficiently unfortunate that the 
 opportunity had been so long overlooked. But it will 
 be nothing less than a national calamity, if, having 
 been recognised in ample time to be employed, that 
 opportunity be altogether neglected. 
 
 Now, after four years delay, time runs short indeed. 
 It is essential that any party intended to observe the 
 transit, should be landed before the Antarctic summer 
 of 1873-74 draws near its end certainly before the 
 middle of February 1874. There may not be time for 
 sending a suitably provided expedition from England. 
 On this point it is for others to speak. I should say, 
 however, that unquestionably there is time for sending 
 an expedition from Tasmania or New Zealand. It was 
 in fact proposed in 1868 by Captain Eichards (Hydro- 
 grapher to the Admiralty) that New Zealand should be 
 made the head-quarters of the expedition then being 
 planned for observing the transit of 1882 from Posses 
 sion Island. One can see no reason why this plan 
 should not now be resumed for securing the more 
 valuable observations which can be made during the 
 transit of 1874. 
 
 If we inquire what has been done towards preparing 
 for observations by Delisle s method, we shall see that 
 by a very slight modification of the Government 
 
THE COMING TRANSIT OF VENUS. 37 
 
 arrangements, Possession Island might be taken as a 
 station without any great additional expense. 
 
 The transit begins earliest at a place in north lati 
 tude 39 45 , and west longitude 143 23 . Woahoo 
 has been selected as a suitable station near this spot ; 
 and in fact the transit begins more than 11 minutes 
 early at Woahoo, while the sun has an elevation at the 
 time of about 20 degrees. Nothing could be more 
 suitable than the station selected by England in this 
 neighbourhood. France takes the Marquesas, while 
 Russia has a station near the mouth of the Amoor 
 River. 
 
 The transit begins latest at a place in 44 27 south 
 latitude, and 26 27 east longitude. The best station 
 hereabout is Crozet Island, so far as astronomical con 
 ditions are concerned ; but bad weather very commonly 
 prevails here. England will send an observing party 
 to Kerguelen s Land, and will also occupy the Mau 
 ritius and Rodriguez Island, which are not so well 
 placed ; since the transit begins 12-J minutes late 
 at Crozet, 11J- minutes late at Kerguelen, only 10J 
 minutes late at Mauritius, and only 10 minutes late at 
 Rodriguez. The party at Mauritius will be that which 
 Lord Lindsay is preparing at his own expense ; and it 
 will be amply provided with all that is required for the 
 purposes of exact observation. Why should not the 
 Government expedition to Rodriguez be given up ? 
 Its cost will certainly not be well repaid, since the cir 
 cumstances of the transit at Mauritius and Rodriguez 
 are almost identical ; and if the money thus saved were 
 
38 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 devoted to an expedition to Possession Island, a good 
 step would have been made towards providing for the 
 cost of such an expedition. 
 
 The transit will end earliest at a place in south 
 latitude 64 47 , and west longitude 114 37 . The 
 best station in this neighbourhood is that very place, 
 Possession Island, which affords the most favourable 
 opportunity for applying Halley s method. For at 
 Possession Island the transit will end 11-|- minutes 
 early. Next in value come several islands between 
 New Zealand and Victoria Land. It was originally 
 proposed to have an English observing party at Auck 
 land or Wellington, New Zealand ; but the station 
 at present selected is Christ Church, where the 
 transit will end 9J minutes early. It is, in my 
 opinion, most unfortunate, that when Possession Island 
 affords the best station for the application of De- 
 lisle s method as well as Halley s, a station inferior 
 in both respects should be selected. Here again 
 expense might be saved which would go far towards 
 the preparation of an expedition (from New Zealand, 
 if need be) to winter in Possession Island. 
 
 Lastly, the transit will end latest at a place in north 
 latitude 62 5 , and east longitude 48 22 . Here the 
 Russians are in great force, as Orsk, Omsk, Tobolsk, 
 and other Russian towns are very suitably placed. 
 The selected station for an English observing party is 
 Alexandria, where the transit begins late by about 10 
 minutes. The sun will only be about 14 degrees high 
 at the time, and a greater elevation would be preferable. 
 
THE COMING TRANSIT OF VENUS. 39 
 
 Amongst the mistakes pointed out by me in 1869 was 
 the complete omission of all notice of stations admir 
 ably placed in Northern India for observing the 
 retarded end of the transit. Thus at Peshawur the 
 transit will begin 10 J minutes late, the sun having an 
 elevation of 31 \ degrees. If Peshawur be not conve 
 niently accessible, then Delhi and the country around 
 would serve nearly as well astronomically. I supposed, 
 until quite recently, that this suggestion, like the more 
 important one relating to Possession Island, would 
 receive no attention. But I was gratified a few weeks 
 ago, by hearing from the Astronomer Eoyal that my 
 discussion of the bubject had induced him to urge that 
 a station should be selected ; somewhere in the North 
 of India. I may be permitted to add (since I do so 
 from no personal gratification, but to give a weight to 
 my present arguments, which otherwise they might 
 not possess) that in the same letter the Astronomer 
 Royal described my researches on the transit of Venus 
 as probably the best of all contributions from 
 Englishmen and foreigners. Apart therefore from the 
 circumstance that though many have discussed my 
 researches not one astronomer has questioned the 
 accuracy of my chief conclusions, I have now the recog 
 nition tardy indeed, but not the less sufficient 
 of the astronomer whose work I criticised. If I use 
 this as a lever to advance my present argument, it is 
 because I feel that the scientific credit of this country 
 is likely to be affected if England does not discharge 
 her duty in this matter. I am satisfied, moreover, that 
 
40 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 whereas the reputation of the eminent man of science 
 who stands at the head of the astronomy of this country 
 will in no degree be affected if the proposed expedition 
 be undertaken somewhat later than was desirable, it 
 will suffer seriously hereafter if that expedition should 
 not be undertaken at all. 
 
 Eraser s Magazine for March 1873. 
 
 THE EVER-WIDENING WORLD OF STARS. 
 
 As the science of astronomy has advanced, the ideas 
 men have formed respecting the extent of the universe 
 have gradually become more and more enlarged. In 
 far-off times, when astronomers were content to judge 
 of the conformation of the universe by the appearances 
 directly presented to their contemplation, the ideas 
 formed respecting the celestial bodies were singularly 
 homely. We read that Theophrastus looked upon the 
 Milky Way as the fastening of the stellar hemispheres, 
 which are < so carelessly knitted together, that the fiery 
 heavens beyond them can be seen through the spaces 
 Anaximenes believed the heavens to be made of a kind 
 fine earthenware, and that the stars are the heads of 
 nails driven through the domed vault formed of this 
 material. And even Lucretius, whose views of nature 
 were so noble, has referred without disapproval to the 
 bizarre theory of Xenophanes that the stars are fiery 
 clouds collected in the upper regions of air 
 
THE EVER- WIDENING WORLD OF STARS. 41 
 
 While the Ptolemaic system of astronomy was ac 
 cepted there were no means of forming any trustworthy 
 views respecting the extent of the stellar universe. If 
 the earth were ever at rest we could never know how 
 far the stars are from us ; and therefore the old astrono 
 mers were free to invent whatever theories they pleased 
 as to the scale on which the sidereal scheme is con 
 structed. It was only when the earth was set free by 
 Copernicus from the imaginary chains which had been 
 conceived as holding it in the centre of the universe 
 that it became possible to form any conception of the 
 distances at which the stars lie from us. Indeed Tycho 
 Brahe immediately pointed this out as an overwhelm 
 ing objection against the new theory. fi Are we to 
 suppose, he argued, that the stars are placed at such 
 enormous distances from us as to seem wholly un 
 changed in position while the earth sweeps round the 
 sun in an orbit millions of miles in diameter ? If this 
 amazing theory were true, the stars would be hundreds 
 of millions of miles from us, a view which is utterly 
 monstrous and incredible. 
 
 But strange as this new view appeared, it gradually 
 gained ground. It became presently so well established 
 that when Cassini discovered that the earth travels in 
 a much wider orbit than Tycho Brahe had supposed 
 so that the stars were at once thrown many hundreds 
 of millions of miles farther from us astronomers still 
 held to the new order of things. With Briarean 
 arms, as Humboldt has described their labours, the 
 fellow-workers of Cassini thrust farther and farther 
 
42 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 away the heaven of the fixed stars, until the immensity 
 of the universe grew so great beneath their labours, 
 that new modes of expressing its dimensions had to be 
 adopted. They were not satisfied with the obvious 
 circumstance that the stars seem to remain unchanged 
 in position as the earth sweeps round the sun. They 
 tested this apparent fixity of position with instruments 
 of greater and greater power, yet always with the 
 same result. They made observations ten, twenty, 
 even fifty times more exact than Tycho Brahe s, and 
 the fact that they still detected no change of position 
 signified nothing less than the universe of the fixed 
 stars is ten, twenty, even fifty times farther from us 
 than Tycho Brahe had imagined. 
 
 Thus when Sir W. Herschel began the noble series 
 of researches amid the stellar depths which has rendered 
 his name illustrious, the world of stars was already 
 of inconceivably enormous extent. Yet so widely did 
 he increase our appreciation of the vastness of the 
 universe, that it has been thought no exaggeration to 
 say of him, that he broke through the barriers of the 
 heavens : Cselorum perrupit claustra, says his monu 
 ment at Upton, and every student of astronomy who 
 has carefully examined HerschePs labours understands 
 the justice of the expression. For consider what 
 Herschel did. When he began his survey of the 
 heavens, astronomers had proved indeed that the nearest 
 of the fixed stars lie at enormous distances from us, and 
 some of the more advanced thinkers had begun to form 
 noble speculations respecting the relations of the stars 
 
THE EVER- WIDENING WORLD OF STARS. 43 
 
 which lie beyond the sphere of those visible to us. 
 But it was reserved for Sir W. Herschel to apply exact 
 observations to the unseen star- systems. He literally 
 gauged the celestial depths. With a telescope whose 
 light-gathering power extended the range of vision to 
 about eight hundred times its natural limit, he swept 
 the whole of the northern heavens. He estimated the 
 depth of the system of stars in every direction by a 
 simple and natural process. For, like all great thinkers, 
 he struck out modes of inquiry which, the moment they 
 were presented to the world, seemed so obvious, that 
 the wonder was how they could have remained so long 
 undetected. He said that precisely as the quantity of 
 water passed through by the sailor s lead-line marks 
 the depth of the sea, so the number of stars which can 
 be seen when a telescope of given power is turned 
 towards any part of the heavens is a measure of the 
 depth of the sidereal system in that direction. In in 
 dividual cases, indeed, the law may not be true, just as 
 the sailor s lead-line may light on the peak of some 
 sunken rock, and so give no true measure of the general 
 depth of the sea in the neighbourhood. But when the 
 average of a great number of such star-gaugings is 
 taken, then we may feel tolerably certain that on 
 applying the simple rule devised by Herschel we shall 
 form no inaccurate estimates of our system s extent in 
 any direction. 
 
 Thence arose his great theory of the stellar system. 
 He showed that our sun is but one of an immense 
 number of suns, distributed in a region of space resem- 
 
44 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 bling a cloven disc in figure. When we look along 
 the thickness of the disc we see the enormous beds of 
 stars, which lie round us in that direction as a cloud 
 of milky light, which so comes to form a cloven ring 
 round the heavens. But when we look out towards 
 the sides of the disc, where the stars are less profusely 
 scattered, we see between them the black background 
 of the sky. 
 
 Then Herschel extended his researches to those 
 strange objects called the nebulae. He showed that 
 where astronomers had reckoned tens of these objects 
 there were in reality thousands. And he found that a 
 large proportion of the nebulae can be resolved into 
 stars. He held that these, therefore, may be looked 
 upon as external universes, resembling that great system 
 of stars of which our sun is a member. We need not, 
 at this point, dwell upon the distinction which Herschel 
 drew between nebulae of this sort, and those objects 
 which he held (and as we now know, justly) to be true 
 clouds, formed of some vaporous substance, of the 
 actual nature of which he forbore to express an opinion. 
 Let it suffice to remark that in whatever mode those 
 vaporous nebulae might be supposed to be formed, it 
 was clear to Herschel that they cannot be held to lie 
 necessarily beyond the system of the fixed stars, as he 
 held to be certainly the case with the stellar nebulae. 
 
 Since Herschel s day a multitude of important dis 
 coveries have been made. His son, the present Sir 
 John Herschel, carried the system of star-gaugings over 
 the southern heavens, having first trained himself for 
 
THE EVER-WIDENING WORLD OF STARS. 45 
 
 the work by verifying Sir William s northern star-gaug- 
 ings. The eminent astronomer Struve and others have 
 applied a series of tests to the basis of Herschel s theory 
 of the universe. Increased telescopic power has been 
 applied to the examination of the nebulae. And lastly, 
 a mode of research more wonderful than the boldest 
 pioneers of science had ventured to hope for has been 
 applied to determine what the stars and nebulae really 
 are, nay even the very elements of which they are con 
 stituted. 
 
 Therefore we stand in a position so far in advance of 
 that to which it was in Herschel s power to attain, that 
 the attempt to modify his theories need no longer be 
 thought to savour of undue boldness. Half a century 
 does not pass without bringing a vast extension of 
 knowledge, and certainly the last half-century has been 
 no exception to this rule ; insomuch that could the 
 great astronomer take his place again among us, and 
 become cognisant of the vast strides which his favourite 
 science has made since he left us, he would be the first 
 to point out that many of his views require to be 
 modified or even to be wholly abandoned. 
 
 For instance, let us consider the meaning of the 
 following observation made by the younger Herschel. 
 While sweeping the southern heavens, this eminent 
 astronomer noticed occasionally the existence of faint 
 outlying streamers belonging to the Milky Way, not 
 only irresolvable into stars, but so exceedingly distant 
 that he could scarcely speak of them as really visible. 
 He was sensible of their existence, but when the eye 
 
46 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 was turned directly upon them they vanished, insomuch 
 that, he says, the idea of illusion has repeatedly arisen 
 subsequently, yet when he came to map down the 
 places where these phantom star-streams had been 
 detected, he found that they formed regular branches 
 of the galactic system. 
 
 Now these outlying star-streams prove first of all 
 that the star-system is not disc-shaped, but spiral in 
 figure. Between the stars which form the ordinary 
 streams of the Milky Way, and those which form the 
 phantom streams, there must lie regions in which stars 
 are either altogether wanting or strewn with much 
 less profusion than in either the nearer or the farther 
 stream. 
 
 But this is not the only nor the chief conclusion 
 which may be drawn from the existence of the almost 
 evanescent star-streams. According to Herschel s views 
 the stars which compose those streams are only faint 
 through enormity of distance. They may be as large 
 as our sun, many of them perhaps far larger. And 
 between them there may yawn distances as large as 
 those which separate us from Arcturus or Aldebaran. 
 Now, this being so the outlying parts of our own 
 sidereal system being removed so far from us as to be 
 all but evanescent in Herschel s splendid reflector 
 how much greater ought to be the faintness of the 
 sidereal systems which lie outside ours ! If the nebulae 
 are really such systems, and made up of suns like our 
 own, then not only ought Herschel s great reflector to 
 fail in rendering them visible, but even Lord Eosse s 
 
THE EVER-WIDENING WORLD OF STARS. 47 
 
 noble mirror would require to be increased a hundred 
 fold in power before we could see them. For clearly 
 the nebulae, which appear as mere tiny specks upon the 
 vault of heaven, must be very much farther away than 
 the confines of our system, if they are comparable with 
 it in size. 
 
 Therefore we must have l of two things one. Either 
 the confines of our sidereal system are constituted very 
 differently from the parts in our neighbourhood ; or 
 the nebulae are constituted very differently from the 
 sidereal system. We say, of two things one, meaning 
 that one of the two views must be true ; but it is plain 
 that there is nothing to prevent both being true. 
 
 We may next come to the inquiry whether these 
 views are severally supported by any special evidence. 
 
 Now as to the first, it happens that the southern 
 heavens surveyed by the younger Herschel afford 
 evidence such as Sir William Herschel was not pos 
 sessed of. The former has seen places in the southern 
 skies where the outline of the Milky Way is so sharply 
 defined that even in the telescope the sudden change 
 from a background of black sky to the sprinkled light 
 of the galaxy is not lost. One half of the field of view 
 will exhibit the former aspect, the other the latter. 
 Now if we consider a cloud, or a dense flight of birds, 
 or any cluster of objects exhibiting a well-defined out 
 line, we see at once what that well-defined outline 
 means. It signifies that the eye is directed along the 
 edge or surface of a distinct cluster of objects in one 
 case globules of water, in another birds, and so on 
 
48 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 and the idea is at once precluded that the eye is within 
 the cluster, of whatever nature that cluster may be. 
 Therefore the theory that the sun forms one of a system 
 of stars spread pretty uniformly over a disc-shaped 
 space must be given up; for were it true, the ap 
 proach to the Milky Way would always be gradual. 
 
 When we add that in the southern skies the Milky 
 Way presents the most fantastic configuration, here 
 expanding into fan-shaped masses, there winding about 
 in a multitude of strange convolutions, here suddenly 
 narrowing into a bright neck or isthmus, there exhi 
 biting a nearly circular vacancy, it becomes clear that 
 the galaxy cannot have the figure assigned to it by Sir 
 W. Herschel. It must consist of streams and sprays 
 of stars at different distances. Such streams by their 
 fantastic convolutions serve to explain all the pecu 
 liarities of the galaxy s structure. 
 
 And next, have we any evidence that the nebulae 
 are not really beyond the galaxy, but are mixed up 
 with the sidereal system ? It appears to me that we 
 have. 
 
 Sir William Herschel noticed that there are places 
 where the nebulae are much more densely crowded than 
 elsewhere, and he was disposed to suspect that pre 
 cisely as the stars by their aggregation form the zone 
 of the Milky Way, so there is a zone of nebulae. But 
 when Sir John Herschel had completed the survey of 
 the heavens it was found that a very different law of 
 distribution made its appearance. Instead of being 
 collected in a zone or band around the heavens, the 
 
THE EVER-WIDENING WORLD OF STARS. 49 
 
 nebulae are arranged in two distinct hut irregular 
 clusters, separated by a well-marked zone almost en 
 tirely free from nebulae. And this zone coincides 
 almost exactly with the Milky Way. 
 
 What are we to understand by so special an arrange 
 ment as this ? A modern astronomer says it clearly 
 proves that the nebulae do not belong to the star- 
 world ; but I can see no escape from an exactly oppo 
 site view. A simple illustration will serve to exhibit 
 the nature of the case. Suppose a person found a 
 space of ground on which gravel was arranged in the 
 form of a ring, and that rough stones were thickly 
 spread over the whole space except the gravel ring, 
 would he conclude that there was no association be 
 tween the arrangement of the gravel and the arrange 
 ment of the stones, because few stones were to be 
 found on or near the gravel ? Would he not rather 
 find in this peculiarity distinct evidence that there 
 was some association ? He would, we think, argue 
 that the gravel had been collected into one place and 
 the stones into another, in pursuance of some one par 
 ticular scheme. The corresponding conclusion in the 
 case of the stars and nebulae would clearly be that the 
 stars had been drawn together in one direction and the 
 nebulae in another, out of a common world of cosmical 
 matter.. In other words we should look on the nebulae 
 as members of the same system or scheme that the 
 stars belong to. 
 
 And here it may be asked how the conclusion thus 
 deduced from the arrangement of stars and nebulae can 
 
 B 
 
50 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 be said to tend to enlarge our views of the world of 
 stars. On the contrary, it might he urged, the views 
 which had prevailed before, presented us with nobler 
 conceptions of the universe. For we were able to 
 recognise in the thousands of nebulae which fleck the 
 dark background of the sky, sidereal systems as noble 
 as that of which our sun is a member ; and in the 
 existence of countless star-systems we had a spectacle 
 to contemplate before which the human intellect was 
 compelled to bow in its utter powerlessness and insig 
 nificance : whereas it seems as though the new views 
 would reduce the scope of our vision to a single 
 galaxy of stars, unless some few members of the nebular 
 system may still be looked on as outer star-schemes. 
 
 But on a closer inspection of the views I have been 
 maintaining, it will appear that they largely enhance 
 our conceptions of the scale on which the world of 
 stars is constructed. Until now it has been held that 
 the telescopes which man has been able to construct 
 enabled us to scan the limits of our sidereal system, 
 and to pass so readily beyond those limits as to become 
 sensible of the existence of thousands of other schemes 
 as noble as our own or nobler. But if the new views 
 should be established, we should be compelled to recog 
 nise in the world of stars a system which our most 
 powerful instruments are not fully able to gauge. The 
 clusters of stars, whose splendour has so worthily ex 
 cited the admiration of the Herschels, the Eosses, the 
 Struves, and the Bonds, must be looked upon as among 
 the glories of our own system, and indicative of the 
 
THE EVER-WIDENING WORLD OF STARS. 51 
 
 multiplied forms of structure or of aggregation to be 
 found within its boundaries. As of late our concep 
 tions of the wealth of the solar system have been 
 enhanced by the discovery of numberless new objects 
 and new forms of matter existing within its range, 
 and co-ordinating themselves in regular relations with 
 the earlier known members of the system, so we seem 
 now called on to recognise in the stellar world an un 
 suspected wealth of material, a hitherto unrecognised 
 variety of cosmical forms, and an extension into regions 
 of space to which our most powerful telescopes have 
 not yet been able to penetrate. 
 
 But now I would call attention to a peculiarity of 
 the southern skies which, while apparently affording 
 conclusive testimony in favour of the new Views, has 
 unaccountably (in my opinion) been urged as an argu 
 ment tending in quite another direction. There are 
 to be seen in those skies two mysterious clouds of light, 
 which were called by the first Europeans who sailed 
 the southern seas the Magellanic clouds, and are now 
 commonly spoken of by astronomers as the Nubeculse. 
 Examined by the powerful telescope of Sir John Her- 
 schel, these objects have been found to consist of small 
 fixed stars and nebulae, grouped together without any 
 evidence of special arrangement, but still obviously 
 intermixed, not merely seen projected on the same 
 field of view. 
 
 These strange objects have given rise to many specu 
 lations ; and among the definite views put forward 
 respecting them is one recently expressed in a most 
 
 2 
 
52 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 valuable communication to the Eoyal Astronomical 
 Society from the pen of Mr. Cleveland Abbe, an 
 astronomer who has laboured in the sound school of the 
 Poulkowa Observatory. Having recognised in the 
 peculiar arrangement of stars and nebulae above re 
 ferred to, an argument that the nebulae lie beyond our 
 system, Mr. Abbe suggests that the Magellanic clouds 
 are two of the nearest of the nebular systems, which 
 thus exhibit larger dimensions than their fellow- 
 schemes. 
 
 The converse of this, which may be termed the positive 
 theory of the Nubeculae, is the hypothesis which may be 
 termed the negative theory. Whatever these objects 
 may be, astronomers have said, they are quite distinct 
 from the sidereal system, nor are the nebulae seen 
 within them to be looked upon as fellows of the other 
 nebulae. For in the Nubeculae we see what we recog 
 nise nowhere else, the combination namely of clustering 
 groups of stars and freely scattered nebulae. It is the 
 characteristic (still I am quoting the theory) of the 
 sidereal system that where its splendours are greatest 
 nebulae are wanting ; it is the characteristic of nebular 
 aggregation that it withdraws itself in appearance from 
 the neighbourhood of clustering star groups. But in 
 the Magellanic clouds neither of these characteristics is 
 to be recognised ; therefore these objects are distinct 
 from either system. 
 
 Nor has another argument been wanting to indicate 
 the distinction that exists between the Magellanic clouds 
 and the other splendours of the celestial vault. Sir 
 
THE EVER-WIDENING WORLD OF STARS. 53 
 
 John Herschel, sweeping over their neighbourhood with 
 his 18-inch reflector, was struck with the singular bar 
 renness of the skies around them. With that expres 
 sive verbiage which gives so great a charm to his astro 
 nomical descriptions, he forces on our attention, again 
 and again, the poverty of the regions which lie around 
 the Nubeculae. Oppressively barren he describes 
 them in one place ; the access to the Nubeculso on all 
 sides is through a desert, he says in another. And this 
 peculiarity, thus established by the certain evidence of 
 an observer so able and trustworthy, has been held by 
 many to imply in the clearest and most distinct manner 
 that there is no connection between the Nubeculae and 
 the stellar system. 
 
 To me the evidence afforded by the barrenness of the 
 regions round the Magellanic clouds points irresistibly 
 in the opposite direction. Why should some outer 
 system, free as is assumed of all association with our 
 own, occupy that peculiarly barren space which so at 
 tracted the attention of Sir John Herschel ? But if 
 we look on the coincidence as striking in the case of 
 one, how much more remarkable will it appear when 
 the only two outer systems of the sort, thus brought 
 within our ken, are associated in this way with the most 
 singularly barren region in the whole heavens ! Surely 
 the more natural conclusion to be drawn from the 
 phenomenon is that the richness of the Magellanic 
 clouds and the poverty of the surrounding districts 
 stand to each other in the most intimate correlation. 
 Is there not reason for concluding that those districts 
 
54 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 are poor because of the action of the same process of 
 aggregation which has attracted within the Nubeculse 
 a larger share than usual of stellar and nebular glories P 1 
 It need hardly be mentioned that the former argu 
 ment, on which the distinction between the Nubeculae 
 and other celestial objects has been founded, is disposed 
 of at once if we recognise the stellar and nebular 
 systems as in reality forming but a single scheme. Not 
 only so, but the Nubeculae afford a striking argument 
 in favour of the latter view. To return to the somewhat 
 homely illustration made use of above. Our conceptions 
 of the original association between the stones and the 
 gravel arranged in the manner indicated would certainly 
 be strengthened, or would even be changed into abso 
 lute certainty, if we perceived in a part of the ground 
 two heaps in which stones and gravel were intermixed. 
 When I add that there are two distinctly marked 
 nebular streams leading towards the Nubeculse, as well 
 as several well-marked star-streams tending in the 
 same direction, the evidence of association seems greatly 
 strengthened. 
 
 If these views be accepted, we shall have to look upon 
 the world of stars as made up of all classes of clustering 
 aggregations, besides strange wisps and sprays extend 
 ing throughout space in the most fantastic convolutions. 
 Then also, while dismissing the idea that the nebulae as 
 
 1 Sir William Herschel has recorded a peculiarity respecting nebulse 
 which is worthy of mention in connection with the facts above consi 
 dered. I have found, he says, that the spaces preceding nebulae were 
 generally quite deprived of stars, so as often to afford many fields 
 without a single star. 
 
THE EVER-WIDENING WORLD OF STARS. 55 
 
 a class are external systems, we may accept as highly 
 probable the conclusion that some of the spiral or whirl 
 pool nebulae really lie far beyond the confines of our 
 system. For we see in these objects the very picture 
 of what the new views show our sidereal system to be. 
 There are the spiral whorls corresponding to the double 
 ring of the Milky Way ; there, are faint outlying 
 streamers corresponding to the phantom star-streams 
 traced by Sir John Herschel; there also, are bright 
 single stars and miniature clusters, nay, there also, may 
 even be recognised large knots or lobes of clustering stars, 
 forming no inapt analogue of the Magellanic clouds. 
 
 Eraser s Magazine for July 1869. 
 
 MOVEMENTS IN THE STAR-DEPTHS. 
 
 AMONG the many striking contrasts between the seeming 
 and the real suggested by the study of astronomy, there 
 is none more startling than the contrast which exists 
 between the apparent repose of the heavens and what is 
 really taking place among the star-depths. On a calm 
 clear night 
 
 When all the winds are laid, 
 And every height comes out, and jutting peak 
 And valley, and the immeasurable heavens 
 Break open to their highest 
 
 the stars seem set as emblems of eternal fixity and rest. 
 As such they have been regarded in all ages by the poet; 
 nor has science, so far as it has been directed to the 
 
56 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 apparent movements of the stars, taught any other 
 lesson. It has, indeed, shown that the stars are even 
 more steadfast than they seem, in so far as it teaches 
 that their diurnal and annual motions are but apparent, 
 while the great precessional swaying of the star-sphere 
 is but the reflexion of the earth s gyration. More and 
 more 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 displayed to our view in the stillness of 
 the calmest and clearest night are, in reality, astir with 
 the most stupendous activity. The least of the orbs we 
 see some star so faint that it is only discerned by 
 momentary gleams is the abode of forces whose action 
 during a single instant surpasses in effect all the forces 
 at work upon the earth during a decade of years. All 
 the wonderful processes taking place within and around 
 the globe of our own sun have their analogues in that 
 distant orb. Let it be remembered also that our sun 
 himself presents an aspect which in no sense suggests 
 his real condition. If we would picture him as he 
 actually is, we must consider the uproar and tumult 
 which prevail where, to our ordinary perceptions, all 
 seems at perfect rest. The least movement on that 
 glowing photosphere represents the action of forces so 
 tremendous that they would be competent to destroy 
 in an instant this earth on which we live. The most 
 hideous turmoil, outvying a million-fold the roar of the 
 hurricane or the crash of the thunderbolt, must prevail 
 for ever in every part of the solar atmosphere. And in 
 
MOVEMENTS IN THE STAR-DEPTHS. 57 
 
 whatever respects other suns may differ from our own, 
 in this at least we know that they resemble him. It is 
 the very charter of their existence as suns as real 
 living centres of energy to schemes of circling worlds 
 that they should thus continually pulsate with their 
 own vitality. Each is the central engine on whose 
 internal motions the life of a system of worlds depends 
 and each must, with persistent activity, work out its 
 purpose, until the fuel which supplies its forces shall be 
 exhausted. 
 
 All the evidence as yet obtained points to the conclu 
 sion that our own sun, wonderful as is his structure and 
 stupendous his energy, is yet very far inferior in splen 
 dour and power to most of his fellow suns. Placed where 
 Sirius is, the sun would appear but as a third-rate star, 
 less bright than hundreds of the stars visible to the 
 unaided eye. But removed to the distance of Alde- 
 baran, or Castor, or Betelgeux, our sun would certainly 
 not shine more brightly than the fourth-magnitude 
 stars, while it is probable that his lustre would be so 
 reduced that he would be barely discernible. There 
 can be little doubt that of all the stars seen on the 
 clearest and darkest night, there are scarce fifty which 
 are not far larger suns than ours, 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 into 
 the movements taking place within the star system, we 
 are yet more startlingly impressed by the contrast be 
 tween the apparent rest prevailing in the star-depths 
 
58 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 and the inconceivable activity really present there. It 
 seems incredible that all those orbs which look so still 
 are speeding through space with a velocity compared 
 with which every form of motion familiar to us on 
 earth must be regarded as almost absolute rest. This 
 appears even more surprising when we consider that 
 during all those centuries with which history deals, 
 during the rise and fall of the nations of antiquity, 
 during the darkness of the Middle Ages, during the 
 more familiar scenes of recent centuries, the stars 
 have presented an aspect so constant that if the Chal- 
 dsean astronomers could be restored to life, they would 
 recognise scarce any change in the positions of the stars 
 forming the ancient constellations. Yet there are no 
 astronomical facts more thoroughly established than 
 those which relate to the motions of the stars. The 
 giant orb of Sirius, exceeding our sun a thousand times 
 in volume, Capella and Procyon, the glories of Orion, 
 the clustered Pleiads, Arcturus, Vega, and Aldebaran, 
 all the stars known to the astronomer, are urging their 
 way with inconceivable velocity, each on its own course, 
 though doubtless all these motions are subordinated to 
 some as yet unexplained system of movements whereby 
 all the stars of the galaxy are made to form parts of 
 one harmonious whole. 
 
 Until lately it had only been by one method of 
 observation that the astronomer could assure himself 
 that these motions were taking place. That method is 
 the simplest conceivable. If a star s place were accu 
 rately determined, either with respect to neighbouring 
 
MOVEMENTS IN THE STAR-DEPTHS. 59 
 
 stars or to the imaginary circles and points on the 
 sphere which are determined by the earth s movements 
 of rotation and revolution, then, if the star be really 
 in motion, a change of place must in the long run 
 manifest itself, not indeed to ordinary vision, but to 
 the piercing scrutiny and to the yet more remarkable 
 measuring powers of the astronomical telescope. A 
 hundred years may elapse before the motion is measure- 
 able, yet the astronomer can none the less certainly 
 assure himself that the motion is taking place, since he 
 has the records of those who have gone before him, and 
 the means of satisfying himself that those records are 
 trustworthy. 
 
 It had long been felt, however, that there was an 
 unfortunate gap in the evidence respecting stellar 
 motions. The astronomer could tell how much or how 
 little the stars were shifting on the heavens, but he 
 could obtain no measure whatever of other motions 
 which nevertheless must exist among the stars. If a 
 star were receding or approaching, no trace whatever 
 of such motion could be recognised. No instrumental 
 means could enable the astronomer to measure the 
 change of brightness due to the star s change of dis 
 tance, since such changes must needs be infinitely small 
 compared with the actual lustre of the star. 
 
 So that it seemed as though the astronomer must for 
 ever remain ignorant of one most important portion of 
 the stellar motions. All he could do, as it appeared, 
 was to watch the aspect of the heavens, and, as it slowly 
 changed, to infer in what way the stars were moving 
 
60 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 athwart the line of vision ; and even this he could only 
 do most imperfectly, since his knowledge of the dis 
 tances of the stars is so limited that he can form but 
 inexact notions of the rate at which the stars are so 
 moving. They may be very far away and moving very 
 swiftly, or they may be at a less (though still enormous) 
 distance and moving with a correspondingly reduced 
 velocity. 
 
 This source of difficulty was very strikingly illustrated 
 when the subject of the stellar motions was treated in 
 connection with the ideas respecting the sidereal uni 
 verse promulgated by Sir W. Herschel. In the hy 
 pothesis which regarded the stars as spread with a 
 certain general uniformity through a stratum or slice 
 of space, there was no feature which afforded any pro 
 mise that by the study of the stellar motions the 
 mysteries of the sidereal universe might be interpreted. 
 The very basis of Sir W. Herschel s own researches into 
 the subject is the vague supposition that it is as likely 
 a priori that any given star will move in one direction 
 as in another. Later we find Struve presenting his 
 results in the following form : < One may wager four 
 hundred thousand to one that a portion of the seeming 
 motions of the stars is due to the sun s motion, and it 
 is an even wager (on pent parier un contre uri) that the 
 latter motion takes place at the rate of between 135 and 
 175 millions of miles per annum. The whole question 
 had become one of probabilities, based on more or less 
 trustworthy assumptions. We cannot wonder greatly 
 that, when Sir Gr. Airy undertook the complete re- 
 
MOVEMENTS IN THE STAR-DEPTHS. 6 1 
 
 examination of the matter twenty years ago, the result 
 he obtained, while indicating the general probability 
 of the inferences before obtained, nevertheless exhibited 
 the whole problem as one needing further investigation. 1 
 
 It will be seen presently that we cannot too atten 
 tively regard those earlier researches, if we would fully 
 estimate the importance of the results which have 
 recently been obtained. Let it be carefully noticed 
 that the earlier results flowed directly from the hypo 
 thesis respecting the stars which have so long main 
 tained their ground in our text-books of astronomy 
 If these hypotheses are sound, the results flowing from 
 them, even though only based on the general principles 
 of probability as applied to those hypotheses, might be 
 expected to be somewhat near the truth. If, on the 
 contrary, an independent and trustworthy series of 
 results should show that those earlier results are not 
 correct are indeed very far from correctness then 
 pro tanto the hypotheses which led to those earlier 
 results would be invalidated. 
 
 Let it then be clearly understood that, according to 
 the results in question, the stars were held to be in 
 motion at rates comparable in general with the velocity 
 of our sun, this velocity being estimated at about four 
 and three-quarter miles per second. .We do not include 
 here the result that the sun is moving towards Hercules, 
 because that may be regarded as established, whatever 
 
 1 This part of my subject is fully discussed in a paper called The 
 Sun s Journey through Space, which appeared in Fraser s Magazine for 
 September 1869, and will be found among my Essays on Astronomy. 
 
62 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 opinion we may form as to the distribution of the stars 
 in space. 
 
 Before proceeding to indicate the bearing of recent 
 observations on these theoretical conclusions, I would 
 invite some degree of attention to the circumstance 
 that the view I am here advancing as to the bearing of 
 new facts on the old hypotheses, is not a new one 
 framed to account for the new facts in a way agreeing 
 with my own theories respecting the stars. More than 
 three years ago in Fraser s Magazine, and earlier still 
 in the proceedings of scientific societies, I indicated my 
 belief that the real facts are precisely such as have now 
 been demonstrated. 
 
 Already when I so wrote, promise had been afforded 
 that the astronomer might come in time to know, 1 not 
 merely whether certain stars are approaching or reced 
 ing, but at what rate (in miles per second) these 
 motions are taking place. I need not here enter into 
 an explanation of the method by which this was to be 
 accomplished, inasmuch as a full account of the prin 
 ciple on which the method is based is given in the 
 paper called 6 News from Sirius, in my Essays on Astro 
 nomy. Suffice it to say, that it depends on the observed 
 displacement of some known dark line in the rainbow- 
 tinted streak forming the spectrum of a star, and that 
 when such a line is displaced towards the red end of the 
 spectrum it is known that the star is receding, while 
 
 1 See the closing words of the last paragraph but three in the essay 
 mentioned. 
 
MOVEMENTS IN THE STAR-DEPTHS. 63 
 
 when the displacement is towards the violet end it is 
 known that the star is approaching. 
 
 Dr. Huggins, our great spectroscopist, had success 
 fully applied this method to the star Sirius, and he had 
 found that that star is receding from the earth at the 
 rate of upwards of twenty-five miles per second. But 
 Sirius was the only star which could then be examined 
 by this method. The light of Sirius exceeds more 
 than five times that of the next star in order of bright 
 ness, at least of those visible in our hemisphere ; and 
 with the instrument then at Dr. Huggins disposal (his 
 own eight-inch refractor) it was found impossible to see 
 the dark lines of any other star-spectrum with a spec 
 troscope dispersive enough to give any measurable 
 displacement of the lines. 
 
 But the importance of the inquiry (as well as of 
 those other spectroscopic researches in which Dr. Hug- 
 gins had been so successful) was manifest to our 
 scientific societies ; and accordingly a large sum was 
 granted by the Eoyal Society for the construction of a 
 refracting telescope, fifteen inches in aperture, to enable 
 Dr. Huggins to extend his researches to the leading 
 stars of our northern heavens. This fine instrument 
 was ready for use in the spring of this year, and before 
 many weeks had passed Dr. Huggins had obtained 
 results of surpassing interest and importance. He had 
 recognised motions of recession and approach in no less 
 than thirty stars, and had traced laws before unknown 
 in the phenomena of these stellar motions. 
 
 One of the most striking features in the series of 
 
64 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 star-motions observed and measured by Dr. Huggins, 
 is the amazing velocity with which some of the stars 
 are moving. Astronomers had ascertained that Sirius 
 is moving athwart the line of vision much more rapidly 
 than the sun is travelling through space. But Sirius 
 is so exceptional both in his brightness and in his esti 
 mated bulk, that his enormous velocity did not appeal- 
 altogether surprising. It did not lead the generality 
 of astronomers to consider that the sun s velocity and 
 the average velocity of the stars had been greatly under 
 estimated. But now we learn from a method of research 
 which is far more trustworthy than any applied to the 
 measurement of thwart motions, that some of the stars 
 are moving from or towards the earth with a velocity 
 far exceeding that of Sirius. If we take the thwart 
 motion of Sirius at twenty-five miles per second, and 
 his motion of recession at twenty miles (this being the 
 value assigned by the latest and best measurements), 
 we find for this absolute motion the amazing velocity 
 of about thirty-two miles per second. But Dr. Huggins 
 finds that Arcturus is receding from the sun at the rate 
 of 55 miles per second, Vega at the rate of about 50 
 miles, Arided (the chief brilliant of the Swan) at the 
 rate of 39 miles, Pollux 49 miles, and Dubhe of the 
 Great Bear at the rate of from 46 to 60 miles per 
 second. Beside such motions as these, our sun s esti 
 mated velocity of about 4| miles per second, which had 
 seemed so imposing when it was considered that he 
 bore with him at this enormous rate his whole family 
 of planets, sinks into relative insignificance. We here 
 
MOVEMENTS IN THE STAR-DEPTHS. 6^ 
 
 \j 
 
 recognise stellar rates of motion nearly equalling that 
 at which our earth circuits around the sun. But a 
 velocity which, considered with reference to a minute 
 orb like the earth, is intelligible, becomes altogether 
 startling in the case of orbs like Arcturus and Vega, 
 which undoubtedly exceed our own sun many times in 
 volume. I use the word intelligible with a purpose ; 
 for I am not considering here what is conceivable or 
 the reverse. We can in reality understand why the 
 earth should be possessed of the velocity she actually 
 displays. We know that the sun s attraction is com 
 petent to generate such a velocity, or a much greater 
 velocity. But in the case of a star these swift motions 
 cannot be thus explained. The stars are too far apart 
 to be so influenced by their mutual attractions that 
 great velocities would be generated. And thus the 
 thoughtful mind cannot but recognise in the stellar 
 motions a subject of contemplation far more impressive 
 than the subordinate, though even swifter motions of 
 the Earth, Venus, or Mercury. Whence sprang that 
 amazing energy which is represented by the proper 
 motions of the suns ? If we admit the possibility that 
 forces of eruption or expulsion could account for the 
 observed motions, we shall have to answer the startling 
 question, Of what order are the orbs whence the giant 
 suns are expelled ? and the yet more difficult questions, 
 Where are these orbs? and, How is it that, inordinately 
 large though they must be, we are yet unable to distin 
 guish them from ordinary suns ? If, on the other 
 hand, we prefer to regard the stellar velocities as gene- 
 
 F 
 
66 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 rated by the attractions of larger orders of bodies than 
 the stars (as planetary velocities may be regarded as 
 generated by their parent suns), we still have the last 
 two questions to answer; and, so far as can be judged, 
 these questions are at present unanswerable. 1 
 
 Another striking feature in the results announced by 
 Dr. Huggins is the absence of any systematic agree 
 ment between the stellar motions he has recognised, 
 and the motion of our sun towards Hercules. It is 
 manifest that if our sun were alone in motion, the 
 actual rates of approach and recession of all the stars 
 in the heavens would be at once determined when the 
 rate of the sun s motion was determined. If, for ex 
 ample, he were moving at the rate of twenty miles per 
 second towards the star Lambda of Hercules, he would 
 be approaching every star lying in that direction at 
 
 1 In passing, however, I would venture to touch on this question of 
 central suns, or of central but opaque orbs round which the stars may 
 revolve, in order to remove a very prevalent misconception. It seems 
 to be commonly supposed that we cannot imagine such orbs to lie far 
 enough away to account for their not being discernible either as orbs of 
 light or by hiding more distant stars, without depriving them of the 
 attractive influence necessary to sway the motions of the stars. This, 
 however, is not the case. An orb looking as bright as Sirius, but ten 
 times as far away, if of equal density and inherent brightness, would be 
 a thousand times more massive, while the effect of distance would only 
 be to reduce its attraction one hundred times. It would, therefore, 
 attract our sun ten times as strongly as Sirius actually does. In like 
 manner, an orb one hundred times as far away as Sirius, but so large 
 as to appear as bright, would attract our sun one hundred times as 
 strongly, and so on. So that it cannot be positively asserted that 
 among the stars visible to us there may not be the central sun of the 
 sidereal scheme inordinately large and massive compared with the 
 rest, but reduced by distance to the same order of brightness. 
 
MOVEMENTS IN THE STAR-DEPTHS. 67 
 
 the same rate ; lie would be receding from all stars 
 lying in the opposite direction at the same rate; and 
 he would be approaching or receding from stars lying 
 in opposite directions at a less rate (readily calculable). 
 A certain half of the heavens would contain all the 
 stars which the sun was approaching ; the other half 
 would contain all the stars from which he was receding ; 
 and the circle separating these halves would mark the 
 place of stars which the sun was neither receding from 
 nor approaching. But nothing of this sort can be 
 recognised in the observed stellar rates of approach 
 and recession. Sirius (which lies nearly opposite to 
 Hercules) is receding at the rate of about 20 miles per 
 second; but Vega (which lies close to Hercules), instead 
 of approaching at about the same rate, is actually 
 approaching at the rate of about 50 miles per second. 
 Castor, which is very near the border line between the 
 two hemispheres just mentioned, and should therefore 
 neither be approaching nor receding, is in fact reced 
 ing at the rate of about 25 miles per second ; while 
 Pollux, though similarly placed, is approaching the sun 
 at the rate of about 49 miles per second. Again, of the 
 seven bright stars forming Charles s Wain, six are 
 approaching (five of them at the rate of about 20 miles 
 per second), while the seventh is receding at a rate 
 probably exceeding 50 miles per second. 
 
 Thus we see that the sun cannot be regarded as an 
 orb moving within the scheme of stars, and by his own 
 movement causing the chief apparent motions of the 
 surrounding orbs. His motion is but part of a grand 
 
 F 2 
 
68 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 scheme of motions, whose laws are as yet unknown to 
 us. We may recognise in the method of research which 
 has now been so successfully applied, the sole means of 
 determining what those laws may be. We can now tell 
 the very rate, in miles per annum, at which the suns 
 are approaching or receding from us ; and though we 
 have no reason for believing that our sun occupies in 
 any sense a central position so that we have yet to 
 learn at what rate and in what way the stars move 
 around the true centre of their system, yet it is far 
 from unlikely that if we can but ascertain the motions 
 of a sufficient number of stars, we shall have the 
 means of judging where the centre lies round which 
 these motions are taking place. 
 
 The astronomer may well look with doubt, however, 
 on the efforts which are being made to solve this stu 
 pendous problem. If we may judge from the analogy 
 of our own solar system, we can see that in the far 
 more complicated scheme of the stars there must exist 
 innumerable features to perplex the observer. If we 
 imagine a being placed in the midst of the solar system, 
 and enabled to study the various apparent motions 
 visible from his stand-point, and if we further suppose 
 him gifted with the power of measuring the rate at 
 which the various orbs are approaching him or receding 
 from him, then we know that if his scrutiny were but 
 continued long enough, he could not fail to recognise 
 the laws which exist within that system and regulate 
 all those motions. Where at first all had seemed con 
 fusion, our imaginary observer would recognise in the 
 
MOVEMENTS IN THE STAR-DEPTHS. 69 
 
 course of time a beautiful harmony; motions which had 
 appeared discordant would be found to be in reality 
 subordinated into one grand scheme. But if we suppose 
 our observer to occupy his imaginary stand-point for a 
 few hours, or even for a few days only, how imperfect 
 would be his ideas of the harmony of the celestial mo 
 tions! He would see the primary planets moving 
 apparently in diverse directions and at inconsistent 
 rates ; the secondary planets apparently travelling with 
 non-accordant motions and on different paths ; the 
 asteroids would perplex him by their wide range of 
 apparent distribution ; meteoric systems would appear 
 to conform to no recognisable law ; and the movements 
 of comets would seem altogether inexplicable. 
 
 Yet the terrestrial observer of the infinitely more 
 complicated sidereal system is in reality even less 
 favourably circumstanced than our imaginary observer 
 of the planetary scheme. The motions which come 
 within his ken are more minute, compared with the 
 real dimensions of the stellar paths, than the motion of 
 Saturn or Jupiter in a single second compared with the 
 wide orbits traversed by these planets. We cannot tell 
 whether the observed motion of a star is that by which 
 it is carried on some vast independent orbit ; or is its 
 motion within some subordinate scheme ; or, lastly, is 
 for the most part due to the sun s own motion within 
 the sidereal system. When we see the stars of the 
 same constellation carried in different directions, we 
 cannot tell whether the real motions are diverse in 
 character, or whether the diversity is but apparent, 
 
70 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 like the apparent advance and retrogression of planets 
 which, nevertheless, are travelling in a common direc 
 tion around a common centre. 
 
 But precisely because the difficulties which surround 
 the problem of the stellar motions are so stupendous, 
 we must so much the more carefully examine every 
 feature which observation may reveal to us. To do 
 otherwise were to abandon the problem as altogether 
 hopeless. 
 
 Now it cannot but be recognised that in this respect 
 the new method of research is peculiarly promising. 
 For whereas all former methods have dealt only with 
 apparent motion, this method tells us of the real rate 
 of stellar displacements. We have seen how it has 
 disposed of the inferences which had been formed as to 
 the sun s velocity, and the average velocities of stellar 
 motion ; let us inquire what has been its bearing on 
 the views of astronomers respecting the stellar universe 
 regarded as a scheme or system. 
 
 Other methods of dealing with the motions of the 
 stars had related chiefly to the question of the sun s 
 journey through space, until Madler was led to inquire 
 whether the motions of the stars might not afford the 
 means of determining where the centre of the stellar 
 system may lie. Limiting his range of inquiry, in the 
 first instance, by certain preliminary considerations, he 
 proceeded to examine the direction of the apparent 
 stellar motions in a particular region of the heavens. 
 It seemed likely to him that the centre of the universe 
 would be near the Milky Way, and probably on that 
 
MOVEMENTS IN THE STAR-DEPTHS. Jl 
 
 band of conspicuous stars which extends over the 
 Greater Dog, Orion, the Bull, Perseus, and Cassiopeia. 
 Still further, he reasoned that if the sun is circling 
 around the central orb, this body must lie on a line 
 square to the sun s path ; so that if we imagine a line 
 extending from the point in the heavens from which 
 the sun is travelling to the point towards which he is 
 travelling, then the central orb must lie somewhere on 
 or near to a plane through the sun and square to that 
 line. Now such a plane would cut the Milky Way in 
 two places, one in the northern heavens in Perseus, the 
 other in the southern heavens between the Altar and 
 the Centaur. Madler further indicates reasons for 
 believing that the centre of the sidereal universe lies 
 towards the northern region of the Milky Way. Lastly, 
 seeing that not far from the northern region there is a 
 remarkable star cluster, the Pleiades, he was led to 
 examine the region around the Pleiades for those signs 
 which he thought likely to exist towards that part of the 
 heavens where lies the centre of the sidereal universe. 
 We do not enter here into a consideration of the reason 
 ing which led Madler to conclude that in that part of 
 the heavens the stars would all appear to be moving in 
 the same general direction, for they are rather recondite. 
 That, however, was his anticipation ; and as he found 
 that the stars in the constellation Taurus are nearly all 
 moving southwards, he was satisfied that he had not 
 been mistaken in setting the Pleiades as the central 
 region of the universe, and the star Alcyone, the 
 
72 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 brightest of the Pleiades, as the central orb around 
 which all the stars revolve. 
 
 Now to such a problem as this a problem whose 
 grandeur cannot but be recognised even by those who 
 reject the conclusions adopted by Madler the new 
 method of research is applicable with peculiar force. 
 For instance, if the stars of Taurus are circling round 
 a particular orb also in Taurus, it will be manifest, on 
 a moment s consideration, that they can have only a 
 slight motion either of recession or approach with re 
 spect to the sun. When from our station on the earth 
 we see Venus or Mercury nearly in the same direction 
 as the sun, we know that at the moment either planet 
 has only a thwart motion, being then either at its 
 greatest or least distance from us. So that if the new 
 method were applied to stars in Taurus, and showed 
 that swift motions of recession or approach are there in 
 progress, it would at once dispose of the attractive but 
 too speculative theory of the German astronomer. 
 
 This has not yet been accomplished; in fact, since 
 Dr. Huggins instrument was mounted and in order, 
 the constellation Taurus has not been well placed for 
 observation by the new method. But in the meantime, 
 evidence of the most convincing nature has been ob 
 tained to show that Madler s theory is unsound. 
 
 We have seen that the theory was based, in the main, 
 on a certain general community of apparent motion 
 among the stars in Taurus. Madler took it for granted 
 that this community of motion is exceptional. It did 
 not occur to him to examine the motions of stars in 
 
MOVEMENTS IN THE STAR-DEPTHS. 73 
 
 other parts of the heavens, to see whether perchance a 
 like feature might not present itself elsewhere. 
 
 Having been myself led by other inquiries than 
 Mauler s to the conclusion that the stellar motions 
 might afford useful information as to the structure of 
 the heavens, I thought it desirable to make a chart 
 showing all the known stellar motions in such a way 
 that wherever a community of direction exists it would 
 be at once apparent in the chart. Little arrows affixed 
 to the star-discs on the map, showed by their direction 
 and length the nature and amount of the stellar thvart 
 motions. When the map was completed, it was easy 
 to see that the community of motion in Taurus was 
 only one instance, and by no means the most striking 
 which could be recognised, of a phenomenon which I 
 have since called star-drift. Certain sets of stars are 
 seen to be moving athwart the heavens, nearly in the 
 same direction, and nearly at the same rate, in such 
 sort as to show that they form distinct families of suns, 
 travelling onwards each family as a single group 
 through the celestial spaces. 
 
 If this view is just, Madler s theory is at once shown 
 to be unsound ; since the stars in Taurus thus appear 
 as simply a drifting family of stars, one among several 
 such families. 
 
 All that was required to make the proof convincing 
 was, that one of these sets of drifting stars should be 
 shown to be either approaching the earth or receding 
 from it as a single group. 
 
 Now, among the instances of star-drift, there was 
 
74 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 one in the Great Bear which presented some very 
 striking features. Five stars in this constellation, 
 known as Beta, Grarnma, Delta, Eta, and Zeta, were 
 seen to be travelling, not merely at the same rate and 
 in the same direction, but on a course precisely oppo 
 site to that which they would have had if their apparent 
 motion had been due to the sun s motion in space. 
 Moreover, all these stars are large and conspicuous ; 
 while one of them, Zeta, is distinguished by having 
 two companions, one very close to it, and the other so 
 far away that its motion around Zeta is only completed 
 (according to Madler s computation) in a period of 
 about 2,000 years ; so that, if all the five large stars 
 form a single system, the cyclic revolutions of the 
 system must require millions of millions of years for 
 their completion. 
 
 I selected this family of stars as affording a con 
 venient means of testing (crucially) the accuracy of 
 my theory of star-drift. If that theory is just, all 
 these stars must be either approaching or receding at 
 a common rate. If the theory is unsound, the chances 
 are enormous against their possessing a common motion 
 of approach or recession. I expressed a strong feeling 
 of confidence that whenever Dr. Huggins applied the 
 new method of research to these stars, he would find 
 that they are either all approaching or all receding, 
 and at one and the same rate. When I expressed this 
 opinion, I knew that before many months had passed, 
 the matter would be decided one way or the other. 
 
 Nothing could be more complete than the confirma- 
 
MOVEMENTS IN THE STAR-DEPTHS. 75 
 
 tion of my views by Dr. Hug-gins observations. In 
 his table of stellar motions, Dr. Huggins brackets 
 together the five stars in question as possessing a com 
 mon motion of recession at the rate of about twenty 
 miles per second. Moreover he finds, from the nature 
 of their spectra, that they are all alike in physical 
 constitution. 
 
 It is hardly necessary to insist upon the importance 
 of this result. It proves, first, that in this instance 
 and therefore presumably in the other instances of 
 apparent star-drift, there is a distinct family or group 
 of stars, travelling bodily onwards amidst the star- 
 depths. It is shown that the motions taking place 
 within this star-family are small compared with the 
 common motion of the group. It can be inferred that 
 the group is relatively isolated, since otherwise we 
 should find other stars in the Great Bear sharing in 
 the motion of these five ; and also, if there had been a 
 disturbing orb at a moderate distance from the group, 
 the members of the family would ere this have lost 
 their uniformity of motion. Whatever may be the 
 centre around which these five stars are moving as a 
 single group, the distance of that centre must exceed 
 enormously the dimensions of the group, precisely as 
 the distance of the sun from Jupiter s satellite family 
 enormously exceeds the dimensions of that system. 
 Yet the distances separating the stars of the Great 
 Bear are themselves amazingly vast. The distance 
 between Beta and Zeta of the Great Bear cannot be 
 less than 100,000 times the distance separating our 
 
76 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 earth from the sun, and is probably far vaster. What 
 then must be the distance of the centre of motion, as 
 seen from which this enormous space is reduced to an 
 almost evanescent arc ! 
 
 It seems not unlikely that we ought to regard the 
 family of stars here recognised as bearing the same 
 general relation to the stellar universe (or to that por 
 tion of it to which our sun belongs) that a group of 
 meteors bears to the solar system. All the drifting 
 star-families may not indeed travel around one and the 
 same centre ; or there may be no true centre, but only 
 a central region, round which these movements take 
 place : but it is impossible to consider thoughtfully 
 any instance of community of stellar motions without 
 feeling that it implies a common influence affecting in 
 the same or nearly the same way each member of the 
 drifting star-family. If there is but one such centre, 
 whether it be a single orb, or a central region of thickly 
 clustering stars, there now seems to be at least a pos 
 sibility that we may find where this centre lies. When 
 only a few more star-families have been recognised, 
 and their motions of approach or recession determined, 
 it will be a problem of no inordinate difficulty to 
 deduce the position in space of the regions round 
 which these motions are taking place, or else to prove 
 (which would equally be a solution of the problem now 
 before us) that no such region exists, and that the stars 
 drift around more centres than one. 
 
 Whatever success may attend the efforts made to 
 explain the stellar motions, there can be no doubt that 
 
MOVEMENTS IN THE STAR-DEPTHS. 77 
 
 the problem is well worthy of the most thorough in 
 vestigation. There is, indeed, something startling in 
 the thought that man, placed as he is on a tiny orb 
 an orb rotating swiftly on its axis, carried swiftly round 
 the sun, and borne along with him in his swift motion 
 through space man, shortlived and weak, and unable 
 by his unaided vision to perceive a thousandth part of 
 the star-system, should yet attempt (and not unhope- 
 fully) to master the secret of its structure and motions. 
 It may be that what has hitherto been done is but the 
 beginning of the series of labours by which, if ever, 
 that end will be accomplished ; or it may be that we 
 are nearer to the mastery of the problem than we at 
 present imagine : but, in any case, there is but one 
 course by which success can be achieved. Piece by 
 piece the facts on which our reasoning is to depend 
 must be gathered together ; while at every stage of the 
 inquiry, the full meaning of observed facts must be as 
 far as possible evolved. Success will not be obtained 
 by observation alone, nor by theorising alone ; but by 
 that combination only of observation and theory to 
 which we owe all the most important discoveries 
 hitherto effected by astronomers. 
 
 Eraser s Magazine for November 1872. 
 
78 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 THE GREAT NEBULA IN ORION. 
 
 DURING the first four months of the year, the constella 
 tion Orion is very favourably situated for observation 
 in the evening. This magnificent asterism is more 
 easily recognised than the Great Bear, Cassiopeia s 
 Chair, or the fine festoon of stars which adorns the 
 constellation Perseus. There is, indeed, a peculiarity 
 about Orion which tends considerably to facilitate 
 recognition. The other constellations named above, 
 gyrate round the pole in a manner which presents 
 them to us in continually varying positions. It is not 
 so with Orion. Divided centrally by the equator, the 
 mighty hunter continues twelve hours above and twelve 
 hours below the horizon. His shoulders are visible 
 somewhat more, his feet somewhat less, than twelve 
 hours. When he is in the south, he is seen as a giant 
 with upraised arms, erect, and having one knee bent, 
 as if he were ascending a height. Before him, as if 
 raised on his left arm, is a curve of small stars, forming 
 the shield, or target of lion s skin, which he is repre 
 sented as uprearing in the face of Taurus. When Orion 
 is in the east, his figure is inclined backwards ; when 
 he is setting, he seems to be bent forwards, as if rush 
 ing down a height ; but he is never seen in an inverted 
 position, like the northern constellations. 
 
 And we may note in passing, that the figure of Orion, 
 
THE GREAT NEBULA IN ORION. 79 
 
 as he sets, does not exactly correspond with the image 
 presented in that fine passage in Maud : 
 
 I arose, and all by myself, in my own dark garden ground, 
 Listening now to the tide, in its broadflung shipwrecking roar, 
 Now to the scream of a maddened beach dragged down by the wave, 
 Walked in a wintry wind, by a ghastly glimmer, and found 
 The shining Daffodil dead, and Orion low in his grave ; 
 
 and again, towards the end of the poem : 
 
 It fell on a time of year 
 
 When the face of night is fair on the dewy downs, 
 And the shining Daffodil dies, and the charioteer 
 And starry Gemini hang like glorious crowns 
 Over Orion s grave low down in the West. 
 
 I would not, however, for one moment be understood 
 as finding fault with these passages of Tennyson s finest 
 poem. Detached from the context, the image is un 
 doubtedly faulty ; but there is a correctness in the very 
 incorrectness of the image, pla-ced as it is in the mouth 
 of one 
 
 Eaging alone as his father raged in his mood ; 
 
 brooding evermore on his father s self-murder : 
 
 On a horror of shattered limbs .... 
 Mangled and flattened and crushed. 
 
 Let us pass on, however, to the subject of our paper. 
 
 Beneath the three bright stars which form the belt 
 of Orion, are several small stars, ranged, when Orion is 
 in the south, in a vertical direction. These form the 
 sword of the giant. On a clear night it is easy to see 
 that the middle star of the sword presents a peculiarity 
 of appearance : it shines as through a diffused haze. 
 
80 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 In an opera-glass this phenomenon is yet more easily 
 recognisable. A very small telescope exhibits the 
 cause of the peculiarity, for it is at once seen, that 
 what seemed a star is in reality a mass 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, failed 
 to detect 
 
 That marvellous round of milky light 
 Below Orion. 
 
 It would not, indeed, have been very remarkable if he 
 had simply failed to notice this object. But he would 
 seem to have directed his attention for some time 
 especially to the region in the midst of which Orion s 
 nebula is found. He says: At first I meant to de 
 lineate the whole of this constellation ; but on account 
 of the immense multitude of stars being also hampered 
 through want of leisure- -I left the completion of this 
 design till I should have another opportunity. He 
 therefore directed his attention wholly to a space of 
 about ten square degrees, between the belt and sword, 
 in which space he counted no less than four hundred 
 stars. What is yet more remarkable, he mentions the 
 fact that there are many small spots on the heavens 
 shining with a light resembling that of the Milky Way 
 (oomplurea similis coloris areolce sparsim per cethera 
 subfulgeant) ; and he even speaks of nebulae of this 
 sort in the head and belt and sword of Orion. He 
 asserts, however, that by means of his telescope, these 
 
THE GREAT NEBULA IN ORION, 8 1 
 
 nebulae were distinctly resolved into stars- -a circum 
 stance which, as we shall see presently, renders his 
 description wholly inapplicable to the great nebula. 
 Yet the very star around which (in the naked-eye view) 
 this nebula appears to cling, is figured in Galileo s 
 drawing of the belt and sword of Orion ! 
 
 It seems almost inconceivable that Galileo should 
 have overlooked the nebula, assuming its appearance 
 in his day to have resembled that which it has at pre 
 sent. And as it appears to have been established, that 
 if the nebula has changed at all during the past century 
 it has changed very slowly indeed, one can scarcely 
 believe that in Galileo s time it should have presented 
 a very different aspect. Is it possible that the view 
 suggested by Humboldt is correct that Galileo did 
 not see the nebula because he did not wish to see it ? 
 4 Galileo, says Humboldt, was disinclined to admit or 
 assume the existence of starless nebulae. Long after 
 the discovery of the great nebula in Andromeda- 
 known as the transcendently beautiful queen of the 
 nebulae Galileo omitted all mention in his works of 
 any but starry nebulse. The last-named nebula was 
 discovered in 1614, by Simon Marius, whose claims to 
 the discovery of Jupiter s satellites had greatly angered 
 Galileo, and had called forth a torrent of invective, in 
 which the Protestant German was abused as a heretic 
 by Galileo, little aware that he would himself before 
 long incur the displeasure of the Church. If we could 
 suppose that an unwillingness, either to confirm his 
 rival s discovery of a starless nebula, or to acknowledge 
 
82 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 that he had himself fallen into an error on the subject 
 of nebulse, prevented Galileo from speaking about the 
 great nebula in Orion, we should be compelled to form 
 but a low opinion of his honesty. It happens too 
 frequently that 
 
 The man of science himself is fonder of glory, and vain 
 An eye well practised in nature, a spirit bounded and poor. 
 
 That Hevelius, the star-cataloguer, should have 
 failed to detect the Orion nebula is far less remarkable; 
 for Hevelius objected to the use of telescopes in the 
 work of cataloguing stars. He determined the position 
 of each star by looking at the star through minute 
 holes or pinnules, at the ends of a long rod attached to 
 an instrument resembling the quadrant. 
 
 The actual discoverer of the great nebula was Huy- 
 ghens, in 1656. The description he gives of the dis 
 covery is so animated and interesting, that we shall 
 translate it at length. He says : 
 
 6 While I was observing the variable belts of Jupiter, 
 a dark band across the centre of Mars, and some indis 
 tinct phenomena on his disc, I detected among the 
 fixed stars an appearance resembling nothing which 
 had ever been seen before, so far as I am aware. 
 This phenomenon can only be seen with large tele 
 scopes such as I myself make use of. Astronomers 
 reckon that there are three stars in the sword of Orion, 
 which lie very close to each other. But as I was look 
 ing, in the year 1656, through my 23-feet telescope, at 
 the middle of the sword, I saw, in place of one star, 
 no less than twelve stars which indeed is no unusual 
 
THE GREAT NEBULA IN ORION. 83 
 
 occurrence with powerful telescopes. Three of these 
 stars seemed to be almost in contact, and with these 
 were four others which shone as through a haze, so 
 that the space around shone much more brightly than 
 the rest of the sky. And as the heavens were serene 
 and appeared very dark, there seemed to be a gap in 
 this part, through which a view was disclosed of 
 brighter heavens beyond. All this I have continued 
 to see up to the present time [the work in which these 
 remarks appear the Sy sterna Saturnium was publish 
 ed in 1659], so that this singular object, whatever it is, 
 may be inferred to remain constantly in that part of 
 the sky. I certainly have never seen anything resem 
 bling it in any other of the fixed stars. For other 
 objects once thought to be nebulous, and the Milky 
 Way itself, show no mistiness when looked at through 
 telescopes, nor are they anything but congeries of stars 
 thickly clustered together. 
 
 Huyghens does not seem to have noticed that the 
 space between the three stars he described as close 
 together is perfectly free from nebulous light insomuch 
 that if the nebula itself is rightly compared to a gap in 
 the darker heavens, this spot resembles a gap within 
 the nebula. And indeed, it is not uninteresting to 
 notice how comparatively inefficient was Huyghens 
 telescope, though it was nearly eight yards in focal 
 length. A good achromatic telescope two feet long 
 would reveal more than Huyghens was able to detect 
 with his unwieldy instrument. 
 
 Dominic Cassini soon after discovered a fourth star 
 
 G 2 
 
84 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 near the three seen by Huyghens. The four form the 
 celebrated trapezium, an object interesting to the pos 
 sessors of moderately good telescopes, and which has 
 also been a subject of close investigation by professed 
 astronomers. Besides the four stars seen by Cassini, 
 there have been found five minute stars within and 
 around the trapezium. These tiny objects seem to 
 shine with variable brilliancy ; for sometimes one will 
 surpass the rest, while at others it will be almost 
 invisible. 
 
 After Cassini s discovery, pictures were made of the 
 great nebula by Picard, Le Gentil, and Messier, These 
 present no features of special interest. It is as we 
 approach the present time, and find the great nebula 
 the centre of quite a little warfare among astronomers 
 now claimed as an ally by one party, now by their 
 opponents that we begin to attach an almost romantic 
 interest to the investigation of this remarkable object. 
 
 In the year 1811, Sir W. Herschel announced that 
 he had (as he supposed) detected changes in the Orion 
 nebula. The announcement appeared in connection 
 with a very remarkable theory respecting nebulas gene 
 rally Herschel s celebrated hypothesis of the conver 
 sion of some nebulae into stars. The astronomical 
 world now heard for the first time of that self-luminous 
 nebulous matter, distributed in a highly attenuated 
 form throughout the celestial regions, which Herschel 
 looked upon as the material from which the stars have 
 been originally formed. There is an allusion to this 
 theory in those words of the Princess Ida : 
 
THE GREAT NEBULA IN ORION. 85 
 
 There sinks the nebulous star we call the Sun, 
 If that hypothesis of theirs be sound. 
 
 And in the teaching of comely Psyche : 
 
 This world was once a fluid haze of light, 
 Till toward the centre set the starry tides, 
 And eddied into suns, that wheeling cast 
 The, planets. 
 
 Few theories have met with a stranger fate. Received 
 respectfully at first on the authority of the great astro 
 nomer who propounded it then in the zenith of his 
 fame the theory gradually found a place in nearly all 
 astronomical works. But, in the words of a distinguished 
 living astronomer, The bold hypothesis did not receive 
 that confirmation from the labours of subsequent in 
 quirers which is so remarkable in the case of many of 
 Herschel s other speculations. It came to pass at length 
 that the theory was looked upon by nearly all English 
 astronomers as wholly untenable. In Grermany it was 
 never abandoned, however, and a great modern discovery 
 has suddenly brought it into general favour, and has in 
 this, as in so many other instances, vindicated Herschel s 
 claim to be looked upon as the most clear-sighted, as 
 well as the boldest and most original of astronomical 
 theorisers. 
 
 Herschel had pointed out various circumstances 
 which, in his opinion, justified a belief in the existence 
 of a nebulous substance fire-mist or star-mist, as it 
 has been termed throughout interstellar space. He 
 had discovered and observed several thousand nebulae, 
 and he considered that amongst these he could detect 
 traces of progressive development. Some nebulae were, 
 
86 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 he supposed, comparatively young ; they showed no 
 signs of systematic aggregation or of central condensa 
 tion. In some nebulae he traced the approach towards 
 the formation of subordinate centres of attraction ; 
 while in others, again, a single centre began to be 
 noticeable. He showed the various steps by which 
 aggregation of the former kind might be supposed to 
 result in the formation of star-clusters, and condensa 
 tion of the latter kind into the formation of conspicuous 
 single stars. 
 
 But it was felt that the strongest part of Herschel s 
 case lay in his reference to the great nebula of Orion. 
 He pointed out that amongst all the nebulse which 
 might be reasonably assumed to be star-systems, a cer 
 tain proportionality had always been found to exist 
 between the telescope which first detected a nebula and 
 that which effected its resolution into stars. And this 
 was what might be expected to happen with star- 
 systems lying beyond our galactic system. But how 
 different is this from what was seen in the case of the 
 Orion nebula. Here is an object so brilliant as to be 
 visible to the naked eye, and which is found on exami 
 nation to cover a large region of the heavens. And 
 yet the most powerful telescopes had failed to show the 
 slightest symptom of resolution. Were we to believe 
 that we saw here a system of suns so far off that no 
 telescope could exhibit the separate existence of the 
 component luminaries, and therefore (considering merely 
 the observed extent of the nebula, which is undoubtedly 
 but a faint indication of its real dimensions) so incon- 
 
THE GREAT NEBULA IN ORION. 8 7 
 
 ceivably enormous in extent that the star-system of 
 which our sun is a member shrinks into nothingness in 
 comparison ? Surely it seemed far more reasonable to 
 recognise in the Orion nebula but a portion of our 
 galaxy, a portion very vast in extent, but far inferior 
 to that limitless ocean of universes presented to us 
 by the other view. 
 
 And when Sir W. Herschel was able, as he thought, 
 to point to changes taking place within the Orion 
 nebula, it seemed yet more improbable that the object 
 was a star-system. 
 
 But now telescopes more powerful than those with 
 which the elder Herschel had scanned the great nebula 
 were directed to its examination. Sir John Herschel, 
 following in his father s footsteps, applied himself to 
 the diligent survey of the marvellous nebula with a 
 reflecting telescope eighteen inches in aperture. He 
 presented the nebula to us as an object of which the 
 revelation of the ten-feet telescope was but the mere 
 rudiment. Strange outlying wisps and streamers of 
 light were seen, extending far out into space. Yet 
 
 I more strange seemed the internal constitution of the 
 object. So strange, indeed, did the nebula appear, so 
 unlike what had hitherto been known of collections of 
 stars, that Sir John Herschel considered the evidence 
 afforded by its appearance as sufficient to warrant the 
 conclusion of a non-stellar substance. 
 
 But this eminent astronomer obtained a yet better 
 view of the great nebula when he transported to the 
 Cape of Good Hope an instrument equal in power to 
 
88 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 tliat which he had applied to the northern heavens. 
 In the clear skies of the southern hemisphere the nebula 
 shines with a splendour far surpassing that which it has 
 in northern climes. It is also seen far higher above the 
 horizon. Thus the drawing which Sir J. Herschel was 
 able to execute during his three years residence at the 
 Cape is among the best views of the great nebula that 
 have ever been taken. But even under these favourable 
 circumstances, Sir John records that the nebula, 
 through his great reflector, showed not a symptom of 
 resolution. 
 
 Then Lassell turned his powerful mirror, two feet in 
 diameter, upon the unintelligible nebula. But though 
 he was able to execute a remarkable drawing of the 
 
 O 
 
 object, he could discern no trace of stellar constitution. 
 
 In 1845 Lord Eosse interrogated the great nebula 
 with his three-feet mirror. Marvellous was the com 
 plexity and splendour of the object revealed to him, but 
 not the trace of a star could be seen. 
 
 The end was not yet, however. Encouraged by the 
 success of the three-feet telescope, Lord Rosse com 
 menced the construction of one four times as powerful. 
 After long and persistent labours, and at a cost, it is 
 said, of thirty thousand pounds, the great Parsonstown 
 reflector, 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 turned 
 towards the Orion nebula. Professor Nichol was the 
 first who saw the mysterious object as pictured by the 
 
THE GREAT NEBULA IN ORION. 89 
 
 great mirror. Although the observation was not suc 
 cessful so far as the resolution of the nebula was con 
 cerned, yet Nichol s graphic account of the telescope s 
 performance is well worth reading : 
 
 4 Strongly attracted in youth by the lofty conceptions 
 of Herschel [he writes], I may be apt to surround the 
 incident I have to narrate with feelings in so far of a 
 personal origin and interest : but, unless I greatly 
 deceive myself, there are few who would view it other 
 wise than I. With an anxiety natural and profound, 
 the scientific world watched the examination of Orion 
 by the six-feet mirror; for the result had either to 
 confirm Herschel s hypothesis, in so far as human 
 insight ever could confirm it; or unfold among the 
 stellar groups a variety of constitution not indicated 
 by those in the neighbourhood of our galaxy. Although 
 Lord Eosse warned me that the circumstances of the 
 moment would not permit me to regard the decision 
 then given as absolutely final, I went in breathless 
 interest to the inspection. Not yet the veriest trace of 
 a star ! Unintelligible as ever, there the nebula lay ; 
 but how gorgeous its brighter parts ! How countless 
 those streamers branching from it on every side ! How 
 strange, especially that large horn on the north, rising 
 in relief from the black skies like a vast cumulous cloud ! 
 It was thus still possible that the nebula was irresolv 
 able by human art ; and so doubt remained. Why the 
 concurrence of every favourable condition is requisite for 
 success in such inquiries may be readily comprehended. 
 The object in view is to discern, singly, sparkling 
 
90 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 points, small as the point of a needle, and close as the 
 particles of a handful of sand ; so that it needs but the 
 smallest unsteadiness in the air, or imperfection in the 
 shape of the reflecting surface, to scatter the light of 
 each point, to merge them into each other, and present 
 them as one mass. 
 
 Before long Lord Eosse, after regrinding the great 
 mirror, obtained better views of the mysterious nebula. 
 Even now, however, he could use but half the power of 
 the telescope, yet at length the doubts of astronomers 
 as to the resolvability of the nebula were removed. 
 We could plainly see, he wrote to Professor Nichol, 
 that all about the trapezium was a mass of stars, the 
 rest of the nebula also abounding with stars, and ex 
 hibiting the characteristics of resolvability strongly 
 marked. These views were abundantly confirmed by 
 subsequent observations with the great mirror. 
 
 It will surprise many to learn that even Lord Eosse s 
 great reflector is surpassed in certain respects by some of 
 the exquisite refractors now constructed by opticians. 
 As a light-gatherer the mirror is, of course, unapproach 
 able by refractors. Even if it were possible to construct 
 an achromatic lens six feet in diameter, yet, to prevent 
 flexure, a thickness would have to be given to the glass 
 which would render it almost impervious to light and 
 therefore utterly useless. But refractors have a power 
 of definition not possessed by large reflectors. With a 
 refractor eight inches only in aperture, for instance, 
 Mr. Dawes has obtained better views of the planets (and 
 specially of Mars), than Lord Eosse s six-feet speculum 
 
THE GREAT NEBULA IN ORION. 91 
 
 would give under the most favourable circumstances. 
 And in like manner, the performance of Lord Rosse s 
 telescope on the Orion nebula has been surpassed so 
 far as resolution into discrete stars is concerned by 
 the exquisite denning power of the noble refractor of 
 Harvard College (U.S.). By means of this instrument 
 hundreds of stars have been counted within the confines 
 of the once intractable nebula. 
 
 It seemed, therefore, that all doubt had vanished 
 from the subject which had so long perplexed astrono 
 mers. That was proved to be real, Nichol wrote, 
 which, with conceptions of space enlarged even as 
 Herschel s, we deemed incomprehensible. 1 
 
 Yet even at this stage of the inquiry there were 
 found minds bold enough to question whether a per 
 fectly satisfactory solution of the great problem had 
 really been attained. Nor is it difficult, I think, to 
 point out strong reasons for such doubts. I shall con 
 tent myself by naming one which has always appeared 
 to me irresistible. 
 
 The Orion nebula as seen in powerful telescopes 
 covers a large extent of the celestial sphere. According 
 to the Padre Secchi, who observed it with the great 
 Merz refractor of the observatory at Rome, the nebulous 
 region covers a triangular space, the width of whose 
 base is some eight times, while its height is more than 
 ten times as great as the moon s apparent diameter, a 
 space more than fifty times greater than that covered 
 by the moon. Now, I do not say that it is inconceivable 
 that an outlying star-system, so far off as to be irre- 
 
92 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 solvable by any but the most powerful telescopes, should 
 cover so large a space on the heavens. On the contrary, 
 I do not believe that a galaxy resembling our own 
 would be resolvable at all, unless it were so near as to 
 appear much larger than the Orion nebula. I believe 
 astronomers have been wholly mistaken in considering 
 any of the nebulae to be such systems as our own. 
 There may be millions of such systems in space, but I 
 am very certain no telescope we could make would 
 suffice to resolve any of them. But what I do consider 
 inconceivable, is, that a nebula extending so widely, and 
 placed (as supposed) beyond our system, should yet 
 appear to cling (as the Orion nebula undoubtedly does) 
 around the fixed stars seen in the same field with it. 
 So strongly marked is this characteristic, that Sir John 
 Herschel (who failed, apparently, to see its meaning) 
 mentions amongst others no less than four stars, one of 
 which is the bright middle star of the belt as < involved 
 in strong nebulosity, while the intermediate nebulosity 
 is only just traceable. The probability that this 
 arrangement is accidental is so small as to be almost 
 evanescent. 
 
 However, as I have said, English astronomers, almost 
 without a dissentient voice, accepted the resolution of 
 the nebula as a proof that it represents a distant star- 
 system resembling our own galactic system, but far 
 surpassing it in magnitude. 
 
 The time came, however, when a new instrument, 
 more telling even than the telescope, was to be directed 
 upon the Orion nebula, and with very startling results. 
 
THE GREAT NEBULA IN ORION. 93 
 
 The spectroscope had revealed much respecting the 
 constitution of the fixed stars. We had learned that they 
 are suns resembling our own. It remained only to 
 show that the Orion nebula consists of similar suns, in 
 order to establish beyond all possibility of doubt the 
 theories which had been so complacently accepted. A 
 very different result rewarded the attempt, however. 
 When Dr. Huggins turned his spectroscope towards the 
 great nebula, he saw, in place of a spectrum resembling 
 the sun s, three bright lines only ! A spectrum of this 
 sort indicates that the source of light is a luminous 
 gas, so that whatever the Orion nebula may be, it is 
 most certainly not a congeries of suns resembling our 
 own. 
 
 It would be unwise to theorise at present on a result 
 so remarkable. Nor can we assert that Herschel s 
 speculations have been confirmed, though his general 
 reasoning has been abundantly justified. Astronomers 
 have yet to do much before they can interpret the 
 mysterious entity which adorns Orion s sword. On every 
 side, however, observations are being made which give 
 promise of the solution of this and kindred difficulties. 
 We have seen the light of comets analysed by the same 
 powerful instrument ; and we learn that the light from 
 the tail and coma is similar in quality (so far as obser 
 vation has yet extended) to that emitted from the 
 Orion nebula. We see, therefore, that in our own solar 
 system we have analogues of what has been revealed in 
 external space. I would not, indeed, go so far as to 
 assert that the Orion nebula is a nest of cometic 
 
94 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 systems ; but I may safely allege that there is now 
 not a particle of evidence that the nebula lies beyond 
 our galaxy. 
 
 Nor need we doubt the accuracy of Lord Rosse s 
 observations. More than a year before his death, 
 indeed, he mentioned to Dr. Huggins that the matter 
 of the great nebula in Orion had not been resolved by 
 his telescope. In some parts of the nebula he observed 
 a large number of exceedingly minute red stars. These 
 red stars, however, though apparently connected with 
 the irresolvable blue material of the nebula, yet seemed 
 to be distinct from it. 
 
 The whole subject seems to be as perplexing as any 
 that has ever been submitted to astronomers. Time, 
 however, will doubtless unravel the thread of the 
 mystery. We may safely leave the inquiry in the hands 
 of the able observers and physicists whose attention has 
 been for a long time directed towards it. And we need 
 only note, in conclusion, that in the southern hemi 
 sphere there exists an object equally mysterious the 
 great nebula round t\ Argus which has yielded similar 
 results when tested with the spectroscope. The examina 
 tion of this mysterious nebula, associated with the most 
 remarkable variable in the heavens a star which at one 
 time shines but as a fifth magnitude star, and at another 
 exceeds even the brilliant Canopus in splendour may, 
 for aught that is known, throw a new light on the con 
 stitution of the great Orion nebula. 
 
 From Fraser s Magazine for February 1869. 
 
THE SUN S TRUE ATMOSPHERE. 95 
 
 THE SUN S TRUE ATMOSPHERE. 
 
 So much attention was directed to the solar corona 
 during the discussions which preceded and followed 
 the late eclipse, that a discovery of extreme import 
 ance but not at all associated with the corona has 
 received far less attention than it deserves. The dis 
 covery I refer to is, in fact, more important in its bear 
 ing on problems of solar physics than any which has 
 been made since Kirchhoff first told us how to inter 
 pret the solar spectrum. It is also intimately con 
 nected with the labours of that eminent physicist. I 
 propose briefly to describe the nature of the discovery, 
 and then to discuss some of the results to which it 
 seems to point. 
 
 Astronomers have long seen reason to believe that 
 the sun has an atmosphere. And by the word atmo 
 sphere I mean something more than mere vaporous or 
 gaseous masses, such as the prominences have been 
 shown to be. A solar envelope, complete and con 
 tinuous as our own atmosphere, seems undoubtedly 
 suggested by the appearance which the sun s image 
 presents when thrown on a suitably prepared screen in 
 a darkened room; for then the disc is seen to be 
 shaded off continuously towards the edge, where its 
 brilliancy is scarcely half as great as at the centre. 
 The phenomenon is so readily seen, and so unmistake- 
 
96 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 able, that it is with a sense of wonder one hears that 
 Arago called it in question. To use the words of Sir 
 John Herschel, the fact is so palpable that it is a 
 matter of some astonishment that it could ever fail to 
 strike the most superficial observer. And, again, not 
 only the light but the heat of the outer portions of the 
 sun s image has been estimated. In this case we do 
 not depend upon the perhaps fallible evidence of the 
 eye, but on that of heat-measuring instruments. Fr. 
 Secchi, measuring the heat of different parts of the solar 
 image, has found that of the part near the centre nearly 
 double that from the borders. Lastly, photography 
 gives unmistakable evidence on the subject. 
 
 Now, when Kirchhoff discovered the meaning of the 
 solar spectrum, it seemed clear to him that he had 
 determined the nature and constitution of the solar 
 atmosphere. Let us consider the nature of Kirchhoff s 
 discovery. 
 
 He found that the dark lines across the rainbow- 
 tinted streak forming the background (as it were) of 
 the solar spectrum, are due to the action of absorbing 
 vapours. The vapours necessarily lie outside the source 
 of that part of the sun s light which produces the rain 
 bow-tinted streak. If those vapours could be removed 
 for a while, we should see a simple rainbow-riband of 
 light. Or if the vapours could be so heated as to be 
 no less hot than the matter beneath them which pro 
 duces the rainbow spectrum, they would no longer 
 cause any dark lines to appear ; but being cooler, and 
 so giving out less light than they intercept, they cut 
 
THE SUN S TRUE ATMOSPHERE. 97 
 
 out the dark spaces corresponding to their special 
 absorptive powers. To use Mr. Lockyer s striking, 
 though perhaps not strictly poetical, description of 
 their action, these vapours gobble up the light on its 
 way to the observer, so that it comes out with a 
 balance on the wrong side of the account. Each 
 vapour produces its own special set of lines, occupying 
 precisely those parts of the spectrum which the vapour s 
 light would illuminate if the vapour shone alone. For 
 these vapours, notwithstanding their action in inter 
 cepting or absorbing portions of the sunlight, are 
 themselves in reality glowing with a light so intense 
 that the human eye could not bear to rest upon it. If 
 we could examine the vapours we supposed just now 
 removed from the sun, we should obtain the very lines 
 of 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 light- 
 globe of the sun, he judged that they form the solar 
 atmosphere. Because, although his mode of observa 
 tion was not such as to assure him that these vapours 
 completely envelope the sun, yet the telescopic aspect 
 of the sun, and especially that darkening near the edge 
 to which I have just referred, seemed to leave room 
 for no other conclusion. But at this stage of the 
 inquiry Kirchhoff fell into a mistake. He judged that 
 the solar corona was the atmosphere which produced 
 the solar dark lines, as well as the darkening of the 
 sun s disc near the edge. The mistake is one which, 
 
 H 
 
98 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 as it seems to me, he would have avoided had he taken 
 into account the enormous pressure at which an atmo 
 sphere so extensive as the corona would necessarily 
 exist under the influence of the sun s mighty attractive 
 energies. It may easily be shown that if the outer 
 parts of the corona were as rare as the contents of our 
 so-called vacuum-tubes, or even a thousand times rarer, 
 yet according to the la,ws which regulate atmospheric 
 pressure, the density even at vast heights above the 
 sun s surface would attain to many hundred times that of 
 our heaviest gases. The pressure would, indeed, be so 
 great that we can see no way of escaping the conclu 
 sion that, despite the enormous heat, the gases com 
 posing the imagined atmosphere would be liquefied or 
 even solidified. 
 
 When the observers of the Indian eclipse of 1868 
 found that the coloured prominences are masses of 
 glowing hydrogen, with other gases intermixed, and 
 when the prominence-spectrum was found to show the 
 hydrogen lines as these appear when hydrogen exists 
 at very moderate pressures, KirchhorFs view had to be 
 abandoned as altogether untenable. Wherever the 
 vapours exist which produce the solar dark lines, they 
 are undoubtedly not to be looked for in the corona. 
 
 But there the lines are. The absorptive action is 
 exerted somewhere. The question is Where are the 
 absorptive vapours ? 
 
 At this stage of the inquiry, a very strange view was 
 expressed by Mr. Lockyer a view which appears to 
 have been founded on a slight misapprehension of the 
 
THE SUN S TRUE ATMOSPHERE. 99 
 
 principles of spectrum analysis. He put forward the 
 theory that the absorptive action takes place below the 
 level of the sun s surface as we see it. 
 
 But observations made by Fr. Secchi at Eome pointed 
 to a view so different from Mr. Lockyer s, as to lead to 
 a controversy which filled many pages of the Comptes 
 Rendus, of the Philosopical Magazine, and of other 
 publications a controversy qonducted, as too many 
 philosophical discussions have been, with a somewhat 
 unphilosophical acrimony. 
 
 Fr. Secchi had noticed that when the very edge of 
 the sun s disc is examined with the spectroscope, the 
 dark lines disappear from the spectrum, which thus 
 becomes a simple rainbow-tinted streak. He judged, 
 accordingly, that the absorbing atmosphere exists above 
 the sun s real surface ; for he believed that just at the 
 edge the bright lines corresponding to the light from 
 the vapours themselves so nearly equal in intensity the 
 light of the solar spectrum, that no signs of difference 
 can be detected ; or, in other words, that the dark lines 
 are obliterated. On the other hand, the glowing atmo 
 sphere cannot, he argued, reach much above the sun s 
 surface, since otherwise the spectroscope would show 
 the bright lines belonging to that atmosphere s light. 
 Now, no such lines are visible. So far as the spectro- 
 scopic evidence is concerned, it would appear as though 
 immediately above the sun s surface, as we see it, there 
 came the sierra that low range of prominence-matter, 
 which, strangely enough, some have regarded as an 
 atmospheric envelope. The spectrum of the sierra 
 
 H 2 
 
IOO LIGHT SCIENCE FOR LEISURE HOURS. 
 
 shows beyond all question that, like the prominences, 
 this region consists of glowing hydrogen, mixed up 
 with a few, and at times with several other gases, but 
 certainly not capable of accounting for the thousands 
 of dark lines in the solar spectrum. It seems quite 
 clear, also, that the sierra is not of the nature of an 
 envelope at all. 
 
 Over the narrow layer which Secchi supposed to 
 exist between the sun s surface and the coloured sierra, 
 began, and presently waxed warm, the controversy 
 above referred to. Fr. Secchi was positive that he 
 could see the narrow continuous spectrum on which he 
 founded his view; Mr. Lockyer was equally positive 
 that the worthy father could see nothing of the kind. 
 Fr. Secchi urged that his telescope was better than 
 Mr. Lockyer s, and that he worked in a better atmo 
 sphere; Mr. Lockyer retorted that his spectroscope 
 was better than Fr. Secchi s, and that the imagined 
 superiority of the Eoman atmosphere was a myth. 
 Something was said, too, by the London observer about 
 a large speculum, which was to decide the question, 
 though this mirror does not seem to have been actually 
 brought into action. Both the disputants expressed 
 full confidence that time would prove the justice of 
 their several views. 
 
 Soon after, an observation was made by Mr. Lockyer, 
 which seemed to prove the justice of Fr. Secchi s 
 opinion ; for, on a very favourable day for observations, 
 Mr. Lockyer was able to detect, not the narrow rain 
 bow-tinted spectrum seen by Secchi, but a narrow strip 
 
THE SUN S TRUE ATMOSPHERE. IOI 
 
 of spectrum belonging to the region just outside the 
 sun s edge, which showed hundreds of bright lines. 
 Here seemed to be conclusive evidence of that shallow 
 atmosphere of glowing vapours in which Fr. Secchi 
 had faith. But Mr. Lockyer interpreted his observa 
 tion differently. The presence of these vapours on 
 this particular occasion he regarded as wholly excep 
 tional, and the cause of the exception he held to be 
 the energetic injection of vapours from beneath the 
 surface of the sun. 
 
 At about this stage of the controversy I had occasion 
 to consider the problems associated with the physical 
 condition of the sun and his surroundings ; and although 
 I took no part in the discussion between Fr. Secchi and 
 Mr. Lockyer, I expressed (in papers which I wrote upon 
 the subject) opinions which agreed with the views of 
 the Italian astronomer. It is necessary for me to pre 
 sent in this place my own reasoning on the question at 
 issue, because it not only serves to introduce the special 
 observation made last December, by which the problem 
 has been finally solved, but also presents certain con 
 siderations which must be attended to in interpreting 
 that observation. 
 
 In the first place, I noted that the darkening of the 
 sun s disc near the edge, or rather the marked nature 
 of that darkening, instead of showing (as had been so 
 often stated) that the sun has a very deep atmosphere, 
 proves, on the contrary, that his atmosphere must be 
 exceedingly shallow by comparison with the dimen 
 sions of his globe. It is easy to show why this is ; and 
 
102 LIGHT SCIENCE FOR LEISURE HOURS. 
 although the considerations on which the matter de- 
 
 O 
 
 pends are exceedingly simple, yet the case is by no 
 means the first in which exceedingly simple considera 
 tions have been lost sight of by students of science. 
 Suppose we have a brightly-white globe encased sym 
 metrically within a globe of some imperfectly trans 
 parent substance as green glass. Now, if the white 
 globe is an inch in diameter and the green glass globe 
 a yard in diameter, the brightness of the white globe 
 will be more or less impaired according to the trans 
 parency of the glass ; but it will not be much more 
 impaired at the edge of the inner globe s disc than 
 near the middle. For clearly, when we look at the 
 middle, we look through a foot and a half of glass 
 (wanting only half an inch), and when we look at the 
 edge of the inner globe s disc, we also look through a 
 foot and a half of glass (wanting only a small fraction 
 of 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 enclosing 
 globe of glass cuts off as much light when we look at 
 the centre of the inner globe s disc as when we look at 
 the edge. But now suppose that the enclosing globe 
 forms a mere shell around the inner one. Suppose, for 
 instance, that the inner globe is a yard in diameter, 
 and the shell of glass only half an inch thick. Then 
 in this case, as in the former, the brightness of the 
 inner globe will be more or less impaired according to 
 the transparency of the glass ; but it will no longer be 
 affected equally whether we look at the middle or at 
 
THE SUN S TRUE ATMOSPHERE. 103 
 
 the edge of the inner globe s disc. In the former case 
 we only look though half an inch of glass, in the latter 
 we look through a much greater range of glass ; as the 
 reader will see at once if he draw two concentric circles 
 nearly equal in size to represent the inner globe and 
 its enclosing shell. It is easy to calculate how long 
 the range of glass actually is in the latter case. I have 
 just gone through the calculation, and find that when 
 the eye is directed to the edge of the enclosed globe, 
 its line of sight passes through rather more than four 
 inches and a quarter, so that more than eight times as 
 much light is absorbed as in the case where the eye 
 looks at the middle of the inner globe s disc, or directly 
 through half an inch of glass. 
 
 Now we cannot tell what proportion holds in the 
 case of the sun s disc, because we do not know how 
 much light has been absorbed where we look at the 
 middle of the disc. All we know is that whatever re 
 mains after such absorption is about twice as much as 
 we receive from near the edge of the disc. It is easily 
 seen that this knowledge is insufficient for our require 
 ments. But there can be no question whatever that 
 the total absorption near the edge exceeds many times 
 that near the middle of the disc ; and on very reason 
 able assumptions as to this excess, it may readily be 
 shown that the absorbing atmosphere cannot exceed 
 some five or six hundred miles in depth. Probably it 
 is even shallower. 
 
 Now, there is a circumstance which perfectly ac 
 counts for the non-recognition by spectroscopists of 
 
104 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 an atmosphere relatively so shallow as this. Let it be 
 remembered, in passing, that the average height of the 
 sierra may be set at about five thousand miles ; so that 
 the atmosphere we are dealing with would be at the 
 outside but one-fifth as high as that fine rim of red 
 light with saw-like edge which astronomers detected 
 around the eclipsed sun in the total eclipses of 1842, 
 1851, and 1860. Still it might be thought that 
 patience only would be needed to detect the signs of 
 such an atmosphere, shallow though it be. But there 
 is a peculiarity of telescopic observation which renders 
 the recognition of such an atmosphere, if of less than a 
 certain depth, not difficult merely, but impossible. It 
 may be well to exhibit the nature of the peculiarity at 
 length, because it is of considerable interest to all who 
 possess or use telescopes. I take an illustrative case, 
 which seems, at first, to have little connection with my 
 subject. 
 
 Every reader of this work has heard of the double 
 stars, and I dare say most of those who read this parti 
 cular article have seen many of these beautiful objects. 
 It is known that some double stars are much closer 
 than others, and we commonly hear it mentioned as a 
 proof of the excellence of a telescope that it will divide 
 such and such a double star. But it might seem that 
 if a telescope of a certain size were constructed with 
 extreme care, it should be capable of dividing any 
 double star ; because we might use an eye-piece of any 
 magnifying power we pleased, and so, as it were, force 
 apart the two star-images formed by the object-glass. 
 
THE SUN S TRUE ATMOSPHERE. 105 
 
 Instead of this being the case, however, there is a limit 
 for every object-glass, beyond which no separation is 
 possible ; for this reason, simply, that the star-images 
 formed by the object-glass are not points of light, as 
 they would be if they correctly represented the stars of 
 which they are the optical images. The larger the 
 object-glass (assumed to be perfect in construction) the 
 smaller is the star-image ; 1 but it has always a definite 
 size, and if this size is such that the two images of the 
 stars forming a pair actually touch or overlap, we can 
 not separate them by using highly-magnifying eye 
 pieces. 
 
 Now what is true of a star is true of every point of 
 any object we examine with a telescope. The image 
 of the point is always a circle of light, which, though 
 minute, has yet appreciable dimensions. The image 
 of the object is made up of all these circles, which 
 necessarily overlap. Nor let the reader suppose that 
 on this account telescopic observation is untrustworthy. 
 Precisely the same peculiarity affects ordinary vision. 
 There is no such thing as a perfect optical image of an 
 object; though neither eyesight nor telescopic vision 
 need be regarded as deceptive, on this account. Our 
 power of seeing minute details is limited by this 
 peculiarity, but we are not actually deceived. If 
 
 1 A curious illustration of this is given by the fact that a certain 
 astronomer of old, having reduced the aperture of his telescope to a 
 mere pin-hole, announced that he was thus enabled to measure the real 
 globes of the stars, for, instead of seeing the stars through his telescope 
 as minute points of light, he now saw them with discs like the planets. 
 He thought he was improving the defining qualities of his telescope, 
 instead of altogether destroying them. 
 
106 LIGHT SCIENCE FOE LEISURE HOURS. 
 
 microscopic writing be shown us, for instance, we may 
 find ourselves, after poring over it for some time, 
 unable to make out its meaning, the letters seeming 
 all blended together; but we know what our failure 
 really means, and do not fall into the mistake of 
 concluding that there are no details because the actual 
 details are inscrutable. 
 
 Let us apply this consideration to the sun, and more 
 particularly to the appearance presented by the edge 
 of the sun s disc. The image of every point of this 
 edge is a small circle ; the combination of all these 
 small circles must produce a ring of light all round 
 the true outline of the disc. If the sun s atmosphere 
 did not reach beyond this ring, then no contrivance 
 whatever could render the atmosphere discernible, let 
 the telescope be ever so perfect and the observer 
 ever so clear-sighted or skilful. Now, the actual ex 
 tension of this ring will be greater or less according as 
 the object-glass of the telescope is less or greater. It 
 may readily be shown that neither Mr. Lockyer s tele 
 scope nor Fr. Secchi s could possibly show any signs of 
 a solar atmosphere under two hundred miles in depth, 
 while in all probability an atmosphere four or five 
 times as deep would escape their scrutiny. 
 
 Are we then to remain altogether in ignorance of 
 such an atmosphere, supposing that it actually exists, 
 and that the dark lines in the solar spectrum are due 
 to its absorptive power ? Is there no way of obviating 
 the difficulty which has just been dealt with ? 
 
 So far as the method of observing the sun when 
 
THE SUN S TRUE ATMOSPHERE. 107 
 
 uneclipsed is concerned, the answer to these questions 
 must be negative ; or, rather, it must be answered that 
 our only hope of meeting the difficulty consists in 
 increasing the size of the telescopes with which the 
 sun is spectroscopically studied. And inasmuch as 
 Dr. Huggins is preparing to apply the powers of a much 
 larger telescope than either Mr. Lockyer s or Fr. 
 Secchi s, we may possibly still hope to hear that the 
 relatively shallow atmosphere can be studied when 
 the sun is not eclipsed. For we may now speak 
 of the existence of this atmosphere as a demonstrated 
 fact. The difficulty which seemed to present insuper 
 able obstacles to the observers who study the uneclipsed 
 sun, has been overcome by the ingenuity of one of the 
 most skilful of those very observers Professor Young, 
 of America when studying the solar eclipse of last 
 December. 
 
 If during any total eclipse of the sun, the moon just 
 concealed the whole of the sun s disc (as may well 
 happen), and if our satellite were only complaisant 
 enough to stay still for a few minutes in such a position, 
 so that one of these exact total eclipses could be 
 studied as readily as one of greater extent (which never 
 can happen), then the shallow atmosphere I have been 
 speaking of could be recognised. The difficulty above 
 considered would no longer exist. For the ring of 
 light which actually hides the shallow atmosphere when 
 the sun is not eclipsed, is an extension of the bright 
 rim of the disc outwards : if the disc is completely 
 hidden, there is no bright rim to be extended, and any- 
 
IOS LIGHT SCIENCE FOR LEISURE HOURS. 
 
 thing existing close by the sun s globe can be recog 
 nised. 
 
 But then, unfortunately, no total eclipse can present 
 these desirable features. If a total eclipse is to be 
 worth seeing at all, the moon s disc as seen at the time 
 must be appreciably larger than the sun s. When 
 totality begins the outlines of the two discs just touch 
 at a single point, and when totality ends the two discs 
 just touch at another point ; but during all the rest of 
 the totality the two outlines do not touch at all, that 
 of the moon surrounding without touching that of the 
 sun. The outlines of the two discs do twice touch, 
 however, in each case for one moment and at one point. 
 What Professor Young determined to do, therefore, was 
 to bring under special examination that one point 
 where the outlines touch at the exact moment when 
 totality begins. In other words, he directed his special 
 attention to the point where the last trace of the sun s 
 disc was about to disappear. It is perhaps scarcely 
 necessary to say that he did not trust to the powers of 
 his telescope, but that he employed a powerful spec 
 troscope. And further, he did not depend on his own 
 observations alone, but had adjusted a spectroscope for 
 the use of Mr. Pye, an English gentleman residing in 
 the part of Spain where the eclipse-observing parties 
 were stationed, so that that gentleman also might make 
 the required observations. 
 
 In his account, Professor Young does not mention 
 what he expected to see. It is probable that he had 
 m his thoughts the observations of Fr. Secchi, and 
 
THE SUN S TRUE ATMOSPHERE. 109 
 
 hoped to obtain evidence respecting that shallow at 
 mospheric envelope which Secchi believed in and 
 Lockyer rejected ; though it is quite possible he merely 
 desired to ascertain whether the constitution of the 
 lower part of the sierra differed in any marked respect 
 from that of the upper portion. As the moment 
 approached when the last fine sickle of sunlight was to 
 be obscured, the solar spectrum which was visible in 
 the spectroscopic field of view grew rapidly fainter. 
 The region actually examined by Professor Young was 
 in reality a narrow, almost linear space, touching the 
 edge of the sun s disc; so that before totality had com 
 menced he had the light from our own illuminated 
 atmosphere, and not direct sunlight, to deal with. 
 Thus he had just such a solar spectrum as is seen when 
 a spectroscope is directed to the sky in the daytime. 
 But as the moment of totality drew near, the illumina 
 tion of the atmosphere, and with it the brightness of 
 the rainbow-tinted streak, rapidly diminished. At last 
 the solar spectrum vanished ; and then What was it 
 replaced 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 crossed by thousands 
 and thousands of dark lines, there appeared a new and 
 most beautiful spectrum a riband of rainbow-tinted 
 lines, thousands in number and of all degrees of thick 
 ness, hundreds of red lines, and then, in order, 
 hundreds of orange lines, hundreds of yellow, green, 
 indigo, and violet lines, like coloured cross-threads on a 
 black riband, only infinitely more beautiful. A charm- 
 
110 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 ing spectacle, truly, but so short-lived that no man can 
 ever hope, though he lived to four-score years and ten, 
 to let his eyes rest in all his life for more than ten or 
 twelve seconds on the beautiful array of coloured lines 
 which two men only have as yet beheld. We may in 
 crease the dimensions and power of our telescopes until 
 the existence of these lines can be recognised without 
 the aid of eclipse-darkness, but the lines can never be 
 seen, save during eclipse, as Young and his colleague 
 saw them last December. And these observers tell us 
 that in a second or two the lines vanished, the ad 
 vancing moon hiding the shallow solar atmosphere. If 
 it should ever be given to any man to see six total 
 eclipses (which has never yet happened to any), and to 
 successfully apply in each instance the method em 
 ployed by Professor Young, then in all, during his life, 
 that man would have seen the beautiful line-spectrum 
 to perfection for some ten or twelve seconds ; but not 
 otherwise can even so long a total period of observation 
 be secured. No single observer, then, can hope to 
 lear^i much about the thousands of lines which have 
 still to be mapped during eclipse opportunities. 
 
 But now let us consider the import of the observa 
 tion. What are these myriads of coloured lines? 
 Every dark line of the solar spectrum, says Professor 
 Young, seemed to have its representative in this 
 bright-line spectrum. Many of the groups of lines 
 which had flashed so quickly into view and endured 
 but so brief a period, were familiar to him ; in other 
 words, his study of the solar spectrum had made him 
 
THE SUN S TRUE ATMOSPHERE. ill 
 
 conversant with the corresponding groups of dark lines. 
 It follows, then, beyond all possibility of question, that 
 the source of light was a highly complex atmosphere, 
 formed of those very vapours which, by their absorptive 
 power, produce the dark lines formed, that is, of the 
 vapours of iron and of copper, of zinc, sodium, magne 
 sium, and of all those elements whose presence in the 
 sun s substance had been inferred from the study of 
 the solar spectrum. 
 
 Here, then, at length we have the true solar atmo 
 sphere, an atmosphere of a highly complex nature, and 
 doubtless exceedingly dense near the visible surface of 
 the sun, because subject to a pressure so enormous. 
 The upper limit of this atmosphere cannot lie very far 
 above the sun s surface, at least not very far compared 
 with the sun s dimensions. Supposing the actual time 
 during which the line-spectrum was visible to have been 
 two seconds, then it is easy to tell how deep the atmo 
 sphere is. For in two seconds the moon must have 
 traversed a space corresponding to about three hun 
 dred miles at the sun s distance. An atmosphere three 
 hundred miles deep is, therefore, indicated by Pro 
 fessor Young s observations. It need hardly be said, 
 however, that in the excitement of eclipse observation, 
 the estimate of minute intervals of time can scarcely 
 be relied upon, unless checked by instrumental arrange 
 ments, which was not the case in the present instance. 
 We may fairly conclude that the depth of the solar 
 atmosphere lies between some such limits as a hundred 
 miles and five hundred miles. 
 
112 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 In the above estimate, I have supposed the measure 
 ment to be made from the sun s visible surface. But 
 it is very unlikely that that surface is the true lower 
 limit of the atmosphere. It seems far more probable 
 that the surface we see is merely a layer of clouds (as 
 Sir William Herschel suggested so long ago) in the 
 solar atmosphere, and that the actual depth of the 
 atmosphere is more truly indicated by the appearances 
 seen when large sun-spots are examined. That these 
 spots are cavities has been abundantly established. 
 That they are openings through layers of solar clouds 
 has not been indeed demonstrated, yet it is difficult to 
 conceive how they can otherwise be interpreted. As 
 to the way in which the spots are formed, theorists are 
 at issue, some urging that there is an uprush from 
 depths beneath the solar surface ; others, that there is 
 a downrush of matter from without. But neither of 
 these views is in any way incompatible with Herschel s 
 theory, that the spots are openings in solar cloud- 
 layers. 
 
 We might thus be led to compare the solar atmo 
 sphere with our own, though it will of course be 
 obvious that there are many marked points of differ 
 ence. But in our own atmosphere we have at least 
 two distinct cloud-levels, the region, namely, where the 
 cumulus or wool-pack clouds are formed, and that 
 where the cirrus or feathery clouds, make their ap 
 pearance. There is air above the cirrus clouds, air be 
 tween the cirrus and cumulus layers, and air between 
 the cumulus clouds and the earth. And precisely in 
 
THE SUN S TRUE ATMOSPHERE. 113 
 
 the same way we may conceive that there exists at all 
 times a solar atmospheric region beneath as well as 
 above the cloud-layer which forms the sun s visible 
 surface, and beneath and between the other cloud- 
 layers revealed by telescopic observations. 
 
 But passing from the very difficult question sug 
 gested by the consideration of regions below the sun s 
 visible surface, let us discuss briefly the bearing of 
 Professor Young s discovery upon our views respecting 
 those outer regions the coloured prominences and 
 sierra, the corona itself, and, in fine, all the portions 
 of space which lie above the true atmosphere. 
 
 In the first place, it seems to me that the observa 
 tions made during the late eclipse dispose finally of 
 the theory that the coloured sierra is an atmospheric 
 envelope, properly so-called. I had long since been 
 led to question whether the sierra could be so regarded. 
 Let me remind the reader that the sierra is nothing- 
 more nor less than the region which Lockyer redis 
 covered in 1868. It had, in fact, been recognised by 
 telescopists since 1806, the name sierra having been 
 given to it by the observers of the eclipse of 1842. It 
 is a red region, having (as its name implies) a serrated 
 upper surface, as seen in the telescope, and seemingly 
 extending all round the sun s disc. The red pro 
 minences appear to spring from its upper surface. 
 Strangely enough, when Lockyer made his ingenious 
 observations of the coloured prominences, he had not 
 heard of this discovery, or had forgotten it. Accord 
 ingly, finding traces of prominence-matter all round the 
 
 I 
 
114 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 sun, he concluded that there was a continuous envelope 
 of hydrogen (mixed with some other gases) surround 
 ing the whole of the sun s globe. It was probably 
 through being misled by this supposition that he gave 
 to the sierra a new name entitling it the chromo 
 sphere announcing at the same time that its upper 
 surface was smooth in outline. Eespighi, the eminent 
 Italian spectroscopist also working, it would seem, 
 in ignorance or forgetfulness of the prior recognition 
 of the layer announced presently that the upper 
 surface of the so-called chromosphere 1 was altogether 
 irregular more irregular, in fact, than the surface of 
 a tempest-tossed sea. On re-examining the sierra, 
 Mr. Lockyer found this to be the case. But perhaps 
 the most striking evidence as to the real aspect of the 
 sierra was 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 complete 
 semicircle of sierra, and noted that this beautiful 
 coloured crescent was formed of multitudes of minute 
 
 1 It affords strange evidence of the caution with which new names 
 should be suggested, that this name, embodying, as we see, an erroneous 
 theory, and also perpetuating the remembrance of a mistaken claim, is 
 scarcely yet beginning to fall into disuse. Perhaps its Greek origin and 
 its length may have something to do with this ; for although astronomy 
 at least descriptive astronomy has hitherto not been disfigured by 
 the hideous nomenclature which botanists and geologists seem to rejoice 
 in, yet there is always a large class of science students who delight in 
 sesqui pedal names, as giving an air of profundity to their discourse. 
 It may even be dangerous to hint that the true form of the compound 
 for a colour-sphere is not chromo -sphere, but chromato- sphere, since the 
 extra syllable will multiply tenfold the favour with which the compound 
 is accepted. When will the tyro learn that the true lover of science 
 Projicit ampullas et sesquipedalia verba ? 
 
THE SUN S TRUE ATMOSPHERE. 115 
 
 prominences. This agrees very satisfactorily with my 
 own anticipatory description of the probable nature of 
 the sierra, when I suggested that the sun s surface is 
 probably covered at all times with small prominences, 
 bearing somewhat the same relation to the gigantic 
 " horns " and " boomerangs " seen during eclipses 
 that the bushes covering certain forest regions bear to 
 the trees. 
 
 But the larger prominences have been shown by 
 Zollner and Eespighi to be phenomena of eruption. 
 They are masses of glowing gas, which have been flung 
 from great depths beneath the visible surface of the 
 sun. May we not conclude that the smaller prominences 
 which constitute the sierra are of like nature ? that they 
 also have been flung from beneath the sun s visible sur 
 face ? As respects the larger prominences we can have 
 no manner of doubt, because they have been seen to be 
 flung out in eruptive sort. And this refers to all orders 
 of prominences, except only those very numerous and 
 relatively very small prominences which crowd together 
 so as to form the seemingly continuous coloured sierra. 
 These cannot be watched as the others have been. But 
 it seems highly probable that those among them whicli 
 are not the remains of loftier prominences, are, like 
 their larger fellows, phenomena of eruption. 
 
 Again, as respects the corona, all the evidence we 
 have is opposed to the conception that the phenomenon 
 is atmospheric. It shows two regions, which, though 
 not separated by well-defined limits from each other, 
 may yet be regarded as, in a sense, distinct. There is 
 
Il6 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 an inner and brighter portion, which the sesquipedalians 
 have proposed to call the leucosphere, apparently on 
 the lucus a non lucendo principle, for it is neither 
 white nor spherical. And there is the outer portion, 
 much less brilliant, and much more strikingly radiated. 
 Neither one part nor the other presents a single feature 
 suggestive of an atmospheric nature ; * and the cer 
 tainty that the two portions belong to a single object 
 affords yet more conclusive evidence against this in 
 terpretation of the corona. But the rays of the corona 
 are of a somewhat remarkable nature. When well seen, 
 as during the eclipse of 1868, they are pointed ; and 
 even during so unfavourable an eclipse as that of 
 December last, the dark spaces between the rays are 
 seen to widen rapidly with increased distance from the 
 sun. These pointed radiations serve to show that 
 coronal rays must be, in reality, shaped somewhat as 
 cones, having their bases towards the sun. The idea 
 is startling enough, but, admitting the accuracy of the 
 pictures made during well-seen eclipses, and of the 
 Astronomer-Royal s account of the corona during 
 the eclipses of 1851 and 1860, there is no escape 
 from the conclusion here stated. It is not more certain 
 that the sun is a globe, and not a flat disc as he 
 seems to be, than that the coronal radiations are not 
 flat pointed rays, but cone-shaped. Yet no one will 
 suppose that there are a number of monstrous cone- 
 
 1 I am here referring to the possibility that the corona may be due to 
 some species of solar atmosphere. The theory that the corona is due 
 to light in our own atmosphere has now at length been definitely aban 
 doned by all astronomers. 
 
THE SUN S TRUE ATMOSPHERE. 117 
 
 shaped masses atmospheric or otherwise standing, 
 as it were, upon the sun s surface. I can see no other 
 way of accounting for these conical extensions than by 
 regarding them as phenomena indicating some form of 
 repulsive action exerted by the sun. 
 
 But whatever opinion we may form on this and 
 kindred problems, it seems clear that we must regard 
 the envelope discovered by Professor Young as the only 
 true solar atmosphere : and a very strange and com 
 plex atmosphere it is. Nothing yet learned respecting 
 the sun s surroundings surpasses in interest this fiery 
 envelope, in which some of the most familiar of our 
 metals appear as glowing vapours. If anything could 
 add to the interest attaching to the coloured pro 
 minences and sierra, it is the fact now revealed that 
 they are propelled through this wonderful envelope, 
 over which they float for a while with strangely chang 
 ing figure. Truly the study of solar physics, which 
 twenty years ago seemed at a stand-still, is advancing 
 with rapid strides ; and it seems scarcely possible to 
 exaggerate the interest either of what has been already 
 revealed, or of the discoveries which are likely to be 
 effected during the approaching eclipse. 
 
 From the St. PauTs Magazine for May 1871. 
 
 ADDENDUM. Doubts were urged, for some time after 
 this paper appeared, as to the reality of Young s dis 
 covery. But during the total eclipse of December 1871, 
 and yet again during the annular eclipse of 1872, 
 decisive evidence was obtained in its favour, and it is 
 now received by all. 
 
Il8 LIGHT SCIENCE FOR LEISURE HOURS, 
 
 SOMETHING WRONG WITH THE SUN. 
 
 WHEN we consider the intense heat which has prevailed 
 in Europe during July, and the circumstance that in 
 America also the heat has been excessive, insomuch 
 that in New York the number of deaths during the 
 week ending July 6 was three times greater than the 
 average, we are naturally led to the conclusion that 
 the sun himself is giving out more heat than usual. 
 Though not endorsing such an opinion, which, indeed, 
 is not warranted by the facts, since terrestrial causes 
 are quite sufficient to explain the recent unusual heats, 
 we cannot refrain from noting, as at least a curious 
 coincidence, that at the very time when the heat has 
 been so great, the great central luminary of the solar 
 system has been the scene of a very remarkable dis 
 turbance an event, in fact, altogether unlike any 
 which astronomers have hitherto observed. 
 
 Now certain Italian spectroscopists Respighi, Sec- 
 chi, Tacchini, and others have set themselves the 
 task of keeping a continual watch upon the chromato- 
 sphere. They draw pictures of it, and of the mighty 
 coloured prominences which are from time to time 
 upreared out of, or through, the chromatospheric en 
 velope. They note the vapours which are present, as 
 well as what can be learned of the heat at which these 
 vapours exist, their pressure, their rate of motion, and 
 
SOMETHING WRONG WITH THE SUN. 119 
 
 other like circumstances. It was while engaged in 
 some of the more difficult and delicate of these tasks 
 that Tacchini noticed the strange occurrence now to 
 be described. 
 
 6 1 have observed a phenomenon, he says, which is 
 altogether new in the whole series of my observations. 
 Since May 6, I had found certain regions in the sun 
 remarkable for the presence of magnesium. Some of 
 these extended half-way round the sun. This state of 
 things continued, the extension of these magnesium 
 regions gradually growing greater, until at length, on 
 June 18, says Tacchini, I was able to recognise the 
 presence of magnesium quite round the sun that is 
 to say, the chromatosphere was completely invaded 
 by the vapour of this metal. This ebullition was ac 
 companied by an absence of the coloured prominences, 
 while, on the contrary, the flames of the chromato 
 sphere were very marked and brilliant. It seemed to 
 me as though I could see the surface of our great 
 source of light renewing itself. While this was going 
 on Tacchini noticed (as had frequently happened before 
 in his experience) that the bright streaks on the sun 
 which are called faculas were particularly brilliant 
 close to those parts of the edge of the disc where the 
 flames of the chromatosphere were most splendid and 
 characteristic. The granulations also, which the as 
 tronomer can recognise all over the sun, when a large 
 telescope is employed, were unusually distinct. 
 
 Tacchini concludes (and the inference seems just) 
 that there had not been a number of local eruptions of 
 
120 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 magnesium vapour, but complete expulsions. Only 
 we would venture to substitute for the word expul 
 sion the expression outflow or uprising, since it 
 may well be that these vapours rise by a quiet process 
 resembling evaporation, and not by any action so violent 
 that it could properly be regarded as expulsive. 
 
 In whatever way, however, the glowing vapour of 
 magnesium thus streamed into the envelope of the sun, 
 it would seem that the aspect of our luminary was 
 modified by the process not indeed in a very striking 
 manner, or our observers in England would have noticed 
 the change, yet appreciably. 6 More than one person, 
 says Tacchini, has told me that the light of the sun 
 has not at present its ordinary aspect ; and at the 
 Observatory we have judged that we might make the 
 same remark. The change must be attributed to mag 
 nesium. 
 
 It is impossible to consider attentively the remark 
 able occurrence recorded by Tacchini without being 
 struck by the evidence which it affords of solar muta 
 bility. We know that during thousands of years our 
 sun has poured forth his light and heat upon the 
 worlds which circle around him, and that there has 
 been no marked intermittence of the supply. We hear, 
 indeed, of occasions when the sun has been darkened 
 for a while ; and we have abundant reasons for be 
 lieving that he has at times been so spot-covered that 
 there has been a notable diminution of the supply of 
 light and heat for several days together. Yet we 
 have had no reasons for anticipating that our sun 
 
SOMETHING WRONG WITH THE SUN. 121 
 
 might permanently lose so much of his heat and lustre 
 that the inhabitants of earth would suffer. Tacchini s 
 observation reminds us, however, that processes are at 
 work upon the sun which admit of being checked or 
 increased, interrupted altogether or exaggerated so 
 violently, that the whole aspect of the sun, his condition 
 as the fire and lamp of the planetary system, may 
 be seriously affected. 
 
 If we only remember that our sun is one of the stars, 
 not in any way distinguished, unless perhaps by relative 
 insignificance, from the greater number of the stars 
 which illuminate our skies at night 01 are revealed 
 by the telescope, we shall learn to recognise the possi 
 bility that he may undergo marked changes. There 
 are stars which after shining with apparent steadiness 
 for thousands of years (possibly for millions of years 
 before astronomy was thought of), have become sud 
 denly much reduced in brightness, or after a few 
 flickering^ (as it were) have gone out altogether. 
 There are others which have shone with equal steadi 
 ness, and have then suddenly blazed out for a while 
 with a lustre exceeding a hundredfold that which they 
 formerly possessed. It would be equally unpleasant 
 for ourselves whether the sun suddenly lost the best 
 part of his light, and presently went out altogether, or 
 whether he suddenly grew fiftyfold brighter and hotter 
 than he now is. Yet in the present position of sidereal 
 astronomy, it is quite impossible to assert confidently 
 that one event or the other might not take place at 
 any time. 
 
122 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 Fortunately, we may view this matter (just as 
 astronomers have learned to view the prospect of mis 
 chievous collisions with comets) as a question of proba 
 bilities. Among so many thousands of stars there have 
 been so many sudden outbursts of light and fire, so 
 many sudden defalcations of splendour. Our sun is 
 one of those thousands, and so far as we know takes 
 his chance with the rest. 
 sssssae^^ From the Spectator for August 1872. 
 
 ~NEWS FROM IIERSCHEL S PLANET. 
 
 SATURN the altissimus planeta of the ancients re 
 mains still the most distant planet respecting whose 
 physical condition astronomers can obtain satisfactory 
 information. The most powerful telescopes yet con 
 structed have been turned in vain towards those two 
 mighty orbs which circle outside the path of distant 
 Saturn : from beyond the vast depths which separate us 
 from Uranus and Neptune, telescopists can obtain little 
 intelligence respecting the physical habitudes of either 
 planet. Nor need we be surprised at the failure of 
 astronomers, when we consider the difficulties under 
 which the inquiry has been conducted. In comparing 
 the telescopic aspect of Uranus with that of Saturn (for 
 example) we must remember that Uranus is not only 
 twice as far from the earth but also twice as far from 
 
NEWS FROM HERSCHEL S PLANET. 12$ 
 
 the sun as Saturn is. So that the features of Uranus 
 are not merely reduced in seeming dimensions, in the 
 proportion of about one to four, but they are less 
 brilliantly illuminated in the same proportion. And 
 therefore (roughly) any given portion of the surface of 
 Uranus say a hundred miles square near the middle of 
 his visible disc sends to us but about one-sixteenth 
 part of the light which an equal and similarly-placed 
 portion of the surface of Saturn would send to us. 
 Now every astronomer knows how difficult it is, even 
 with very powerful telescopes, to study the physical 
 features of Saturn. A telescope of moderate power will 
 show us his ring-system and some of his satellites ; but 
 to study the belts which mark his surface, the aspect of 
 his polar regions, and in particular those delicate tints 
 which characterise various portions of his disc, requires 
 a telescope of great power. It will be understood, 
 therefore, that in the case of Uranus, which receives so 
 much less light from the sun, and is so much farther 
 from us, even the best telescopes yet made by man must 
 fail to reveal any features of interest. We may add 
 also that Uranus is a much smaller planet than Saturn, 
 though far larger than the combined volume of all the 
 four planets, Mars, Venus, the Earth, and Mercury. If 
 Saturn (without his rings) and Uranus were both visible 
 together in the same telescopic field (a circumstance 
 which may from time to time happen) the Herschelian 
 planet would appear so small and faint that it might 
 readily be taken for one of Saturn s moons, the ringed 
 
124 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 planet sending us altogether some sixty times as much 
 light as Uranus. 
 
 But what the telescope had hitherto failed to accom 
 plish, has just been achieved by means of that wonderful 
 ally of the telescope, the spectroscope, in the able hands 
 of the eminent astronomer and physicist, Dr. Huggins. 
 News has been received about the constitution of the 
 atmosphere of Uranus, and news so strange (apart from 
 the strangeness of the mere fact that any information 
 could be gained at all respecting a vaporous envelope 
 so far away) as to lead us to speculate somewhat curi 
 ously respecting the conditions under which the Ura- 
 nians, if there are any, have their being. 
 
 Before describing the results of Dr. Huggins s late 
 study of the planet, it may be well to give a brief 
 account of what is known respecting Uranus. 
 
 The question has been raised whether Uranus was 
 known to the astronomers of old times. There 
 is nothing altogether improbable in the supposi 
 tion that in countries where the skies are unusually 
 clear, the planet might have been detected by its 
 motions. Even in our latitude Uranus can be quite 
 readily seen on clear and moonless nights, when favour 
 ably situated. He shines at such times as a star of 
 about the fifth magnitude that is, somewhat more 
 brightly than the faintest stars visible to the naked 
 eye. In the clear skies of more southerly latitudes he 
 would appear a sufficiently conspicuous object, though, 
 of course, it would be wholly impossible for even the 
 most keen-sighted observer to recognise any difference 
 
NEWS FROM HERSCHEL S PLANET. 125 
 
 between the aspect of the planet and that of a star of 
 equal brightness. The steadiness of the light of Saturn 
 causes this planet to present a very marked contrast 
 with the first magnitude stars whose lustre nearly 
 equals his own. But although the stars of the lower 
 orders of magnitude scintillate like the leading orbs, 
 their scintillations are not equally distinguishable by 
 the unaided eye. Nor is it unlikely that if Uranus 
 were carefully watched (without telescopic aid) he 
 would appear to scintillate slightly. Uranus would 
 only be recognisable as a planet by his movements. 
 There seems little reason for doubting, however, that 
 even the motions of so faint a star might have been 
 recognised by some of the ancient astronomers, whose 
 chief occupation consisted in the actual study of the 
 star groups. We might thus understand the Burmese 
 tradition that there are eight planets, the sun, the moon, 
 Mercury, Venus, Jupiter, and Saturn, and another 
 named Rahu which is invisible. If Uranus was actually 
 discovered by ancient astronomers, it seems far from un 
 likely that the planet was only discovered to be lost 
 again, and perhaps within a very short time. For if any 
 thing positive had been learned respecting the revolu 
 tion of this distant orb, the same tradition which 
 recorded discovery of the planet would probably have 
 recorded 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 tradition 
 relates to Uranus, Sir William Herschel must be 
 stripped of his honours. The rediscovery of a lost 
 
126 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 planet, especially of one which had remained concealed 
 for so many centuries, must be regarded as at least as 
 interesting as the discovery of a planet altogether un 
 known. Nor was there any circumstance in the actual 
 discovery of Uranus, which would lose its interest, even 
 though w r e accepted quite certainly the conclusion that 
 the Herschelian planet was no other than old Bahu. 1 
 
 Let us turn to Herschel s own narrative of his detec 
 tion of Uranus. It is in many respects very instructive. 
 
 In the first place, we must note the nature of the 
 work he was engaged upon. He had conceived the idea 
 of measuring the distances of the stars, or at least of 
 the nearer stars, by noting whether as the earth circles 
 around the sun the relative positions of stars lying very 
 close to each other seemed to vary in any degree. To 
 this end he was searching the heavens for those objects 
 which we now call double stars, most of which were in 
 his day supposed to be not in reality pairs of stars 
 that is, not physically associated together but seen 
 near together only because lying nearly in the same 
 direction. The brighter star of a pair was in fact sup- 
 
 1 It is, after all, at least as likely that Bahu assuming there really 
 was a planet known under this name might have been Vesta, the 
 brightest of the small planets which circle between Mars and Jupiter, 
 as the distant and slow-moving Uranus. For although Vesta is not 
 nearly so bright as Uranus, shining, indeed, only as a star of the 
 seventh magnitude, yet she can at times be seen without telescopic aid 
 by persons of extremely good sight ; and her movements are far more 
 rapid than those of Uranus. In the high table-lands of those eastern, 
 countries, where some place the birth of astronomy, keen-sighted ob 
 servers might quite readily have discovered her planetary nature, 
 whereas the slow movements of Uranus would probably have escaped 
 their notice. 
 
NEWS FROM HERSCHEL S PLANET. 127 
 
 posed to lie very much nearer than the fainter ; and it 
 was because, being so much nearer, the brighter star 
 should be much more affected (seemingly) by the 
 earth s motion around the sun, that Herschel hoped to 
 learn much by studying the aspect of these unequal 
 double stars at different seasons of the year. He hoped 
 yet more from the study of such bright orbs as are 
 surrounded by several very faint stars. It was a case 
 of this kind that he was dealing with, when accident 
 led him to the discovery of Uranus. On Tuesday, the 
 13th of March (1781), he writes, between ten and 
 eleven in the evening, while I was examining the small 
 stars in the neighbourhood of Eta in Gemini, I per 
 ceived one that appeared visibly larger than the rest. 
 Being struck with its uncommon magnitude, I com 
 pared it to Eta and the small stars in the quartile 
 between Auriga and Gemini, and finding it so much 
 larger than either of them, suspected it to be a comet. 
 I was then engaged in a series of observations (which 
 I hope soon to have the opportunity of laying before 
 the Royal Society) requiring very high powers, and I 
 had ready at hand the several magnifiers of 227, 660, 
 932, 1,536, 2,010, &c., all of which I have successfully 
 used on that occasion. The power I had on when I 
 first saw the (supposed) comet was 227. From experi 
 ence I knew that the diameters of the fixed stars are 
 not proportionally magnified with higher powers, as 
 those of the planets are ; therefore I now put on the 
 powers of 660 and 932, and found the diameter of the 
 comet increased in proportion to the power, as it ought 
 
128 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 to be on a supposition of its not being a fixed star, 
 while the diameters of the stars to which I compared 
 it were not increased in the same ratio. Moreover, the 
 comet being magnified much beyond what its light 
 would admit of, appeared hazy and ill-defined with 
 these great powers, while the stars presented that lustre 
 and distinctness which from many thousand observa 
 tions I knew they would retain. The sequel has shown 
 that my surmises were well-founded. 
 
 There are three points to be specially noted in this 
 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 properly 
 regarded as accidental, although Herschel was not at 
 the time on the look-out for as yet unknown planets. 
 Secondly, the instruments he was employing were of 
 his own construction and device, and probably no 
 other in existence in his day would have led him to 
 the discovery that the strange orb was not a fixed star. 
 And thirdly, without the experience he had acquired 
 in the study of the heavens he would not have been 
 able to apply the test which, as we have seen, he found 
 so decisive. The fact that the stars are not magnified 
 by increased telescopic power to the same extent as 
 planets or comets, is, as Professor Pritchard has justly 
 remarked, an important result of the undulatory theory 
 of light, and was unsuspected in Sir William Herschel s 
 day. So that whether we consider the work Herschel 
 was engaged upon, the instruments he used, or the ex 
 perience he had acquired, we recognise the fact that he 
 
NEWS FROM HERSCHEL S PLANET. 129 
 
 alone of the astronomers of his time was capable of dis 
 covering Uranus otherwise than by a fortunate accident. 
 Others might have lighted on the discovery indeed, 
 we shall presently see that the real wonder is that 
 Uranus had not been for many years a recognised mem 
 ber of the solar system but no one except Herschel 
 could within a few minutes of his first view of the planet 
 have pronounced confidently that the strange orb (what 
 ever it might be) was not a fixed star. 
 
 I do not propose to enter here, at length, into the 
 series of researches by which it was finally demonstrated 
 that the newly-discovered body was not a comet but a 
 planet, travelling on a nearly circular path around the 
 sun, at about twice Saturn s distance from that orb. 
 With this part of the work Herschel had very little to 
 do. To use Professor Pritchard s words, having ascer 
 tained the apparent size, position, and motion of the 
 stranger, Herschel very properly consigned it to the 
 care of those professional astronomers who possessed 
 fixed instruments of precision in properly constituted 
 observatories to Dr. Maskelyne, for instance, who was 
 then the Astronomer-Royal at Greenwich, and to 
 Lalande, who presided over the observatory in Paris. 
 As the newly-discovered body travelled onwards upon 
 its apparent path, astronomers gradually acquired the 
 means of determining what its real path might be. 
 At first they were misled by erroneous measures of the 
 stranger s apparent size, which suggested that the sup 
 posed comet had in the course of the first month after 
 its discovery approached to within half its original 
 
130 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 distance. At length, setting aside all these measures, 
 and considering only the movements of the stranger, 
 Professor Saron was led to the belief that it was no 
 comet, but a member of the solar system. It was 
 eventually proved, chiefly by the labours of Lexell, 
 Lalande, and the great mathematician Laplace, that 
 this theory fully explained all the observed motions of 
 the newly-discovered body, and before long (so complete 
 is the mastery which the Newtonian system gives astro 
 nomers over the motions of the heavenly bodies) all the 
 circumstances of the new planet s real motions became 
 very accurately known. It was now possible, not only 
 to predict the future movements of the stranger, but 
 to calculate his motions during former years. This last 
 process was quickly applied to the planet, with the 
 object 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 surpris 
 ing. The new planet had actually been observed no 
 less than nineteen times before that night when Herschel 
 first showed that it was not a fixed star, and those 
 observations were made by astronomers no less eminent 
 than Flamstead, Bradley, Mayer, and Lemonnier. 
 Flamstead had seen the planet five several times, each 
 time cataloguing it as a star of the sixth magnitude, so 
 that five such stars had to be dismissed from Flam- 
 stead s lists. But the case of Lemonnier was even more 
 singular ; for he had actually observed the planet no 
 less than twelve times, several of his observations having 
 
NEWS FROM HERSCHEL S PLANET. 131 
 
 been made within the space of a few weeks. M. Arago 
 naturally comments, says Professor Pritchard, on the 
 want of system displayed by Lemonnier in 1769 : had 
 he but reduced and arranged his observations in a 
 properly-constructed register, his name instead of 
 Herschel s would have been attached for all time to one 
 of the starry host. But Lemonnier was not a man of 
 order ; his astronomical papers are said to have been a 
 very picture of chaos ; and M. Bouvard, to whom we 
 have long been indebted for the best tables of the new 
 planet, narrates that he had seen one of Lemonnier s 
 observations of this very star written on a paper bag 
 which had contained hair powder ! 
 
 In our days, when fresh planets are being discovered 
 and named in the course of each year that passes, it 
 may appear strange that much difficulty was found in 
 assigning a suitable name to the stranger. But we 
 must remember that for ages the planetary system had 
 been supposed to comprise no other primary members 
 than those known to the ancients. The discovery of 
 Uranus was an altogether novel and unlooked-for cir 
 cumstance. It was not supposed that fresh discoveries 
 of like nature would be made, still less that a planet 
 would hereafter be discovered under circumstances far 
 more interesting even than those which attended the 
 discovery of Uranus. Accordingly a mighty work was 
 made before Uranus was fitted with a name. Lalande 
 proposed the name of the discoverer, and the new planet 
 was indeed long known on the Continent by the name 
 of Herschel. The symbol of the planet ( $ ), the initial 
 
 K 2 
 
132 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 letter of Herschel s name with a small globe attached 
 to the cross-stroke, still reminds us of the honour which 
 Continental astronomers generously proposed to render 
 to their fellow-worker in England. 1 Lichtenberg pro 
 posed the name of Astrsea, the goddess of justice for 
 this exquisite reason, that since justice had failed to 
 establish her reign upon earth, she might be supposed 
 to have removed herself as far as possible from our 
 unworthy planet. Poinsinet suggested that Cybele 
 would be a suitable name ; for since Saturn and Jupiter, 
 to whom the gods owed their origin, had long held their 
 seat in the heavens, it was time to find a place for Cybele, 
 the great mother of the gods. Had the supposed 
 Greek representative of Cybele Khsea been selected 
 for the honour, the name of the planet would have 
 approached somewhat nearly in sound, and perhaps in 
 signification, to the old name Eahu. But neither 
 Astrsea nor Cybele were regarded as of sufficient dignity 
 and importance among the ancient deities to supply a 
 name for the new planet. 2 Prosperin proposed Neptune 
 as a suitable name, because Saturn would thus have the 
 
 1 There is a certain incongruity, accordingly, among the symbols of 
 the primary planets. Mercury is symbolised by his caduceus, Venus by 
 her looking-glass (I suppose), Mars by his spear and shield, Jupiter by 
 his throne, Saturn by his sickle ; and again, when we pass to the sym 
 bols assigned to the planets discovered in the present century, we find 
 Neptune symbolized by his trident, Vesta by her altar, Ceres by her 
 sickle, Minerva by a sword, and Juno by a star-tipped sceptre. Uranus 
 alone is represented by a symbol which has no relation to his position 
 among the deities of mythology. 
 
 2 Both these names are found among the asteroids, the fifth of these 
 bodies (in order of discovery) being called Astraea, the eighty-ninth 
 being named after the great mother of gods and goddesses. 
 
NEWS FROM HERSCHEL S PLANET. 133 
 
 eldest of his sons on one side of him, and his second 
 son on the other. Bode at length suggested the name 
 of Uranus, the most ancient of the deities ; and as 
 Saturn, the father of Jupiter, travels on a wider orbit 
 than Jupiter, so it was judged fitting that an even wider 
 orbit than Saturn s should be adjudged to Jupiter s 
 grandfather. In accepting the name of Uranus for the 
 new planet, astronomers seemed to assert a belief that 
 no planet would be found to travel on a yet wider path ; 
 and accordingly when a more distant planet was dis 
 covered, the suggestion of Prosperin had to be recon 
 sidered; but it was too late to change the accepted 
 nomenclature, and accordingly the younger brother of 
 Jupiter has had assigned to him a planet circling out 
 side the paths of the planets assigned to their father and 
 grandfather. It may be noted, also, that a more appro 
 priate name for the new planet would have been Ccelus, 
 since all the other planets have received the Latin 
 names of the deities. 
 
 Herschel himself proposed another name. As Gralileo 
 had called the satellites of Jupiter the Medicean planets, 
 while French astronomers proposed to call the spots on 
 the sun the Bourbonian stars, so Herschel, grateful for 
 the kindness which he had received at the hands of 
 George III., proposed that the new planet should be 
 called Greorgium Sidus. On account of the interest 
 attaching to all Herschel s remarks respecting his dis 
 covery, I quote in full the letter in which he submitted 
 this proposition to Sir Joseph Banks, then the President 
 of the Eoyal Society. By the observations of the most 
 
134 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 eminent astronomers in Europe, he remarks, it appears 
 that the new star, which I had the honour of pointing 
 out to them in March 1781, is a primary planet of our 
 solar system. A body so nearly related to us by its 
 similar condition and situation in the unbounded ex 
 panse of the starry heavens, must often be the subject 
 of the conversation, not only of astronomers, but of 
 every lover of science in general. This consideration, 
 then, makes it necessary to give it a name, whereby it 
 may be distinguished from the rest of the planets and 
 fixed stars. In the fabulous ages of ancient times, the 
 appellations of Mercury, Venus, Mars, Jupiter, and 
 Saturn, were given to the planets, as being their princi 
 pal heroes and divinities. In the present more philo 
 sophical era, it would be hardly allowable to have 
 recourse to the same method, and call on Juno, Pallas, 
 Apollo, or Minerva, for a name to our new planet. 
 The first consideration in any particular event or re 
 markable incident seems to be its chronology ; if, in 
 any future age it should be asked when this last-found 
 planet was discovered, it would be very satisfactory to 
 say, " In the reign of Greorge III." As a philosopher, 
 then, the name of Greorgium Sidus presents itself to me 
 as an appellation which will conveniently convey the 
 information of the time and country where and when 
 it was brought to view. But as a subject of the best 
 of kings, who is the liberal protector of every art and 
 science ; as a native of the country from whence this 
 illustrious family was called to the British throne ; as 
 a member of that society which flourishes by the dis- 
 
NEWS FROM HERSCHEL S PLANET. 135 
 
 tinguished liberality of its royal patron ; and last of all 
 as a person now more immediately under the protection 
 of this excellent monarch, and owing everything to his 
 unlimited bounty, I cannot but wish to take this oppor 
 tunity of expressing my gratitude by giving the name 
 of Greorgium Sidus 
 
 Georgium sidus 
 -jam nunc assuesce Yocari, 
 
 to a star which, with respect to us, first began to shine 
 under his auspicious reign. Herschel concludes by 
 remarking that, by addressing this letter to the Presi 
 dent of the Eoyal Society, he takes the most effectual 
 method of communicating the proposed name to the 
 literati of Europe, which he hopes 6 they will receive 
 with pleasure. 
 
 Herschel s proposition found little favour, however, 
 among Continental astronomers. Indeed it is some 
 what singular that for some time two names came into 
 general use one in Great Britain and the other on 
 the Continent, neither being the name eventually 
 adopted for the planet. In books published in Eng 
 land for more than a quarter of a century after the 
 discovery of Uranus we find the planet called either 
 the Greorgium Sidus, or the Georgian. For a shorter 
 season the planet was called on the Continent either 
 the Herschelian planet, or simply Herschel. Many 
 years elapsed before the present usage was definitely 
 established. 
 
 In considering Herschel s telescopic study of the 
 planet, we must remember that, owing to the enormous 
 
136 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 length of time occupied by Uranus in circling round 
 his orbit, the astronomer labours under a difficulty 
 distinct in character from the difficulties which have 
 already been considered. As Jupiter and Saturn circle 
 on their wide orbits they exhibit to us the former 
 in the course of eleven years, the latter in the course 
 of twenty-nine and a half years all those varying pre 
 sentations which correspond to the seasons of these 
 planets. Jupiter, indeed, owing to the uprightness of 
 his axis (with reference to his path) presents but slight 
 changes. But Saturn s globe is at one time bowed 
 towards us, so that a large portion of his north polar 
 regions can be seen, and anon (fifteen years later) is so 
 bowed, that a large portion of his southern polar 
 regions can be seen ; while between these epochs we 
 see the globe of Saturn so posed that both poles are 
 .on the edge of his disc, and then only does the shape 
 of his disc indicate truly the compression or polar 
 flattening of the planet. 
 
 But although similar changes occur in the case of 
 Uranus, they occupy no less than eighty-four years in 
 running through their cycle, or forty-two years in com 
 pleting a half cycle during which, necessarily, all 
 possible presentations of the planet are exhibited.. 
 Now it is commonly recognised among telescopists that 
 the observing time of an astronomer s life that is, the 
 period during which he retains not merely his full skill, 
 but the energy necessary for difficult researches 
 continues but about twenty-five years at the outside. 
 So that few astronomers can hope to study Uranus in 
 
NEWS FROM HERSCHEL S PLANET. 137 
 
 all his presentations, as they can study Mars, or Jupiter, 
 or Saturn. 
 
 When we add to this circumstance the extreme 
 faintness of Uranus, we cannot wonder that Herschel 
 should have been unable to speak very confidently on 
 many points of interest. His measures of the planet s 
 globe were sufficiently satisfactory, and, combined with 
 modern researches, show that Uranus has a diameter 
 exceeding the earth s rather less than four and a half 
 times. Thus the surface of Uranus exceeds that of our 
 globe about twenty times, and his bulk is more than 
 eighty times as great as the earth s. His volume, in 
 fact, exceeds the combined volume of Mercury, Venus, 
 the Earth, and Mars, almost exactly forty times. But 
 Sir W. Herschel was unable to measure the disc of 
 Uranus in such a way as to determine whether the 
 planet is compressed in the same marked degree as 
 Jupiter and Saturn. All that he felt competent to 
 say was that the disc of the planet seemed to him to 
 be oval, whether he used his seven-feet, or his ten-feet, 
 or his twenty-feet reflector. Arago has expressed some 
 surprise that Herschel should have been content with 
 such a statement. But in reality the circumstance is 
 in no way surprising. For as a matter of fact 
 Herschel had been almost foiled by the difficulty of 
 measuring even the planet s mean diameter. The dis 
 cordance between his earliest measures is somewhat 
 startling. His first estimate of the diameter made it 
 ten thousand miles too small (its actual value being 
 about thirty-four thousand miles); his next made it 
 
138 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 nearly three thousand miles too great ; while his third 
 made it ten thousand miles too great. His contem 
 poraries were even less successful. Maskelyne, after a 
 long and careful series of observations, assigned to the 
 planet a diameter eight thousand miles too small ; the 
 astronomers of Milan gave the planet a diameter more 
 than twenty thousand miles too great ; and Mayer, of 
 Mannheim, was even more unfortunate, for he assigned 
 to the planet a diameter exceeding its actual diameter 
 of thirty-four thousand miles, by rather more than 
 fifty thousand miles. It will be understood, therefore, 
 that Herschel might well leave unattempted the task 
 of comparing the different diameters of the planet. 
 This task required that he should estimate a quantity 
 (the difference between the greatest and the least 
 diameters) which was small even by comparison with 
 the errors of his former measurements. 
 
 But besides this, a peculiarity in the axial pose of 
 Uranus has to be taken into account. I have spoken 
 of the uprightness of Jupiter s axis with reference to 
 his path ; and by this I have intended to indicate the 
 fact that if we regard Jupiter s path as a great level 
 surface, and compare Jupiter to a gigantic top spinning 
 upon that surface, this mighty top spins with a nearly 
 upright axis. In the case of Uranus the state of 
 things is altogether different. The axis of Uranus is 
 so bowed down from uprightness as to be nearly in the 
 level of the planet s path. The result of this is that 
 when Uranus is in one part of his path his northern 
 pole is turned almost directly towards us. At such a 
 
NEWS FROM HERSCHEL S PLANET. 139 
 
 time we should be able to detect no sign of polar 
 flattening even though Uranus were shaped like a 
 watch-case. At the opposite part the other pole is as 
 directly turned towards the earth. Only at the parts 
 of his path between these two can any signs of com 
 pression be expected to manifest themselves; and Uranus 
 occupies these portions of his path only at intervals of 
 forty-two years. 
 
 Herschel would have failed altogether in determining 
 the pose of Uranus but for his discovery that the 
 planet has moons. For the moons of the larger planets 
 travel for the most part near the level of their planet s 
 equator. We can, indeed, only infer this in the case 
 of Uranus (for even the best modern measurements 
 cannot be regarded as satisfactorily determining the 
 figure of his globe), but the inference is tolerably safe. 
 
 For six years Herschel looked in vain for Uranian 
 satellites. His largest telescopes, supplemented by his 
 wonderful eyesight and his long practice in detecting 
 minute points of light, failed to reveal any trace of 
 such bodies. At length he devised a plan by which 
 the light-gathering power of his telescopes was largely 
 increased. On the llth of January, 1787, he detected 
 two satellites, though several days elapsed before he 
 felt justified in announcing the discovery. At intervals, 
 during the years 1790-1798, he repeated his observa 
 tions; and he supposed that he had discovered four 
 other satellites. He expresses so much confidence as to 
 the real existence of these four bodies, that it is very 
 difficult for those who appreciate his skill to understand 
 
140 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 how he could have been deceived. But he admits that 
 he was unable to watch any of these satellites through 
 a considerable part of its path, or to identify any of 
 them on different nights. All he felt sure about was 
 that certain points of light were seen which did not 
 remain stationary, as would have happened had they 
 been fixed stars. No astronomer, however, has since 
 seen any of these four additional satellites, though Mr. 
 Lassell has discovered two which Herschel could not 
 see (probably owing to their nearness to the body of the 
 planet). As Mr. Lassell has employed a telescope more 
 powerful 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 of 
 the four additional satellites. Since, therefore, he is very 
 confident that they have no existence, I feel bound to 
 represent that view as the most probable ; yet I am 
 unable to pass from the subject without expressing a 
 hope that one of these days new Uranian satellites will 
 be revealed. 
 
 The four known moons travel backwards; that is, 
 they circle in a direction opposed to that in which all 
 the planets of the solar system, and all the moons of 
 Jupiter and Saturn, as well as our own moon, are 
 observed to travel. Much importance has been attached 
 to this peculiarity; but in reality the paths of the 
 Uranian moons are so strangely situated with respect 
 to the path of Uranus, that the direction in which they 
 travel can hardly be compared with the common direc 
 tion of the planetary motions. Imagine the path of 
 
NEWS FROM HERSCHEL S PLANET. 141 
 
 Uranus to be represented by a very large wooden hoop 
 floating on a sheet of water ; then, if a small wooden 
 hoop were so weighted as to float almost upright, with 
 one half out of the water, the position of that hoop 
 would represent the position of the path of one of the 
 planet s satellites. It will be seen at once that if we 
 suppose a body to travel round (and upon) the former hoop 
 in a certain direction, then a body travelling round the 
 latter hoop could scarcely be said to travel in the same 
 direction, whether it circled one way or the other. 
 Or to employ another illustration if a watch be laid 
 face upward on a table, we should correctly say that its 
 hands move from east through south to west ; but, if 
 it be held nearly upright and the face rather upwards, 
 we should scarcely say that the hands moved from 
 east through south to west ; nor if the face were tilted 
 a little further forward, so as to be inclined rather 
 downwards, should we say that the hands move from 
 east through north to west. 
 
 The great slope or tilt of the paths is undoubtedly a 
 more singular feature than the direction of motion. 
 Implying as it does that the planet s globe is similarly 
 tilted, it suggests the strangest conceptions as to the 
 seasonal changes of the planet. It seems impossible 
 to suppose that the inhabitants of Uranus, if there are 
 any, can depend on the sun for their supply of heat. 
 The vast distance of Uranus from the sun, although 
 reducing the heat-supply to much less than the three- 
 hundredth part of that which we receive, is yet an 
 insignificant circumstance by comparison with the axial 
 
142 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 tilt. One can understand at least the possibility that 
 some peculiarity in the atmosphere of the planet 
 might serve to remedy the effects of the former cir 
 cumstance; precisely as our English climate is tempered 
 by the abundant moisture with which the air is ordi 
 narily laden. But while we can conceive that the 
 minute and almost starlike sun of the Uranian skies 
 may supply much more heat than its mere dimensions 
 would lead us to expect, it is difficult indeed to under 
 stand how the absence of that sun for years from the 
 Uranian sky can be adequately compensated. Yet in 
 Uranian latitudes corresponding to the latitude of 
 London the sun remains below the horizon for about 
 twenty-three of our years in succession. Such is the 
 Arctic * night of regions in Uranus occupying a posi 
 tion corresponding to that of places in our temperate 
 zone. 
 
 But the most important result of the discovery of 
 the satellites has been the determination of the mass 
 or weight of the planet, whence also the mean density 
 of its substance has been ascertained. It has been thus 
 discovered that, like Jupiter and Saturn, Uranus is con 
 structed of much lighter materials than the earth. Our 
 earth would outweigh almost exactly six times a globe as 
 large as the earth but no denser than Uranus. It is to be 
 
 1 It has been remarked that there is some incongruity in the name 
 Arctic planets which I have assigned in my Other Worlds to Uranus 
 and Neptune, when considered with reference to the theory I have enun 
 ciated that these planets still retain an enormous amount of inherent 
 heat. Many seem to imagine that the term arctic implies cold. I have, 
 of course, only used the name as indicating the distance of Uranus and 
 Neptune from the sun. 
 
NEWS FROM HERSCHEL S PLANET. 143 
 
 noticed that in this respect the outer planets resemble 
 the sun, whose density is but about one-fourth that of 
 the earth. It seems impossible that the apparent size 
 of any one of the outer planets can truly indicate the 
 dimensions of its real globe. An atmosphere of 
 enormous extent must needs surround, it would seem, 
 the liquid or solid nucleus which probably exists within 
 the orb we see. 
 
 In the case of Jupiter or Saturn, the telescope has 
 told us much which bears on this point ; and as I have 
 indicated in my ; Other Worlds, and elsewhere, there is 
 an overwhelming mass of evidence in favour of the theory 
 that those orbs are still instinct with their primeval 
 fires. But in the case of Uranus, it might well be 
 deemed hopeless to pursue such inquiries, otherwise 
 than by considering the analogy of the two larger 
 planets. Direct evidence tending to show that the 
 atmosphere of Uranus is in a condition wholly differing 
 from that of our own atmosphere, cannot possibly be 
 obtained by means of any telescopes yet constructed by 
 men. Some astronomers assert that they have seen 
 faint traces of belts across the disc of Uranus ; but the 
 traces must be very faint indeed, since the best tele 
 scopes of our day fail to show any marks whatever 
 upon the planet s face. Even if such belts can be 
 seen, their changes of appearance cannot be studied 
 systematically. 
 
 It is, however, on this very subject the condition 
 of the planet s atmosphere that the discovery I have 
 now to describe throws light. 
 
144 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 Faint as is the light of Uranus, yet, when a telescope 
 of sufficient size is employed, the spectrum of the 
 planet is seen as a faint rainbow-tinted streak. The 
 peculiarities of this streak, if discernible, are the means 
 whereby the spectroscopist is to ascertain what is the 
 condition of the planet s atmosphere. Now, Father 
 Secchi, studying Uranus with the fine eight-inch tele 
 scope of the Eoman Observatory, was able to detect 
 certain peculiarities in its spectrum, though it would 
 now appear that (owing probably to the faintness of 
 the light) he was deceived as to their exact nature. 
 He says : The yellow part of the spectrum is wanting 
 altogether. In the green and the blue there are two 
 bands, very wide and very dark. But he was unable 
 to say what is the nature of the atmosphere of the 
 planet, or to show how these peculiarities might be 
 accounted for. 
 
 Eecently, however, the Eoyal Society placed in the 
 hands of Dr. Huggins a telescope much more powerful 
 than either the Roman telescope or the instrument with 
 which Dr. Huggins had made his celebrated observa 
 tions on sun and planets, stars and star- cloudlets. It 
 is fifteen inches in aperture, and has a light-gathering 
 power fully three times as great as that possessed by 
 either of the instruments just mentioned. 
 
 As seen by the aid of this fine telescope the spectrum 
 of Uranus is found to be complete, no part being 
 wanting, so far as the feebleness of its light permits it 
 to be traced. But there are six dark bands, or strong 
 lines, indicating the absorptive action of the planet s 
 
NEWS FROM HERSCHEL S PLANET. 145 
 
 atmosphere. One of these strong lines corresponds in 
 position with one of the lines of hydrogen. Now it 
 may seem at a first view that since the light of Uranus 
 is reflected solar light, we might expect to find in the 
 spectrum of Uranus the solar lines of hydrogen. But 
 the line in question is too strong to be regarded as 
 merely representing the corresponding line in the solar 
 spectrum; indeed, Dr. Hug-gins distinctly mentions 
 that the bands produced by planetary absorption are 
 broad and strong in comparison with the solar lines. 
 We must conclude, therefore, that there exists in the 
 atmosphere of Uranus the gas hydrogen, sufficiently 
 familiar to us as an element which appears in combi 
 nation with others, but which we by no means recognise 
 as a suitable constituent (at least to any great extent) 
 of an atmosphere which living creatures are to breathe. 1 
 And not only must hydrogen be present in the atmo 
 sphere of Uranus, but in such enormous quantities as 
 to be one of the chief atmospheric constituents. The 
 strength of the hydrogen line cannot otherwise be 
 accounted for. If by the action of tremendous heat 
 all the oceans of our globe could be changed into their 
 constituent elements, hydrogen and oxygen, it is pro 
 bable that the signs by which an inhabitant of Venus 
 or Mercury could recognise that such a change had 
 taken place would be very much less marked than the 
 signs by which Dr. Huggins has discovered that hydro- 
 
 1 Traces of hydrogen can nearly always be detected in the air, but 
 the quantity of hydrogen thus shown to be present is almost infinitesi- 
 mally small compared with the amount of oxygen and nitrogen. 
 
 L 
 
146 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 gen exists in the atmosphere of Uranus. It will indeed 
 be readily inferred that this must be the case, when 
 the fact is noted that no signs whatever of the exist 
 ence of nitrogen can be recognised in the spectrum of 
 Uranus, though it is difficult to suppose that nitrogen 
 is really wanting in the planet s atmosphere. Dr. 
 Huggins also notes that none of the lines in the spec 
 trum of Uranus appear to indicate the presence of 
 carbonic acid. Nor are there any lines in the spectrum 
 of Uranus corresponding to those which make their 
 appearance in the solar spectrum when the sun is low 
 down, and is therefore shining through the denser 
 atmospheric strata. Most of these lines are due to 
 the 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 its 
 quantity must be small compared with that of the free 
 hydrogen. 
 
 Admitting that the line seen by Dr. Huggins is 
 really due to hydrogen a fact of which he himself has 
 very little doubt we certainly have a strange discovery 
 to deal with. If it be remembered that oxygen, the 
 main supporter of such life as we are familiar with, 
 cannot be mixed with -hydrogen without the certainty 
 that the first spark will cause an explosion (in which 
 the whole of one or other of the gases will combine 
 with a due portion of the other to produce water), it 
 is difficult to resist the conclusion that oxygen must be 
 absent from the atmosphere of Uranus. If hydrogen 
 could be added in such quantities to our atmosphere 
 
NEWS FROM HERSCHEVS PLANET. 147 
 
 as to be recognisable from a distant planet by spectro- 
 scopic analysis, then no terrestrial fires could be lighted, 
 for a spark would produce a catastrophe in which all 
 living things upon the earth, if not the solid earth 
 itself, would be destroyed. A single flash of lightning 
 would be competent to leave the earth but a huge 
 cinder, even if its whole frame were not rent into a 
 million fragments by the explosion which would 
 ensue. 
 
 Under what strange conditions then must life exist 
 in Uranus, if there be indeed life upon that distant 
 orb. Either our life- sustaining element, oxygen, is 
 wanting ; or, if it exists in sufficient quantities (ac 
 cording to our notions) for the support of life, then 
 there can be no fire, natural or artificial, on that giant 
 planet. It seems more reasonable to conclude that, 
 as had been suspected for other reasons, the planet is 
 not at present in a condition which renders it a suitable 
 abode for living creatures. 
 
 The St. Paul s Magazine for October 1871. 
 
 THE TWO COMETS OF THE YEAR 1868. 
 PART I. BRORSEN S COMET. 
 
 TEN years ago, all that astronomers could hope to do 
 with comets was to note their appearance and changes 
 of appearance when viewed with high telescopic powers. 
 There was one instrument, indeed, the polariscope, 
 
 L 2 
 
148 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 which afforded doubtful evidence respecting the quality 
 of the light we receive from comets, and thus allowed 
 astronomers to form vague guesses respecting the struc 
 ture of these mysterious wanderers. But beyond the 
 unsatisfactory indications of this instrument, astrono 
 mers had no means whatever of ascertaining the phy 
 sical nature of comets. 
 
 At present, however, an instrument of incomparably 
 higher powers is applicable to the inquiry. The spec 
 troscope has the power of revealing, not only the 
 general character of any substance which is a source 
 of light, but even of exhibiting, in many instances, 
 the elementary constitution of such a substance. The 
 indications of this wonderful instrument of analysis 
 are not affected by the distance or dimensions of the 
 object under examination. So long as the object is 
 luminous the spectroscope will tell us with the utmost 
 certainty whether the light is inherent or reflected ; 
 and if the light is inherent that is, if the object is 
 self-luminous the spectroscope will tell us with the 
 utmost certainty what terrestrial elements (if any) 
 exist in the constitution of the object. It is with the 
 revelations of the spectroscope respecting Brorsen s 
 comet that I now propose to deal. I must make a 
 few preliminary remarks, however, respecting the vari 
 ous peculiarities of structure which have been presented 
 by comets. 
 
 I assume that my readers are familiar with the 
 general appearance presented by comets at least by 
 those which 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 com 
 monly recognised in comets viz. the nucleus, coma, 
 and tail the coma alone is invariably exhibited. A 
 comet which has neither nucleus nor tail presents 
 simply a round mass of vapour slightly condensed 
 towards the centre. The nucleus, when seen, appears 
 as a bright point within the condensed part of a comet. 
 The tail, as every one knows, is a long train of light 
 issuing from the head. 
 
 It was noted in very early times that comets are 
 almost perfectly translucent. This peculiarity has 
 been confirmed by modern and more exact observations. 
 Sir W. Herschel watched the central passage of a comet 
 over the fainter component of a double star ; and he 
 could detect no diminution of the star s brilliancy. 
 Similar observations were made by MM. Olbers and 
 Struve. Sir John Herschel watched the passage of 
 Biela s comet over a small cluster of very faint tele 
 scopic stars. The slightest haze would have oblite 
 rated the cluster, yet no appreciable effect was pro 
 duced by the interposition of cometic matter having a 
 thickness (according to Herschel s estimate) of 50,000 
 miles. And there is another remarkable evidence of 
 tenuity. From recognised optical principles, a star 
 seen through the globular head of a comet, should 
 appear displaced from its true position just as any 
 object seen (non-centrally) through a globular decanter 
 full of water seems thrown out of its true place. The 
 astronomer Bessel made an observation on a star which 
 approached within about eight seconds of the nucleus 
 
150 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 of Halley s comet, and he found that the place of the 
 star was not affected to an appreciable extent. 
 
 Whether the nucleus of a comet is solid or not had 
 long been a disputed point among astronomers. With 
 telescopes of moderate power the bright point within 
 the coma presents an appearance of solidity which 
 might readily deceive the observer. But with an 
 increase of power the nucleus assumes a different 
 appearance. Instead of having a well-defined outline 
 it appears to merge into the coma by a somewhat rapid 
 gradation but not by an abrupt variation of light. 
 Good observers have reported the extinction of tele 
 scopic stars behind the nuclei of comets, but there are 
 peculiar difficulties about an observation of this sort ; 
 and it is very difficult to determine whether a star is 
 really concealed from view by the interposition of the 
 nucleus or simply obliterated by the glare of light. 
 
 The tail of a comet appears nearly always as an 
 extension from the coma, and a dark interval is usually 
 seen between the head and the tail. But there is an 
 immense 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 from 
 the sun. The comet of 1823 had also two tails, but 
 one was turned almost directly towards the sun. Other 
 comets have had lateral tails. 
 
 The processes which seem to be passed through by 
 comets during their approach towards and recession 
 from the sun have proved very perplexing to astrono 
 mers and physicists. When first seen a comet usually 
 
THE TWO COMETS OF THE YEAH 1868. 151 
 
 appears as a light roundish cloud with a point of 
 brighter light near the centre. As it approaches the 
 sun the comet appears to grow considerably brighter 
 on the side turned towards him. An emanation of 
 light seems to proceed towards the sun for a short 
 distance and then to curl backwards and stream out 
 in a contrary direction. Gradually the backward 
 streaming rays extend to a greater distance the 
 nucleus continuing to throw out matter towards the 
 sun. Thus the tail is formed ; and it is often thrown 
 out to a distance of many millions of miles in a few 
 hours. 
 
 One of the most singular facts connected with the 
 approach and recession of a comet, is the peculiarity 
 that the comet grows gradually smaller and smaller as 
 it approaches perihelion, and swells out in a corre 
 sponding manner as it passes away from the sun. The 
 comet of 1652 was observed by Hevelius to increase so 
 rapidly in dimensions as it passed away from the sun, 
 that between December 20 and January 12 its volume 
 had increased in the proportion of about 13,800 to 1. 
 When it was last visible this comet exceeded the sun 
 in volume. This observation, on which much doubt 
 had been thrown, has been confirmed by the researches 
 of the best modern observers. M. Struve measured 
 Encke s comet as it approached the sun towards the 
 end of the year 1828. He found that between October 
 28 and December 24 the comet collapsed to about the 
 sixteen-thousandth part of its original volume. Sir 
 John Herschel found in like manner that Halley s 
 
152 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 comet when passing away from the sun increased in 
 volume upwards of fortyfold in a single week. 
 
 The tremendous heat to which many comets are sub 
 jected during perihelion passage is an important point 
 for consideration, in attempting to form an opinion of 
 the physical structure of comets. Newton calculated 
 that the comet of 1680 was subjected to a heat 2,000 
 times greater than that of red-hot iron. But comets 
 have been known to approach the sun even more closely. 
 Sir John Herschel estimates that the comet of 1843 
 was subjected to a heat exceeding in the proportion of 
 24J to 1 the heat concentrated in the focus of Perkins 
 great lens. Yet the heat thus concentrated had suf 
 ficed to melt agate, rock-crystal, and cornelian. 
 
 We cannot wonder that so great an intensity of heat 
 should have produced remarkable effects upon many 
 comets. The great wonder is that any comet should 
 resist the effects of such heat without being dissipated 
 into space. 
 
 We learn from Seneca that Ephorus, an ancient 
 Greek author, mentions a comet which divided into 
 two distinct comets. Kepler considered that two 
 comets which were seen together in 1618 had been 
 produced by the division of a single comet. Cysatus 
 noticed that the great comet of 1618 showed obvious 
 signs of a tendency to break up into fragments. This 
 comet when first seen appeared as a circular nebulous 
 cloud. A few weeks later it seemed to be divided into 
 several distinct cloudlike masses. On December 20 
 it resembled a multitude of small stars. 
 
THE TWO COMETS OF THE YEAR 1868. 153 
 
 We might doubt whether these observations were 
 entitled to credit were it not that, quite recently, 
 Biela s comet has been seen to separate into two dis 
 tinct comets, each having a nucleus, coma, and tail, 
 and each of which pursued its course independently 
 until distance concealed both from view. 
 
 It is clear that nothing but a long series of careful 
 observations can put us in a position to theorise with 
 confidence, respecting the nature of comets, the pro 
 cesses of change which they undergo, and the functions 
 which they subserve in the economy of the solar system. 
 We may therefore dwell with particular satisfaction on 
 the fact that every comet which has appeared during the 
 last two years has been subjected to careful observa 
 tion, and that at length, by means of spectroscopic 
 analysis we are beginning to get hold of positive facts 
 respecting comets, and have promise of shortly being 
 able to form consistent theories with regard to these 
 singular members of the solar system. 
 
 I have had occasion in other works to speak of 
 the principles on which spectroscopic analysis depends ; 
 but I think it best briefly to restate the most im 
 portant points. When the light from a luminous 
 object is received upon a prism, there is formed what 
 is called the prismatic spectrum. According to the 
 nature of the source of light this spectrum varies in 
 appearance. If the source of light is an incandescent 
 body the spectrum is a continuous, rainbow-tinted 
 streak. Where the light comes from an incandescent mass 
 surrounded with cooler vapours, the streak of rainbow- 
 
154 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 coloured light is crossed by dark lines whose position 
 indicates the nature of the vapours which the light has 
 traversed. When the light comes from luminous 
 vapours the spectrum consists wholly of bright lines ; 
 and these have exactly the same position as the corre 
 sponding dark lines which are seen when the same 
 vapours intercept light from an incandescent solid 
 mass. Lastly, when light is reflected from an opaque 
 substance, the spectrum is the same as that which 
 would be presented by the light before reflection, unless 
 the opaque substance is surrounded by vapours, in 
 which case the spectrum will be crossed by new dark 
 lines corresponding to the absorptive qualities exerted 
 by those particular vapours. 
 
 We see then the wonderful qualities of the new 
 analysis. Applied to the sun and stars it has enabled 
 our physicists and astronomers to pronounce as con 
 fidently that certain elements exist in these far distant 
 orbs, as the chemist can pronounce on the constitution 
 of substances submitted to his direct analysis. The 
 questions, or some of them, which have been at issue 
 respecting comets, will undoubtedly yield to the powers 
 of the spectroscope. The great want, at present, is a 
 brilliant comet to work upon. Donati s comet (1859), 
 or the great comet of 1861 would have served this 
 purpose admirably, but the first came in the very year 
 in which the principles of spectroscopic analysis were 
 first discovered ; and the powers of the spectroscope 
 were only just beginning to be recognised when the 
 comet of 1861 made its brief visit to our northern 
 skies. 
 
THE TWO COMETS OF THE YEAR 1868. 155 
 
 Two small comets have been analysed with the spec 
 troscope, and each presented similar results. The 
 spectrum in each case consisted of thin bright lines on 
 a faint continuous streak of light. And from the fact 
 that the bright lines did not extend across the whole 
 breadth of the faint streak of light, it became evident 
 that they formed the spectrum of the nucleus, the 
 faint continuous spectrum belonging to the coma. 
 Hence it resulted that the nucleus of each of these 
 small comets consisted of self-luminous gas, while the 
 coma either consisted of incandescent solid matter or 
 shone by reflecting the light of the sun. The latter is 
 far the more probable hypothesis. In fact, when we 
 consider the extreme tenuity of the substance of a 
 comet, and therefore the certainty that if composed of 
 solid matter such matter must be dispersed in very 
 minute fragments, we shall recognise the extreme 
 improbability that these fragments should be self- 
 luminous through intensity of heat. If the comets 
 had been brighter, I may remark, there would have 
 been no dubiety respecting this point, since it would 
 have been possible to compare the continuous streak of 
 light with the solar spectrum, and by the resemblance 
 or dissimilarity of the two spectra, to determine whether 
 the coma really shines by reflecting the sun s light or 
 not. 
 
 Brorsen s comet has now been examined with the 
 spectroscope, and with results quite different from 
 those which attended the analysis of the other two. 
 Dr. Huggins, the physicist, who examined the latter, 
 says of Brorsen s comet : 
 
156 i.ianr MUKNUK voit LIUM/UK 
 
 li :i|)|. .n . in Mi- telescope as ;i nearly round nehu- 
 lomfy, in which thn li^ht JIHTCJIHCH rapidly towards Mm 
 coiifrn, where, on Honin occasions, I dotocted) I holievo, 
 i Mi.ill stellar mi. -I. -ii.:. (icnerally, this iniuulc nucleus 
 
 u.i . H"l to ho (Nnl illMlliHllcd In MM- |.n--lil r.-nl i:d p;u j 
 
 of I, IK* num-i. | ln Hprrl.riun (MHiHinlH lor lh<- nio.sl. 
 part of l.lnvn liri^lil, haiidn. The Icn^lli ol l,lic handn 
 in tu6 inni riiincnt ihowi (lint, ilicy arn n<l, due .-ili.in- 
 l.o Mir Hh llar nucleus, hill, arn produced hy Mm li^hl, 
 of MIC hri^lilcr porMons of Mm coma. I took sonic 
 
 pains i.o Irani (he precise character of these luminous 
 
 I .mds. When MIC slil. was wid<^ Micy rcscmhlcd MIC 
 expanded linen HCCII in noino ^MHCH. AM MIC slil. was 
 iiKnln narrow MIC l.vvo fainl.cr hands ap|)c!ircd l,o 1 ado 
 mil, willionl, hccoinino- more defined. I was nnahlo to 
 rosolvc Mi. -in into lines. Tim middle hand, which is so 
 nmcli hiio|,r,M- Mian MIC olh<-rs lhai i(, may h<^ con- 
 Hidored !o rcprcHciil, prohahly Mircc-lonrllis, or nearly 
 HO; of Mm whole of Mie lio|,|, which we receive from Mie 
 eomef, appears l,o possess similar characters. In this 
 hand, however, I dntooUul occaHionally two hri^hl lines 
 which appear to he shorter than Mm hand, and may he 
 <lll(% ln lll iinclens its< lf. .... Besides these bright 
 hands Miere was a very faint continuous spectrum/ 
 
 Inlerpreliiin; those ohserval i(ns according to Mie 
 priiM-ipN H which hav^ heen aJivady Hfa.l<>d, we deduce 
 Mie following iutoroHUit n>Hul(H. 
 
 The ntn-li na >f Mrorsou s comet consists of luminous 
 tfiis. The tvniti is also gaseous in (he ueiohhourhood 
 ll " > nucleus, hut its ouler porMons are of a ditVerent 
 
/ ///; TWO COMETS OF THE YEAR 1808. 157 
 
 character and shine by reflecting the solar light. This 
 part of UK; coma m;iy IMJ either li(juid or solid. There 
 i.s nothing opposed to the supposition that it in of 
 the nature of cloud --that is, that it is produced by 
 the condensation of trim vapour into minute li<[uid 
 globules. 
 
 Returning to the consideration of the gaseous part 
 nf tin; cornet the question will at mice surest itself 
 what the gases may be wliich constitute the substance 
 of the nucleus and coma. Mere our information is not 
 quite so satisfactory as could be desired. 
 
 The brightest band ia in the green part of the spec 
 trum, and agrees very nearly with the brightest line in 
 the spectrum of nitrogen. The want of exact agree 
 ment prevents us from assuming that nitrogen really 
 exists (in any form) in the substance of the comet. 
 The olhcr lines of the spec! rum of nitrogen are not 
 present in the spectrum of the comet: but, this pecu 
 liarity is not, so perplexing as the other, for it is well 
 known that certain lines will disappear from the spectra 
 of hydrogen, nitrogen, and other gases, under particular 
 circumstances of illumination, temperature, and so 
 on. 
 
 Nor is the, circumstance that, there are bands of light 
 instead of well marked lines a peculiarity which need 
 cause perplexity. For under certain circumstances of 
 temperature and pressure, the lines of the spectra of 
 various gases become expanded or diffused uut.il they 
 appear as bands of light. 
 
 The two fainter bauds are yellow and blue, respec- 
 
158 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 lively. They cannot be identified with the lines seen 
 in the spectra of any known terrestrial gases. 
 
 Of whatever gases the nucleus is composed it appears 
 that conditions wholly different from any with which 
 we are familiar on earth prevail in this, and doubtless 
 in all other comets. The gases which form the nucleus, 
 though self-luminous, are probably not incandescent. 
 Remembering that comets are luminous when situated 
 far out in space beyond the orbit of our own earth, we 
 are prevented from assuming the existence of an inten 
 sity of heat (due to solar action) sufficient to account 
 for their inherent light. And if the light of a comet 
 were due to a state of incandescence in the component 
 gases, there would be a rapid consumption of the 
 substance of the comet, and we should be quite 
 unable to account for the fact that Halley s comet 
 has continued to shine, with no appreciable loss of 
 brilliancy, for upwards of three hundred years. We 
 seem forced therefore to surmise that the gases which 
 form the substance of comets owe their light to a species 
 of phosphorescence which is independent of the comet s 
 temperature, or else to some electrical properties the 
 nature of which it would not be easy to divine. 
 
 Our perplexity is increased when we see the gases 
 which form the nuclei assuming either the liquid or 
 the solid form in the outer part of the coma. The 
 change from gaseity to liquidity or solidity is an evi 
 dence of loss of heat, whereas one would expect the 
 outer part of the coma, which is exposed to the full 
 
THE TWO COMETS OF THE YEAR 1868. 159 
 
 intensity of the sun s action, to be the most heated por 
 tion of a comet s volume. 
 
 None of the comets which have been examined have 
 had a tail, so that we are unable as yet to form any 
 certain opinion respecting the nature of this portion of 
 a comet s volume. It seems almost certain, however, 
 that the tail shines by reflected light, because in every 
 known instance the tail has appeared as an extension 
 from the outer part of the coma, and may therefore be 
 expected to resemble that portion of the comet in its 
 general characteristics. 
 
 One of the comets which has been examined with 
 the spectroscope, though it has not a visible tail, has 
 been shown to have an appendage of a very remarkable 
 character, respecting which, also, we have been able to 
 learn several interesting particulars. 
 
 In the year 1866 a telescopic comet was discovered 
 by M. Tempel. This was the first comet examined by 
 Dr. Huggins. Its orbit was carefully calculated by the 
 German astronomer Oppolzer, and found to pass very 
 near the orbit of our own earth. Soon after this, Pro 
 fessor Adams calculated the orbit of the November 
 shooting stars ; and to the surprise of the astronomical 
 world it was found that these minute bodies travel 
 along the very path in space which had been already 
 assigned to Tempel s comet. We need not here discuss 
 the circumstances of this discovery. Let it suffice to 
 state that all astronomers who are competent to form 
 an opinion on the subject are agreed that the Novem 
 ber shooting-stars are certainly due to the existence of 
 
160 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 a long- extended flight of cosmical bodies travelling in 
 the track of Tempel s comet. 
 
 Now it appears clear that this flight of cosmical 
 bodies may be looked upon as constituting an ex 
 tension of the comet an invisible train as it were. 
 But for the accident that the comet s track intersects 
 the earth s path in space, we should have remained 
 for ever ignorant of the fact that the comet has 
 any other extent than that which is indicated by 
 its telescopic figure. Now, however, that we know 
 otherwise, we recognise the probability that other 
 comets which have been looked upon as tailless may 
 have invisible extensions reaching far behind them into 
 space, or even completely around their orbit. 
 
 But the members of the November shooting-star 
 system have been subjected to spectroscopic analysis. 
 We know that they contain several terrestrial elements ; 
 and we recognise the probability that if we could 
 examine one of them before its destruction (in tra 
 versing our own atmosphere) we should find a close 
 resemblance in its constitution to that of those aero 
 lites or meteorites which have reached the surface of 
 the earth. 
 
 But here we encounter a new difficulty. One theory 
 respecting the tails of comets has accounted for them 
 by the supposition of a propulsive effect exerted by the 
 solar rays ; and another theory has ascribed them to the 
 action of vapours ascending in the solar atmosphere. 
 But if the tails of comets really consist of solid matter 
 very widely dispersed, it must be quite evident that 
 
THE TWO COMETS OF THE YEAR 1868. l6l 
 
 neither of these causes could suffice to account for the 
 great extension of these appendages. Then the rapid 
 manner in which the tails seem to be formed remains 
 wholly mysterious. And we are also left without any 
 explanation of the rapid change of position exhibited by 
 the tail while the comet is sweeping around the sun 
 at the time of nearest approach to that luminary. 
 Sir John Herschel compared this motion to that of a 
 stick whirled round by the handle the whole extent 
 of the tail partaking in the movement as if comet and 
 tail formed a rigid mass. 
 
 The difficulties here discussed seem in the present 
 state of our knowledge wholly insoluble. In fact, it 
 seems impossible even to conceive of a solution to the 
 last mentioned phenomenon, so long as we look upon 
 the comet s tail as a distinct unvarying entity. For 
 instance, if the tail, a hundred millions of miles long, 
 which extended backwards from Halley s comet before 
 perihelion passage, consisted of the same matter as the 
 tail which projected forwards to the same extent a few 
 days later, then certainly there is nothing in our pre 
 sent experience of matter and its relations which can 
 enable us to deal with so astounding a phenomenon. 
 It will be understood, of course, Sir John Herschel does 
 not say in so many words, that the tail of Halley s 
 comet was brandished round in the manner described 
 above, but that, although it appeared to move in 
 this manner, it is impossible so to conceive of its 
 motion. 
 
 M 
 
1 62 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 We refrain, however, from speaking further on a 
 point respecting which we have no means of reasoning 
 satisfactorily. Mere guess-work is an altogether 
 unprofitable resource in the discussion of scientific 
 matters. 
 
 Now that we have so powerful an instrument of re 
 search as the spectroscope, there really seems hope 
 that even the hitherto inscrutable mysteries presented 
 by comets tails may one day be interpreted. Each 
 comet which has been subjected to spectroscopic 
 analysis has revealed something new. Observations, 
 such as those which have been made on Brorsen s comet, 
 and on the two telescopic comets previously examined 
 by Dr. Huggins, are not merely valuable in themselves, 
 but as affording promise of what may be achieved when 
 some brilliant comet shall be subjected to spectroscopic 
 analysis. When we consider that all the comets yet 
 examined have been absolutely invisible to the naked 
 eye on the darkest night, whereas several of the great 
 comets have blazed forth as the most conspicuous 
 objects in the heavens, and have even been visible in 
 the full splendour of the midday sun, we see good 
 reason for the hope that far fuller information will be 
 gained respecting the structure of comets so soon as 
 one of the more important members of the family shall 
 have paid us a visit. 
 
 Whenever such an event may happen it is not likely to 
 rind our spectroscopists unprepared. It is probable that, 
 before long, every important observatory will be sup 
 plied with spectroscopes. Already some of the most 
 
THE TWO COMETS OF THE YEAR 1868. 163 
 
 powerful telescopes in use have been fitted with them. 
 We hear also, that the giant reflector of the Parsons- 
 town Observatory commonly known as the Eosse tele 
 scope has been armed with a spectroscope especially 
 constructed for the purpose by Mr. Browning, F.E.A.S., 
 the optician. Not only in England, but at the princi 
 pal Continental observatories, spectroscopic work is in 
 progress, and observers are daily becoming more and 
 more familiar with the powers of the new analysis. 
 Stars which are far too small to be viewed by the 
 naked eye have already been examined with the spec 
 troscope. The Padre Secchi at Rome has just pub 
 lished a list of minute red stars thus examined. It is 
 such delicate work as this which will fit observers to 
 deal with the difficulties involved in the spectroscopic 
 analysis of comets. 
 
 We shall see when we come to deal with the second 
 comet of the year 1868, that we have yet better reason than 
 the analysis of Brorsen s comet has afforded, for hoping 
 that before long we may have interesting and exact 
 information respecting the structure of these mysterious 
 wanderers. We may even hope to gain some know 
 ledge respecting the purposes which comets subserve in 
 the economy of the solar and sidereal systems. 
 
 PAKT II. WINNECKE S COMET. 
 
 IN the preceding pages I have described the principal 
 features presented by comets as they approach and pass 
 away from the neighbourhood of the sun. The various 
 
 M 2 
 
164 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 hypotheses which have been put forward to account for 
 these peculiarities must now for a brief space claim our 
 attention. Although we are far from being in a posi 
 tion to theorise with any confidence respecting the 
 nature of comets, and still less as to the purposes which 
 they subserve in the economy of nature, yet the observa 
 tions made upon the second comet of the year 1868 have 
 resulted in a positive discovery which may serve as a 
 stand-point, so to speak, whence we can examine some 
 what more confidently than of old, the various theories 
 which have suggested themselves to those who have 
 studied cometic phenomena. 
 
 In considering these hypotheses we have to distinguish 
 between the views which have been entertained respect 
 ing the nucleus and coma, and those which regard the 
 less intelligible phenomena presented by the tail. This 
 remark may seem trite and obvious, but in reality 
 the two classes of hypotheses are found singularly con 
 founded together in many works on popular astronomy. 
 Let it be understood then, that when, in speaking of 
 an hypothesis respecting comets no special mention is 
 made of the tail, it is to be assumed that the hypothe 
 sis applies solely to the head of the comet. The same 
 holds, by the way, with reference to the phenomena 
 presented by comets. For instance, when we said in 
 the paper on Comet I. that comets grow smaller as 
 they approach the sun, the remark was to be under 
 stood to apply to the volume of the head, not to the 
 whole space occupied by the head and tail. In fact, it 
 would have been impossible to assert anything with 
 
THE TWO COMETS OF THE YEAR 1868. 165 
 
 respect to the volumes of comets* tails, inasmuch as the 
 apparent extent of these appendages varies according 
 to the atmospheric conditions (humidity, clearness, and 
 so on) under which the comet is observed, and also 
 according to the light-gathering power of the observer s 
 telescope. 
 
 To return, however, to the theories which have been 
 formed respecting comets. 
 
 It has been commonly admitted that the substance 
 of which comets are composed is either wholly or prin 
 cipally gaseous. In no other way, it should seem, can 
 the remarkable variations of appearance which comets 
 present as they approach the sun or recede from him 
 be reasonably accounted for. 
 
 Kepler held that comets are wholly gaseous, and 
 that they are liable to be dissipated in space by the 
 sun s action. He supposed that the process of evapora 
 tion which thus led to the destruction of a comet was 
 carried on through the medium of the tail. It need 
 hardly be said that modern observations are completely 
 opposed to this view. Comets have been seen to return 
 again and again to the neighbourhood of the sun with 
 out any apparent diminution of volume, although at 
 each return a tail of considerable length has been 
 thrown out. For a long time, indeed, it was thought 
 that Halley s comet was gradually diminishing in 
 volume ; but at the last return this magnificent object 
 had recovered all its pristine splendour. 
 
 Newton held, on the contrary, that comets are partly 
 composed of solid matter. He supposed that only the 
 
1 66 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 gaseous matter was affected to any noteworthy extent 
 by the action of the sun s heat. Raised from the solid 
 nucleus the vaporised particles passed first into the 
 coma, he imagined, and were thence carried off into 
 space to form the comet s tail. Others so far modified 
 Newton s views as to suggest that the vaporised matter 
 is not wholly carried off but partially re-precipitated 
 upon the head of the comet, just as the vapours raised 
 from the ocean are precipitated upon the earth in the 
 form of rain. 
 
 We have seen that a comet diminishes in volume as 
 it approaches the sun. It will be noticed that both 
 the theories which have been described would account 
 satisfactorily for the observed decrease of volume. 
 But neither of them gives any satisfactory explanation 
 of the fact that a comet recovers its original volume as 
 it departs from the sun s neighbourhood. Newton, in 
 deed, put forward certain views respecting the emission 
 of smoke from the nucleus during perihelion passage, 
 and he surmised that the true dimensions of the comet 
 might in this manner be veiled to a certain extent : 
 but 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. Valz 
 which accounts for the variation of a comet s volume 
 by the supposition that the solar atmosphere exerts a 
 power of compression, which, varying with that atmo- 
 
THE TWO COMETS OF THE YEAR 1868. 167 
 
 sphere s density, is most effective in the sun s neighbour 
 hood. We know, for instance, that a balloon must not 
 be fully inflated at first rising, because when it reaches 
 the upper regions of air, where there is less compres 
 sion, the enclosed gas expands and would burst the 
 silk if the balloon had been fully filled at first. And 
 certainly, on the somewhat bold assumption that the 
 solar atmosphere extends outwards to those regions in 
 which the observed change of volume takes place, and 
 on the additional and equally bold supposition that 
 comets are surrounded with a film of some sort per 
 forming the same function as the silk of the balloon 
 (or that in some other way the substance of the comet 
 is prevented from intermingiing with the substance of 
 the solar atmosphere) the theory of M. Valz would 
 have a certain air of probability. Even then, however, 
 it would be insufficient to account for the enormous 
 extent to which the variation has been observed to 
 proceed. 
 
 The only probable explanation of the variation in 
 question is that which is put forward by Sir John 
 Herschel in his admirable work on the southern 
 heavens. During his stay at the Cape of (rood Hope 
 he had an opportunity of observing the recession of 
 Halley s comet, and he discusses the phenomena with 
 admirable acumen and judgment. The result at which 
 he arrives appears to afford a simple and rational ex 
 planation of the observed phenomena. He supposes 
 that as a comet approaches the sun the action of the 
 solar heat transforms the nebulous substance of the 
 
1 68 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 comet into invisible vapour. This action progressing 
 from without inwards, of course produces an apparent 
 diminution of volume. The diminution continues as 
 long as the comet is approaching the sun, and for yet 
 a few days after perihelion passage ; but soon after the 
 comet has begun to leave the sun s neighbourhood the 
 transparent vapour begins to return to its original 
 condition, the solar action being insufficient to keep 
 the whole of the vaporised matter in the gaseous state. 
 Thus the comet gradually resumes its original apparent 
 dimensions. 
 
 There are few phenomena which have given rise to 
 more speculation than those presented by the tails of 
 comets. Astronomers who, in dealing with other 
 matters, have exhibited the soundest judgment, and 
 the most logical accuracy of argument, seem to feel free 
 to indulge in the most fanciful, speculations when deal 
 ing with this subject. 
 
 A favourite theory with the earlier astronomers was 
 founded on the observed peculiarity that the tails of 
 comets are usually turned directly from the sun. It was 
 supposed that the tail is not a really existent entity, 
 but merely indicates the passage of the solar rays 
 through space, after their condensation by the spherical 
 head of the comet. Just as a light received into a dark 
 room through a small aperture appears as a long ray 
 extending in a straight line through the room, so, 
 according to this theory, the sun s light, concentrated 
 by the comet s head, throws a long luminous beam into 
 space. Unfortunately for this view there is a want of 
 
THE TWO COMETS OF THE YEAR 1868. 169 
 
 analogy between the two cases thus brought into com 
 parison. The light shining into a room produces the 
 appearance of a ray, because it illuminates the air and 
 the small particles of floating dust which it encounters 
 in its passage. There is nothing corresponding to this 
 in the interplanetary spaces. If there were, the sky 
 would never appear black, since the sun would always 
 be shining on matter capable of reflecting his rays. 
 
 Kepler was the first to form a reasonable hypothesis 
 respecting comets tails. He supposed that the action of 
 the solar heat dissipates and breaks up a comet s substance. 
 The rarer portions are continually swept away, he 
 imagined, by the propulsive energy of the solar rays, 
 and are swept in this way to enormous distances from 
 the comet s tail. The denser portions remain around 
 the nucleus and form the coma. 
 
 The modern theory respecting light (according to 
 which there is no propulsion of matter from the sun, 
 but a simple propagation of wave-like motion), does not 
 affect Kepler s hypothesis so much as might be ima 
 gined. Whatever theory of light we adopt we are 
 forced to assume an extreme tenuity in the matter 
 which forms the tails of comets. And when once we 
 have made this assumption, we are enabled to admit 
 that even the propagation of a wave-like motion through 
 the ether which is supposed to occupy the interplanetary 
 spaces, might suffice to carry off the attenuated nebu 
 lous matter with tremendous rapidity. 
 
 The defect of Kepler s theory is that it appears 
 insufficient to account for those anomalous tail-for- 
 
170 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 mations which were referred to in our paper on 
 Comet I. 
 
 Newton s hypothesis respecting comets tails was 
 somewhat different. He supposed that the intensely 
 heated comet communicated its heat to the surrounding 
 ether, which thus grew rarer and ascended in the solar 
 atmosphere that is, flowed away from the sun pre 
 cisely as heated air ascends from the earth. The ether 
 thus displaced would carry away with it the rarer por 
 tions of the comet s substance, just as smoke is carried 
 upwards by a current of heated air. 
 
 It will be seen at once that Newton s theory^ like 
 Kepler s, affords no explanation of lateral tails, or of tails 
 turned towards the sun. 
 
 In modern times a theory has been founded on the 
 supposition that cometic phenomena may be due to 
 electrical agency. The German astronomer Olbers was 
 one of the first to propound this view, and many emi 
 nent astronomers amongst others the younger Herschel 
 have looked with favour upon the theory. As yet, 
 however, we do not know enough respecting electricity 
 to accept with confidence any theory of comets founded 
 upon its agency. 
 
 The comet respecting which I now have to treat 
 was discovered in the middle of June 1868, by Win- 
 necke. At first it was a telescopic object, but it 
 gradually increased in brilliancy until it became visible 
 to the unaided eye. In the telescope, at the end of 
 June, the comet appeared as a circular cloud rather 
 brighter in the middle, where there was a roundish spot 
 
THE TWO COMETS OF THE YEAR 1868. 1 7 r 
 
 of light. A tail could be traced to a distance of about 
 one degree from the nucleus. 
 
 Dr. Huggins quickly subjected the new arrival to 
 spectroscopic analysis. The result, at first sight, seemed 
 to differ little from that which had been noticed in the 
 case of Brorsen s comet. Indeed the astronomers at 
 the Paris observatory and the Padre Secchi at Rome 
 were led to pronounce the spectra of the two comets to 
 be absolutely identical. The more powerful spectro 
 scopic appliances employed by Dr. Huggins, however, 
 exhibited important differences. 
 
 The spectrum consisted of three bands of light sepa 
 rated by dark intervals. Of these bands two were 
 greenish blue, the other greenish yellow. The two 
 former were tongue-shaped, the last was narrowed off at 
 both extremities. 
 
 From what I have said above respecting the nature 
 of spectroscopic analysis, it will be understood that the 
 distribution of the comet s light along the length of 
 the spectrum is the most important circumstance to be 
 attended to in endeavouring to form an estimate of" the 
 substance of the comet. But as we see that there are, 
 in this instance, peculiarities affecting the breadth of the 
 spectrum, it will be well briefly to consider their mean 
 ing. The matter is, in reality, simple enough, but 
 requires a little attention. 
 
 The breadth of the spectrum corresponds to the 
 breadth of the object which is the source of light. If 
 that object is uniformly bright the spectrum is also 
 uniformly bright across its breadth, whatever variations 
 
172 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 may exist in the direction of its length. But if the 
 object is brighter in some parts of its breadth than in 
 others, the spectrum will show corresponding variations 
 of brilliancy across its breadth. Hitherto we have been 
 assuming that all the light from the object is of the 
 same kind, however it may vary in brilliancy. Suppose, 
 however, that the light from the middle of the object 
 gives one kind of spectrum, the light from the outer 
 parts another ; then the spectrum will vary in character 
 as well as in brilliancy across its breadth. Suppose for 
 example, that the middle of the object is gaseous while 
 the outer parts are solid or liquid, then the appearance 
 presented would be two thin streaks of rainbow-tinted 
 light, separated by a dark space l across which would 
 be seen the bright lines belonging to the gaseous central 
 part of the luminous object. 
 
 Now the breadth of the spectrum seen by Dr. Huggins, 
 corresponded with the breadth of the coma so far as 
 the widest parts of the tongue-shaped bands were con 
 cerned. But the narrower parts were about the width 
 of the nucleus. Therefore the first question to be de 
 cided was this, Is the narrowing of these bands of 
 light towards one extremity, and of the other towards 
 both extremities, to be considered as indicative of any 
 
 1 Our readers will, of course, understand that a slice only of the object 
 is brought under spectroscopic analysis at once. If the whole of a cir 
 cular object, whose centre was gaseous, were examined at once, the 
 middle streak of the spectrum would exhibit the compound spectrum of 
 the edge and centre of the object. Such an arrangement would clearly 
 be unfavourable to the formation of clear views respecting the character 
 of the object s light. 
 
THE TWO COMETS OF THE YEAR 1868. 173 
 
 difference, in character, between the light emitted by 
 the nucleus and that emitted by the coma ? At first 
 sight it seems that no other conclusion could be come 
 to. But a little consideration enabled Dr. Huggins to 
 arrive at a different result. The tongue-shaped bands 
 were not only narrower but very much fainter towards 
 one end. They were also fainter along their outer 
 edges, on account, of course, of the faintness of the 
 coma as compared with the nucleus. Now it was 
 possible that the narrowing down of the bands might 
 be only apparent, and due to the fact that their outer 
 parts, though really existent, became invisible at the 
 fainter end. And there were two modes of attacking 
 the question. First the observer could determine by a 
 careful inspection whether the light at the narrower 
 end of the tongues was so faint that it ought to disap 
 pear at the edges merely by undergoing the same sort 
 of reduction as the brighter light at the broader end of 
 the tongue : this would show that the coma does not 
 differ in constitution from the nucleus. Secondly, if 
 the strip brought under examination were narrowed by 
 any contrivance, it is clear that any difference which 
 might exist in the constitution of the coma and of the 
 nucleus ought to be exhibited in a more marked 
 manner. 
 
 Dr. Huggins applied both methods, and each resulted 
 in showing that the nucleus has the same constitution 
 
 O 
 
 as the coma, excepting only that the exterior part of 
 the coma seems to give a continuous spectrum. In 
 other words, the nucleus and all the coma except its 
 
174 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 outer shell consists of the same incandescent vapour ; 
 but the outer shell of the coma either consists of in 
 candescent solid or liquid matter or shines by reflecting 
 the solar rays. 
 
 So far, however, there is little in the spectroscopic 
 analysis which differs in character from what had been 
 observed respecting Brorsen s comet. But we have now 
 to record one of the most startling discoveries ever made 
 respecting comets. 
 
 Dr. Huggins was reminded by the appearance of the 
 cometic spectrum of a form of the spectrum of carbon 
 which he had observed in the year 1864. It must be 
 premised that the spectrum of an element often assumes 
 a different form according to the circumstances under 
 which it is obtained. Amongst the objects which have 
 spectra thus variable is the element carbon. The par 
 ticular form of carbon-spectrum which resembled that 
 of the comet, is that obtained when an electric spark is 
 taken through olefiant gas a substance which, as many 
 of my readers are doubtless aware, consists of carbon 
 and hydrogen, and is one of the constituents of ordinary 
 coal-gas. 1 Of course the spectrum of olefiant gas 
 exhibits the bright lines belonging to hydrogen ; but as 
 these are well known, the part of the spectrum belong 
 ing to carbon also becomes determinable. 
 
 Having noticed, as we said, the resemblance between 
 the spectrum of the comet and a form of the carbon 
 
 1 The other constituent is fire-damp ; also a compound of carbon 
 and hydrogen. Olefiant gas is commonly called heavy carburetted 
 hydrogen, while fire-damp is termed light carburetted hydrogen. 
 
THE TWO COMETS OF THE YEAR 1868. 175 
 
 spectrum, Dr. Huggins determined to compare the two 
 spectra directly. We have not space to explain the 
 contrivances by which this was effected. Suffice it to 
 say, that when the two spectra were brought side by side 
 it appeared that in place of mere resemblance there was 
 absolute identity. The bands of light which formed 
 the comet s spectrum were found not only to coincide 
 in position with those which appeared in the spectrum 
 of olefiant gas, but to present the same relative bright 
 ness. Two days later the observations were repeated 
 by Dr. Huggins in company with Professor Miller (who 
 had been associated with him in his earlier spectroscopic 
 labours), and both observers agreed in the opinion that 
 the coincidence between the spectra could not be more 
 exact. 
 
 The reader will, of course, understand that the hydro 
 gen lines belonging to the spectrum of olefiant gas are 
 not seen in the spectrum of the comet. 
 
 Now only one interpretation can be put on this re 
 markable result, and that is that Winnecke s comet 
 consists of the incandescent vapour of carbon, not 
 of burning carbon, be it understood, but of volatilised 
 carbon. 
 
 But carbon, as we are acquainted with it on earth, 
 is a substance whose chief peculiarity, perhaps, is its 
 fixity at ordinary temperatures ; and no phenomenon 
 hitherto presented by comets is more perplexing than 
 the existence of volatilised carbon as the main or only 
 constituent of a comet of enormous real bulk, when 
 that comet was not so near to the sun as to be raised 
 
176 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 (one could suppose) to an extraordinarily high tempera 
 ture. There have been cases where comets have been 
 so near to the sun as to account for almost any con 
 ceivable change in the constitution of their elements. 
 An intensity of heat of which we can form no concep 
 tion must have been experienced, for example, by 
 Newton s comet ; and a still fiercer heat dissipated the 
 substance of the comet of 1843. But Winnecke s 
 comet at the time of observation was at far too great a 
 distance from the sun for us to assign to its mass a 
 temperature which under ordinary circumstances would 
 account for the volatilisation of carbon. 
 
 Nor does the rarity of the atmosphere in which the 
 comet was moving serve to help us in our difficulty. 
 Doubtless we are little familiar with the effects which 
 terrestrial elements would experience if they were dis 
 tributed freely in the ether occupying the interplanetary 
 spaces. But so far as our experience enables us to 
 judge, we should rather look for intensity of cold than 
 of heat under such circumstances. We see the heights 
 of the Andes and of the Himalayas clothed in perpetual 
 snow, though day after day the fierce heat of the tropi 
 cal sun pours down upon them, and though there is no 
 winter there (in our sense of the word) during which 
 the snows are accumulated. We know that the explana 
 tion of this peculiarity lies in the extreme rarity of the 
 air at a great height. It seems, therefore, reasonable 
 to conclude that the cold of the interplanetary spaces 
 must be far greater. Yet here we have an object whose 
 light comes from the incandescent vapour of so fixed 
 
THE TWO COMETS OF THE YEAR 1868- 177 
 
 and unchangeable a substance as carbon, and thus, in 
 place of an almost inconceivable intensity of cold we 
 lind the evidence of intense heat. 
 
 It seems impossible, at present, to suggest any ex 
 planation of the observed phenomena. That carbon 
 exists out yonder in space in the state of luminous gas 
 or vapour, is the one fact of which alone we can 
 be certain. Dr. Huggins in his treatment of this fact 
 suggests the possibility that the carbon may be divided 
 into particles so minute, that as the comet approaches 
 the sun, more of the sun s heat is gathered up, so to 
 speak, and that thus the carbon is volatilised. He also 
 points to phenomena of phosphorescence and fluores 
 cence in illustration of the appearance presented by 
 the comet s spectrum ; but without suggesting any 
 association between these phenomena and those pre 
 sented by comets. 
 
 One cannot help associating the new views thus 
 opened out to us respecting comets, with the discovery 
 recently made that the meteoric bodies which flash 
 singly or in showers across our skies belong in reality 
 to the trains of comets. We have now every reason to 
 believe that there is not a single member of the me 
 teoric systems, not a single aerolite, bolide, or fire-ball, 
 that has not belonged once upon a time to a comet. 
 The evidence on which this view is founded, though it 
 may seem insufficient at a first glance, is almost irre 
 sistible to those who can appreciate its significance. 
 Let us briefly recapitulate the facts. 
 
 It has been proved that shooting-stars come from 
 H 
 
178 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 the interplanetary spaces, that they form systems, and 
 that these systems travel in regular elliptical orbits 
 about the sun. Two of the systems produce striking 
 meteoric displays, viz. the system encountered by the 
 earth on or about August 10, and the system encoun 
 tered on or about November 13. Now it had been 
 suggested that the members of the former system 
 follow the track of the conspicuous comet which made 
 its appearance in the year 1862; and it was proved 
 that, assuming the orbit of the meteors to be very 
 eccentric, and assigning to them a period of 147 years 
 (that of the comet), their motions corresponded in the 
 most remarkable manner with the orbital track of the 
 comet. In fact the agreement was so close that very 
 few who had examined the question could believe it to 
 be accidental. But there were two objections on which 
 some stress was laid. First, it had been necessary to 
 make assumptions respecting the motion of the meteors ; 
 secondly, those assumptions were not rendered probable 
 by anything which had been proved respecting any 
 meteoric system. The examination of the November 
 star-shower by a host of eminent mathematicians in 
 1866-7 led to results which at once removed these 
 objections, and brought new evidence and that of 
 the most striking character in favour of the theory 
 that comets and meteors are associated. It had been 
 supposed that the November meteors travelled in a 
 nearly circular orbit within a period of somewhat less 
 than a year. Adams proved that they travel in an orbit 
 extending far out into space beyond the orbit of distant 
 
THE TWO COMETS OF THE YEAR 1868. 179 
 
 Uranus. And the period of this orbit was calculated 
 to be 33 years. Here then was strong confirmatory 
 evidence in favour of the elliptic orbit and the long 
 period assigned, by way of assumption, to the August 
 meteors. But this was far from being all. Astrono 
 mers looked for a comet to be associated with the 
 November meteors ; and they found one a small one, 
 it is true, but with a well-defined character an orbit 
 calculated beyond suspicion of important error, and 
 agreeing so closely in its motions with those of the 
 November meteors that the chances were millions on 
 millions to one against the coincidence being acci 
 dental. It hardly required, after this, that an associa 
 tion should be pointed out between other meteor- 
 systems and other comets. Yet this has been done, 
 and thus that which had already been demonstrated 
 was illustrated by new proofs. We may say that 
 nothing which men of science have dealt with has ever 
 been more satisfactorily proved than the fact that 
 meteors are the attendants on comets. 
 
 Now, how meteors are thrown off from cometic 
 nuclei we are not yet able to say. They differ wholly 
 in character from their source, and thus we learn that 
 the gaseous nature of cometic nuclei is due to the 
 action of causes connected with those to which the 
 nuclear (structure of the comet s head is due. But 
 whether the first formation of meteoric systems is asso 
 ciated in any way with the processes which result in 
 the formation of a comet s tail, is not quite so clear. 
 As yet no comet which has had a brilliant tail has been 
 
 N 2 
 
/8o LIGHT SCIENCE FOR LEISURE HOURS. 
 
 subjected to spectroscopic analysis, so that we cannot 
 pronounce with any certainty respecting the structure 
 of these singular appendages. Some astronomers are 
 disposed to look on the formation of a track of meteors 
 all round the orbit of a comet as due to the action of 
 influences by which parts of the comet s mass are 
 thrown into orbits of slightly longer period than that 
 of the head, though closely resembling that orbit in 
 figure. Be this as it may, it is certain that the great 
 contrast in character between the meteoric bodies 
 which form the train of a comet, and the gaseous 
 nucleus and coma, remains yet among the mysteries 
 which astronomers have been unable to clear up. 
 
 But so soon as it had been shown that a comet s head 
 is formed of a certain well-known terrestrial substance, 
 it was natural that the question should be asked 
 whether this substance is to be found in meteors. 
 Hitherto no great progress has been made in deter 
 mining the elementary constitution of meteors which 
 have not actually fallen upon the earth. It is so diffi 
 cult 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 any 
 appearance of probability to exist in shooting-stars. 
 Certainly carbon is not among the number of those 
 elements which have been detected in this way. But 
 at a recent meeting of the Astronomical Society, it was 
 stated that several aerolites contain carbon in their 
 structure, and Dr. De la Eue offered a fragment of 
 one of these to Dr. Huggins for analysis. Certainly a 
 
THE TWO COMETS OF THE YEAR 1868. 181 
 
 strange circumstance that an astronomer who had 
 analysed the structure of a body millions of miles away 
 from the earth, should take into his hands and subject 
 to chemical analysis a fragment which had once in all 
 probability belonged to a similar comet! 
 
 In conclusion, I must notice that there has been a 
 remarkable absence during the past few years of those 
 brilliant and long-tailed comets which alone seem 
 calculated to afford the spectroscopist the means of 
 answering some of the difficult questions suggested 
 above. The tail of Winnecke s comet was too faint to 
 give a visible spectrum. In fact the comet itself was 
 only just visible to the naked eye. When a blazing 
 object like Donati s comet or the comet of 1861 comes 
 to be subjected to spectroscopic analysis, we may hope 
 for an amount of information compared with which 
 that hitherto obtained is probably altogether insig 
 nificant. 
 
 From Frazer s Magazine for February and June 1869. 
 
 COMETS OF SHORT PERIOD. 
 
 IT is related by Apollonius the Myndian, that the 
 Chaldean astronomers held comets to be bodies which 
 travel in extended orbits around the solar system. 
 The Chaldeans spoke of comets, he says, < as of tra 
 vellers, penetrating far into the upper celestial spaces. 
 He adds, that those ancient astronomers were even able 
 
1 82 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 to predict the return of comets. How far it may be 
 safe to accept the statements of Apollonius is uncer 
 tain. He ascribed other powers to the Chaldeans, of 
 which we may fairly doubt their possession for in 
 stance, the power of predicting earthquakes and floods. 
 In fact, there is so marked a disposition among ancient 
 writers to exaggerate the acquisitions of Chaldean 
 astronomers, that it becomes extremely difficult to dis 
 tinguish truth from falsehood. Still, there is sufficient 
 evidence of their skill and patience as observers, to 
 render it fully possible that they may have discovered 
 the periodicity of one or two comets. 
 
 But until the rise of modern, astronomy, the opinion 
 which was almost universally held respecting comets 
 was that of Aristotle, that they are of the same nature 
 as meteors or shooting-stars, existing either in the air 
 not far above the clouds, or certainly far below the 
 moon. 
 
 The discovery of the periodicity of Halley s cornet 
 following quickly on Newton s announcement of the 
 law of gravitation, led astronomers to examine the 
 orbits of all the comets which became visible, with the 
 hope of finding that some of these bodies may be tra 
 velling in re-entering paths. But inasmuch as none 
 of the brilliant comets of whose appearance records had 
 been preserved seemed to have ever revisited the earth 
 save Halley s alone, while even Halley s travelled in an 
 orbit of enormous extent, an orbit which reached out 
 in space more than three times as far as the orbit of 
 the most distant known planet, astronomers held that 
 
COMETS OF SHORT PERIOD. 183 
 
 the only kind of path which they might expect a comet 
 to pursue was a long oval. They accordingly con 
 fined their calculations, and limited the invention of 
 new mathematical processes, to the case of very eccen 
 tric orbits. 
 
 But in 1770 a comet appeared which led astrono 
 mers to form wholly new views. No orbit which could 
 be devised (subject to the above-mentioned condition) 
 could be reconciled with the motions of the new arrival. 
 At length the astronomer Lexell discovered that the 
 path of the comet was not an oval of extreme eccen 
 tricity, but an ellipse of such a figure that the comet s 
 period of revolution was less than six years. But here 
 a difficulty arose. The comet was sufficiently conspi 
 cuous ; and it was asked, how could such an object 
 have gone on circulating so rapidly around the sun, 
 and yet have remained undiscovered ? A very singular 
 result rewarded the inquiry into this question. It was 
 found that the aphelion of the comet s path lay just 
 outside the orbit of Jupiter ; and, further, that when 
 the comet was last in aphelion, Jupiter was quite close 
 to it. Thus it became clear that the comet had been 
 travelling in another, and doubtless much wider orbit, 
 when its motions had brought it into the neighbour 
 hood of the planet Jupiter the giant of the solar 
 system. The comet had actually approached the planet 
 nearer than his fourth satellite. It had intruded, 
 says Sir J. Herschel, an uninvited member into his- 
 family circle. 
 
 The result of this close appulse may be readily con- 
 
184 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 ceived. Just as Halley s comet, when close to the sun, 
 sweeps rapidly round him that is, in a sharply curved 
 path so the new comet s path was sharply bent around 
 the temporary focus formed by the great planet. But 
 just as Halley s comet, after perihelion passage, moves 
 away from the sun, so Lexell s comet, after what may 
 be termed perijovian passage, moved away from Jupi 
 ter, and passed again within the sun s attraction. From 
 this time the comet began to follow a new orbit around 
 the sun. This new orbit was an oval of moderate 
 eccentricity, round which the comet travelled in about 
 five and a half years. 
 
 At the next return of the comet to perihelion, it was 
 not likely that astronomers would obtain a view of it ; 
 for, on account of the odd half-year in its period, it 
 came to perihelion when the earth held a point in her 
 orbit exactly opposite to that which she had occupied 
 at the comet s former perihelion passage ; therefore, 
 the comet, which before was favourably, was now un 
 favourably situated 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 the 
 faint speck of nebulous light which should have an 
 nounced the return of the wanderer. But days, and 
 weeks, and months passed, until it became certain 
 that either the comet had been shorn of nearly all its 
 former brilliancy, and had thus escaped unnoticed, or 
 that something had happened to deflect it from its 
 course. 
 
COMETS OF SHORT PERIOD. 185 
 
 The last alternative appeared so much the more 
 probable one, that mathematicians began to examine 
 the path of the comet, to see whether it had approached 
 so near to any disturbing body as to have been driven 
 from its recently adopted orbit. The examination was 
 soon rewarded with success. If we consider the nature 
 of orbital motion, we shall at once see that, so long as 
 Lexell s comet was subjected to no new disturbing 
 attractions, it was compelled, once in every revolution, 
 to return to the scene of its former encounter with the 
 planet Jupiter. This return was fraught with danger 
 to the stability of the comet s motions. So long as 
 Jupiter was not near that particular part of his orbit 
 at which the encounter had taken place, the comet was 
 free to pass the point of danger, and return towards 
 the sun ; but if ever it should happen that Jupiter was 
 close at hand when the comet approached his orbit, 
 then the comet would be as liable to have its motions 
 disarranged as at the original encounter. It happened 
 that the period of the comet s motion in its new orbit 
 was almost exactly one-half of Jupiter s period. 
 This was unfortunate ; since it clearly follows that, 
 when the comet had revolved twice, Jupiter had re 
 volved onc,e round the sun. Thus the comet again 
 encountered the planet, with what exact result has 
 never become known ; but certainly with this general 
 result, that the comet s movements were completely 
 disarranged. It has never returned to the neighbour 
 hood of the earth. 
 
 We may look upon Lexell s as the first discovered 
 
1 86 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 comet of short period ; for although it was never seen 
 after its first visit, yet nothing can be more certain 
 than that it did actually return once, and that it 
 went twice round its new orbit. Indeed, if it has 
 not been absorbed by Jupiter a very unlikely con 
 tingency it must still be revolving in space with an 
 orbit which brings it, once in each revolution, to the 
 scene of its former encounters. The figure of its orbit 
 may be altered again and again by encounters with 
 Jupiter ; but each new orbit must traverse this dan 
 gerous point. This follows directly from the laws of 
 orbital motion around an attracting centre. A body 
 will continue to revolve in any orbit along which it 
 has once begun to move, unless it is acted upon by 
 some extraneous force. Accordingly, if at any point 
 of its path an extraneous force suddenly disturb its 
 motion, the disturbed orbit cannot fail to pass through 
 the point of disturbance. Thus the body may again 
 fall under the influence of the disturbing agent, and be 
 caused to move in yet another orbit through the same 
 point. And in the course of millions of years, a body 
 might thus travel in a hundred different orbits, all 
 passing through a common point. There is, indeed, 
 one way in which Lexell s comet might have escaped 
 from Jupiter s control. If after one of its encounters 
 with Jupiter, it happened to pursue a path which 
 brought it very nearly into contact with Saturn or some 
 other large planet, it might be compelled thenceforth 
 to abandon its allegiance to Jupiter. But the proba 
 bility of this happening to a comet which had once got 
 
COMETS OF SHORT PERIOD. 187 
 
 into the toils, may be reckoned almost at naked 
 nothing. 
 
 We have been careful to dwell on this point for a 
 reason which will appear presently. 
 
 The search for Lexell s comet led to the discovery of 
 a considerable number of nebulae ; and the discovery 
 of nebulae led in turn to the discovery of another comet 
 of small period. In 1786 Mechain announced to Mes 
 sier (who had constructed a list of 103 nebulas) that he 
 had discovered a nebulous object. This turned out to 
 be a telescopic comet. It was again seen by Miss 
 Caroline Herschel in 1795, by Thulis in 1805, and by 
 Pons in 1818. All this time no suspicion had arisen 
 that these observers had seen the same object. But in 
 1818 the comet remained in view so long that it became 
 possible to calculate its orbit. This was done by the 
 German mathematician Encke, who found that the 
 orbit is an ellipse, and the period of revolution about 
 three years and four months. He found, after a labori 
 ous process of calculation, that it could be no other 
 than the object that attracted attention in 1786, 1795, 
 and 1805. Encke then applied himself to calculate 
 the next return of the comet, which he did so success 
 fully that astronomers have continued to call by his 
 name the object whose motions he had been the first to 
 interpret. 
 
 Encke s comet was seen by one observer only in 
 1822, as it was not favourably situated for observation 
 in the northern hemisphere that observer was M. 
 Riimker, who followed the comet for three weeks at 
 
1 88 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 the private observatory of Sir T. M. Brisbane at Para- 
 inatta. In 1825, the comet was detected by several 
 independent observers. It was seen again in 1828, 
 being detected by two observers Harding at Gottin- 
 gen, and Gambart at Marseilles. In 1832 and 1835, 
 it was seen from the observatory at the Cape of Good 
 Hope. 
 
 At the next return of the comet, which took place 
 on December 9, it was visible to the naked eye for the 
 first time since its discovery. At this passage, also, a 
 very noteworthy peculiarity was remarked or rather 
 a peculiarity which had been remarked by Encke in 
 1818, was now, for the first time, placed beyond a 
 doubt. Encke had suspected that the comet s period 
 was slowly diminishing. Each return to perihelion oc 
 curred about two and a half hours before the calculated 
 time. Such a discrepancy may appear very trifling, 
 and in fact it might seem that no certainty could be 
 felt respecting it ; and this is the case so far as one or 
 two revolutions are concerned. But when each succes 
 sive revolution shows the same discrepancy, the defi 
 ciency soon mounts up to a period respecting which 
 no doubt can be entertained. For example, between 
 the perihelion passage in 1789 and that of 1865, the 
 comet has made twenty-three revolutions, and each 
 has been less than the preceding by two and a half 
 hours (on the average). Hence, the last revolution of 
 the series occupied two days and a half less than the 
 first. But even this does not express the full effect of 
 the change ; for the comet having gained two and a 
 
COMETS OF SHORT PERIOD. 189 
 
 half hours in the first revolution, five in the next, seven 
 and a half in the next, and so on it is the sum of all 
 these gains (and not the gain made in the last revolution) 
 which expresses the total gain of the comet in point of 
 time. Hence the last perihelion passage occurred 
 twenty-nine days before the time at which it would 
 have taken place, but for some unknown cause which 
 has interfered with the comet s motion. What that 
 cause may be, has not yet been certainly determined ; 
 but it is at least highly probable that Encke has as 
 signed the true cause in suggesting that so light a 
 substance as the comet may be retarded in its passage 
 through the interplanetary spaces by the existence of 
 4 a thin ethereal medium, incapable of perceptibly 
 retarding the motion of the planets. 
 
 At first sight, it may seem strange that we should 
 speak of the acceleration of the comet as being caused 
 by the retarding influence of such a medium as has 
 been conceived to occupy the interplanetary spaces. 
 Yet it is strictly the case that, if a planet or comet be 
 continually checked in its onward course, its velocity 
 will continually grow greater and greater. For instance, 
 if our earth were so checked, it would move in a spiral 
 which would gradually bring its orbit to that of Venus, 
 by which time its motion would be as rapid as that of 
 Venus (which moves one-third faster than the earth); 
 then it would continue revolving in a spiral t ill it 
 reached the orbit of Mercury, when it would be moving 
 as fast as this the swiftest of all the planets. And so 
 
190 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 the earth would continue to approach the sun with 
 continually increasing velocity. 
 
 Returning to Encke s comet, we have to notice yet 
 another important discovery which was effected by its 
 means. The comet passed so near to Mercury in 1835 
 as to enable astronomers to form a much more satis 
 factory estimate of this planet s mass than had hitherto 
 been obtained. It was found that the mass of Mercury 
 had been largely over-estimated. 
 
 No very long interval passed after the discovery of 
 Encke s comet before another comet of short period 
 was detected. M. Pons, who had discovered Encke s 
 comet, it will be remembered, in 1818, observed a faint 
 nebulous object on June 12, 1819. This object turned 
 out to be a comet ; and in this case, as in the former, 
 Encke calculated the stranger s orbit and period. He 
 found that it moves in an ellipse which extends slightly 
 beyond the orbit of Jupiter, and that it has a period of 
 about five and a half years. This object was not seen 
 again, however, until the year 1858, when M. Winnecke 
 discovered it, and at first supposed it to be a new comet. 
 Calculation soon showed the identity of the two objects, 
 and confirmed the results which had been obtained by 
 Encke in 1819. 
 
 The next comet of short period was discovered by 
 M. Biela in 1826. Perhaps nothing in the whole his 
 tory of cometic observation is more surprising than 
 what has been recorded of this singular object. We 
 must premise that the comet had been seen in March 
 1772, and again in November 1805. But it was not 
 
COMETS OF SHOUT PERIOD. 191 
 
 until its re-discovery in 1826 that its orbit and period 
 were computed. An ellipse of moderate eccentricity, 
 extending beyond the orbit of Jupiter, was assigned 
 as the comet s orbit the period of revolution being 
 about six and a half years. The orbit was found to pass 
 within about twenty thousand miles of the earth s 
 orbit; and at the first return of the comet (in 1832), 
 some alarm was experienced lest the near approach of 
 the two bodies should lead to mischief of some sort. 
 The comet returned again in 1839 and 1845. It was 
 at the last-mentioned return that a singular pheno 
 menon occurred, which is nearly unique in the 
 history of comets. On the 19th of December 1845, 
 Hind noticed a certain protuberance on the comet s 
 northern edge. Ten days later, observers in North 
 America noticed that the comet had separated into 
 two distinct comets, similar in form, and each having 
 a nucleus, a coma, and a tail. European observers did 
 not recognise the bi-partition of the comet until the 
 middle of January 1846. The new and smaller comet 
 appears to have sprung into existence from the pro 
 tuberance observed by Hind, since this object moved 
 towards the north of the other. After a while, the new 
 comet became the brighter, but, shortly after, it re 
 sumed its original relative brilliancy. Lieutenant 
 Maury noticed, on one occasion, a faint bridge-like 
 connection between the two comets. The distance 
 between them gradually increased, until first the new 
 comet, and then the old one, had passed out of view. 
 In 1852, Biela s comet was again seen, and the 
 
192 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 Padre Secchi, at Rome, detected a faint comet pre 
 ceding it. If, as is probable, this faint comet is the 
 companion, we may assume that the two bodies are 
 permanently separated. 
 
 At the two next returns the comet was not seen, 
 and much interest was felt by astronomers respecting 
 the anticipated return in January 1866. It was 
 searched for systematically at the principal European 
 observatories. In fact, so closely did Father Secchi 
 examine the calculated track of the comet, that he 
 detected several new nebulas in that region. But the 
 comet itself was not found. Astronomers are unable 
 to assign any satisfactory reasons for its disappearance. 
 It is known to have traversed the zone of the Novem 
 ber meteors where that zone is richest our readers 
 will remember the display of shooting-stars in 1866 
 and Sir J. Herschel surmises that it may have been 
 destroyed in the encounter. Possibly this may be the 
 true solution of the difficulty ; or, it may be that the 
 comet was merely dispersed for a while during the 
 passage of the meteor-zone, and may yet gather itself 
 together and become visible to astronomers. 1 
 
 \Ve pass over three or four comets belonging to this 
 class which present no special features of interest, to 
 
 1 The return of this comet in 1872 was eagerly looked for by astro 
 nomers. But the comet was not seen. On November 27, 1872, there 
 was a fine display of meteors, as the earth passed through the comet s 
 track, and afterwards a cometic object was seen in the direction towards 
 which the meteors had been travelling. But this was not Biela s comet, 
 which, indeed, mubt have passed that place nearly twelve weeks earlier. 
 Indeed, some doubt exists whether the object was travelling in the 
 track either of the comet or of the meteors. 
 
COMETS OF SHORT PERIOD. 193 
 
 come to an object which has recently been rediscovered, 
 and will continue visible (in good telescopes) for 
 several weeks. On February 6, 1846, M. Brorsen 
 discovered a telescopic comet, whose motions soon 
 showed it to belong to the class of objects we are now 
 dealing with. It was found to have an orbit of mode 
 rate eccentricity, extending just beyond Jupiter s orbit, 
 and a period of about five and a half years. It was 
 not seen at its next return to perihelion ; but was re 
 discovered by M. Bruhns on March 18, 1857. In 
 1862, it again escaped undetected; but at its present 
 return, it has been rediscovered (by three observers 
 simultaneously), and it is now being carefully tracked 
 across the northern skies. 
 
 In all, there have been recognised thirteen comets of 
 short period that is, having periods of less than seven 
 years. Amongst these are included several which have 
 only been seen once, and some which are known to have 
 been subjected to such disturbance as no longer to 
 travel in orbits of short period. Of these thirteen 
 comets, no less than ten have the aphelia of their 
 orbits just beyond the orbit of the planet Jupiter ; two 
 have their aphelia just within Jupiter s orbit; and 
 Encke s cornet alone has its aphelion at a safe distance 
 from that orbit. It appears to us that the peculiarity 
 thus exhibited is not without meaning. Eemembering 
 the history of Lexell s comet, we seem to find a satis 
 factory explanation of the peculiarity. We have seen 
 how Lexell s comet was first introduced into the system 
 of short-period comets by the giant planet Jupiter, and 
 
 o 
 
IQ4 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 then summarily dismissed. So long as the comet re 
 mained within that system, the aphelion of its orbit 
 lay just beyond the orbit of Jupiter, and this would 
 be the case with any comet introduced in a similar 
 manner. But for the coincidence which led to its 
 expulsion, Lexell s comet would have continued to 
 revolve as a short-period comet. It seems also clear, 
 that in the course of many ages, its period and orbit 
 would have grown gradually smaller, through the 
 operation of the same cause (whatever that may be) 
 which is now reducing, the period and orbit of Encke s 
 comet. At length it must have attained a path safe 
 within the orbit of the great disturbing planet. In 
 the list of short-period comets, then, we seem to see 
 illustrations of the successive stages through which 
 Lexell s comet would have passed in attaining the sort 
 of orbit in which Encke s comet is now moving. And 
 it seems permissible to assume that all the short-period 
 comets have been introduced to their present position 
 within the solar system by the same cause which led 
 to the temporary appearance of Lexell s comet as a 
 comet of short period that is, by the attractive 
 energy of the planet Jupiter. 
 
 Chambers s Journal, July 1868. 
 
195 
 
 THE GULF STREAM. 
 
 MAJOR EENNELL was the first, I believe, to whom we 
 owe the comparison of ocean-currents to rivers. He 
 spoke of them as ocean-rivers, and pointed out how 
 enormously their dimensions exceed those of such 
 streams even as the Amazon and the Mississippi. Some 
 of the ocean-currents are from 50 to 250 miles in 
 breadth, and flow more swiftly than the largest navi 
 gable rivers. The banks and bottom of these currents 
 are not land, but water ; and so deep are the currents 
 that they are turned aside by shoals and banks whose 
 tops are 40, 50, or even 100 fathoms beneath the 
 surface of the ocean. The outlines of ocean-currents 
 are sharply denned, insomuch that < often, says Captain 
 Maury, one half of a vessel may be seen floating in the 
 current, while the other half is in common water of the 
 sea. The border-line of the Grulf Stream can be traced 
 by the eye. Yet more remarkable is the distinction 
 between the moving water and that which is at rest, 
 when large masses of sea-weed carried along by the 
 former enable one to recognise the rapidity with which 
 it moves. 
 
 Of all the ocean-currents the most important, perhaps, 
 in its bearing on the destinies of men and nations, is 
 the great Grulf Stream. I propose to examine the 
 
 o 2 
 
196 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 course and habitudes of this current, and then to inquire 
 a little into the vexed question of its cause. 
 
 Major Eennell traced the Gulf Stream from a sup 
 posed source in the Indian and Southern Oceans. 
 Modern geographers and physicists prefer to look for 
 the rise of the current somewhere near the Cape of 
 Good Hope. The commencement and first impulse 
 of the mighty Gulf Stream is to be sought, writes 
 Humboldt, 6 southward of the Cape of Good Hope. 
 It appears to me, however, that the true source of the 
 great stream is to be looked for in the equatorial zone 
 of the Atlantic. When we come to inquire into the 
 cause or causes which give birth to the Gulf Stream, 
 we are led, as I imagine, to this region rather than to 
 any other (though, perhaps, in a stream which forms 
 part of a continuous system of circulation, we can 
 hardly speak of any one portion as the source) ; I shall 
 therefore trace the stream, and the system to which it 
 belongs, from the great equatorial waters which move, 
 as Columbus was the first to discover, with the heavens 
 (las aguas van con los cielos), that is, from east to 
 west, following in this the apparent motions of the sun, 
 moon, and stars. 
 
 The map of the Atlantic Ocean on p. 217, is con 
 structed upon one of those forms of isographic pro- 
 jection described in my Essays on Astronomy. It is 
 important, in dealing with the subject of currents, that 
 the question of area should be considered, and, there 
 fore, that our illustrative charts should represent such 
 areas correctly. This Mercator s charts are far from 
 
THE GULF STREAM. 197 
 
 doing. The portion of the Atlantic Ocean between 
 England and the United States of America is unduly 
 magnified, and still more is this the case with the 
 portion between Sweden and Greenland. On the other 
 hand the portion between Africa and the Ghilf of Mexico 
 is unduly diminished. Thus it is scarcely possible to 
 form from such charts just notions of the actual cha 
 racter of the oceanic circulation whereof the Gulf Stream 
 forms a part. (Compare the charts illustrating the 
 Essay on the Climate of Great Britain, pp. 264, 265.) 
 
 We see, in our map (p. 217), 1 that there is a great 
 equatorial stream extending in its eastern portion far to 
 the south of the equator, but passing to the north also 
 even here, and still further to the north between the 
 coasts of Africa and South America. Near here the great 
 equatorial current divides into two portions. One passes 
 southward and then returns towards the east, according 
 to some authorities, but, according to others, continues 
 its course southward until it is lost in the Antarctic 
 Ocean. We shall follow the northern bifurcation, 
 however. The course of this portion of the Atlantic 
 current system has been far more exactly traced out. 
 Taking a north-westerly course, the great current pours 
 itself against the barrier formed by the Leeward and 
 Windward Islands. Passing between these islands, it 
 sweeps around the shores of the Grulf of Mexico, a por- 
 
 1 For the sake of completeness, and also that the present essay may 
 fairly represent my views when it was written, I leave the account of the 
 map and of the course of the Gulf Stream unchanged here. By com 
 paring this essay with the following, it will be noticed that only very 
 few passages are repeated in substance. 
 
198 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 tion, however, of its volume passing probably outside 
 the West Indian Islands, to rejoin the other outside the 
 promontory of Florida. At this point the stream h^s 
 become, probably, somewhat diminished in volume, 
 but being still more diminished in breadth, it flows as 
 a deep, strong, and swift stream, known among sailors 
 as The Narrows of Bernini. From hence the stream, 
 now become the true Gulf Stream, grows gradually 
 wider, less deep, and less swift. Off Hatteras it is 
 already twice as broad as in the Florida Straits, and 
 as it stretches with a wide easterly sweep across the 
 Atlantic towards the shores of Ireland and the Hebrides, 
 the current not only reassumes something of its ori 
 ginal extent of surface, but again bifurcates ; a wide 
 but somewhat sluggish stream is sent southward 
 towards the shores of north-western Africa, to rejoin 
 the equatorial stream. The main portion of the 
 current, however, passes with a north-easterly course 
 up the Atlantic valley, between Iceland and Sweden 
 to the Polar seas. It seems uncertain whether Rennell s 
 current, which passes around the Bay of Biscay, and 
 the current which streams southward past the shores of 
 Spain, are forks of the Gulf Stream. They are usually 
 represented in maps as independent currents, and in 
 Captain Maury s large map of the Gulf Stream the 
 great southern bifurcation already mentioned is repre 
 sented as a current impinging upon the flank of the 
 stream which flows past Sp#in and north-western 
 Africa. Yet, if these streams have not their source in 
 the Gulf Stream, it will be found no easy problem to 
 
THE GULF STREAM. 199 
 
 assign their origin ; and I cannot but think that the 
 Biscay and Guinea currents, as well as the current 
 which flows into the Mediterranean through the Straits 
 of Gibraltar, are as truly bifurcations of the Gulf 
 Stream as the current which laves the shores of Ireland 
 and Sweden. 
 
 There will be noticed also in the map three return 
 streams, one flowing southward outside Iceland, another 
 sweeping round the eastern shores of Greenland, and 
 the third flowing through Baffin s Bay and Davis s 
 Straits. The two last unite south of Davis s Straits, and 
 flow on together to meet the first stream outside New 
 foundland, whence the three flow as a single current 
 past the shores of the United States. It is generally 
 assumed, and in all probability justly, that these three 
 streams are derived from the Gulf Stream, and are 
 different branches of its returning waters. 
 
 Between the single return-stream which laves the 
 shores of the United States and the Gulf Stream there 
 is an unshaded space in the map. It is not to be in 
 ferred, however, that this space represents still (or 
 rather unflowing) water. On the contrary, it is the 
 debatable ground between the opposite currents. In 
 spring the whole of this space is occupied by the south 
 ward flowing waters of the cold return-current. In 
 autumn the whole of the space is occupied with the 
 waters of the Gulf Stream. Backwards and forwards 
 over this space the rival currents are continually sway 
 ing, the period of an oscillation being one year. 
 
 In the widest part of the Atlantic Ocean that, 
 
200 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 namely, which extends between the most westerly part 
 of Africa and the West Indies there is a wide expanse 
 of waters unmoved by the flux or reflux of currents. 
 Surrounded on every side by the circulating waters of 
 the Central Atlantic current-system, this region re 
 mains undisturbed save by winds and the tidal wave. 
 Accordingly its surface is covered with different forms 
 of marine vegetation. My readers will doubtless 
 remember the interest which the Great Sargasso Sea 
 excited in the mind of Christopher Columbus. Oviedo 
 termed this region the seaweed meadow. A host of 
 small marine animals, says Humboldt, inhabit this 
 ever-verdant mass of Fucus natans, one of the most 
 widely-diffused of the social plants of the ocean, con 
 stantly drifted hither and thither by the tepid winds 
 that blow across its surface. 
 
 In the South Atlantic there is a smaller and some 
 what more sharply-defined Sargasso, covered chiefly 
 with rockweed and drift. A weedy space occurs also 
 about the Falkland Islands, but is probably not a true 
 Sargasso. Maury considers that the seaweed reported 
 there probably comes from the Straits of Magellan, 
 where it grows so thickly that steamers find great 
 difficulty in making their way through it ; for it so 
 cumbers their paddles as to make frequent stoppages 
 necessary. 
 
 Such is the distribution of the surface of the Atlantic 
 Ocean. But now the question will at once suggest 
 itself: Is the complete system of oceanic circulation 
 exhibited on the surface ? It seems now quite certain 
 
THE GULF STREAM. 2OI 
 
 that this question must be answered in the negative. 
 We might, indeed, at once point to the existence of the 
 important current which laves the shores of the United 
 States as an answer to the question ; for where can all 
 this water find an outlet ? It does not pass the Penin 
 sula of Florida as a current ; it does not cross the Grulf 
 Stream ; where, then, can it go but underneath the 
 ocean s surface ? But w 7 e have positive evidence of the 
 existence of under- currents. 
 
 In the first place it is found that in deep-sea sound 
 ings in many parts of the ocean, far more line may be 
 paid out without any sign of the bottom being reached 
 than the depth of the ocean in those parts would 
 account for. In places where it has been proved by 
 other methods than ordinary sounding that the depth 
 is not more than three miles, no less than ten miles of 
 line have been paid out, being carried out so strongly 
 that the slightest check in the paying-out apparatus 
 has sufficed to break the sounding-line. 
 
 In the second place, it has been found possible to 
 determine the depth at which a submarine current is 
 flowing, and the direction in which it flows. Thus 
 Lieuts. Walsh and Lee, in the American service, having 
 loaded a block of wood to sinking, and let it down to 
 different depths, had repeatedly the satisfaction of 
 seeing the work of under-currents. It was wonderful, 
 indeed, they write, to see the barrega (a float attached 
 to the upper end of the line) moving off, against wind, 
 sea, and surface current, at the rate of over one knot 
 an hour, as was generally the case, and on one occasion, 
 
202 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 as much as one and three-quarter knots. The men in 
 the boat could not repress exclamations of surprise, for 
 it really appeared as if some monster of the deep had 
 hold of the weight below, and was walking off with it. 
 
 Lastly, we may mention that Captain Wilkes, of the 
 United States Exploring Expedition, established the 
 existence of a cold under-current no less than two hun 
 dred miles broad at the equator. 
 
 We may assume, then, that a complete system of cir 
 culation, vertical as well as horizontal, exists throughout 
 the whole of the waters contained within the great 
 Atlantic valley. 
 
 Where are we to look for the origin of this vast series 
 of movements ? The actual work done in the Atlantic 
 Ocean is so enormous in other words, the transfer of 
 such large volumes of water represents so enormous a 
 force, that we might well expect to be able at once to 
 assign the motive-power of this great machinery. For 
 it would seem that the giant which works such wonders 
 could not readily hide himself from our recognition. 
 
 It has not been found, however, that the solution of 
 the problem has been so simple as was to have been 
 anticipated. 
 
 Passing over the earlier guesses which marked the 
 Grulf Stream as the offspring of the Mississippi Kiver, 
 of the sun s motion in the ecliptic (a mysterious inter 
 pretation of the phenomena), and of the tidal wave, we 
 may remark that but two explanations of the Atlantic 
 currents seem to merit discussion. 
 
 Sir John Herschel is the principal exponent of the 
 
THE GULF STREAM. 203 
 
 first theory, which assigns to the trade- winds the princi 
 pal almost the sole agency in the generation of the 
 Atlantic current-system. He refuses indeed, to look 
 on the subject as one of any doubt or difficulty. The 
 dynamics of the Grulf Stream have of late, he writes, 
 been made a subject of much (we cannot but think 
 misplaced) wonder, as if there could be any possible 
 ground for doubting that it owes its origin entirely to 
 the trade-winds. If there were no atmosphere, there 
 would be no Grulf Stream, or any other considerable 
 oceanic current (as distinguished from a mere surface- 
 drift) whatever. He presents his solution somewhat 
 as follows : The trade-winds are an actually existent 
 cause for an easterly motion in the tropical seas ; we 
 cannot ignore their action ; we know, also, that when 
 the trade-winds arrive at the equator, they have lost 
 their easterly momentum; and we know, therefore, 
 that that momentum must have been imparted to the 
 surface of the water (for where else can it have gone ?) ; 
 hence there arises the great easterly movement which 
 generates the whole system of circulation. 
 
 The second view, which attributes oceanic circulation 
 to differences of temperature and saltness in different 
 parts of the ocean, is supported by Humboldt and others, 
 but is taken up most unflinchingly by Captain Maury, 
 who assigns it as practically the sole cause of all oceanic 
 circulation. The Gulf Stream, he writes, as well as 
 all the constant currents of the sea, is due mainly to this 
 cause. Such differences are inconsistent with aqueous 
 equilibrium, and to maintain this equilibrium the 
 
204 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 great currents are set in motion. The agents which 
 derange equilibrium in the waters of the sea, by altering 
 specific gravity, reach from the equator to the poles, 
 and in their operations they are as ceaseless as heat and 
 cold ; consequently, they call for a system of perpetual 
 currents to undo their perpetual work. Other causes 
 help to cause currents, he says, but the currents created 
 by them are ephemeral? 
 
 Here we have what is a very pretty quarrel as it 
 stands. Each of the disputants points to causes of 
 acknowledged importance, and also (whether efficient 
 or not in the particular matter under question) of 
 acknowledged general efficiency. Each has much to 
 say in favour of his own view, and each considers his 
 antagonist s agent as utterly insufficient for the work 
 ascribed to it. Each has heard his opponent s argu 
 ments, and reiterates his own statement. Nor can it be 
 said that the opponents are unequally matched ; for, if 
 we must place Sir John Herschel far before Maury as a 
 mathematician and physicist, and if we must undoubt 
 edly look upon the former as the more practised 
 reasoner, yet we must remember, in turn, the special 
 attention which Captain Maury has given to the subject 
 under discussion, and the practical acquaintance with it 
 which his experience as a seaman has given to him. 
 
 Let us briefly state the arguments adduced by 
 Herschel against Maury s view, and by Maury against 
 Herschel s. 
 
 Sir John Herschel asserts that, inasmuch as the sun s 
 heat warms the surface of the ocean most intensely, so 
 
THE G ULF STREAM. 205 
 
 that the water of least specific gravity is already upper 
 most, there can be no tendency to motion. For the 
 heated water cannot descend^ being buoyant ; nor 
 ascend, being uppermost ; nor move laterally, having 
 no impulse to motion of that sort, and being only able 
 to move laterally * by reason of a general declivity of 
 surface, the dilated portion occupying a higher level. 
 He then applies to this declivity the test of quanti 
 tative analysis. Taking a column of water at the 
 equator having at the base a temperature of 39 (at 
 which temperature fresh water attains its greatest 
 density, and which is also the temperature of water 
 7,200 feet beneath the surface at the equator), while its 
 top has a temperature of 84 (the warmth of equatorial 
 surface-water), he finds that such a column is 10 feet 
 higher than a similar column in latitude 56, where 
 39 is the surface temperature. And since from 
 the equator to latitude 56 the distance is 3,360 
 geographical miles, we have a declivity of barely one- 
 twenty-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 the 
 existence of a strong current from the equator towards 
 the tropics ; while, so far from giving any account of 
 the motion of the equatorial current from east to west, 
 it would tend to form a north-easterly current. 
 
 This seems strongly opposed to Maury s view, and I 
 do not find that he does much to get over the force of 
 Herschel s reasoning. He points out, indeed, that sea- 
 water does not attain its greatest density at a tempera- 
 
206 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 ture of 39, but some 12 or 14 lower. This, however, 
 does not affect Herschel s argument. If he had taken 
 a column whose base had a temperature of 25 instead 
 of 39, he would have had to extend, also, the range of 
 the water-slope in latitude ; and, in fact, he would have 
 obtained a yet smaller declivity in this way than that 
 actually deduced by him. Maury does not seem to 
 have noticed the really weak point in Herschel s argu 
 ment. I shall presently show where this seems to me 
 to lie. 
 
 But if Maury fails in efficiently defending his own 
 views, he certainly is sufficiently effective in his attack 
 upon Sir John Herschel s. 
 
 He asks, in the first place, the pertinent question 
 How can the north-easterly trade-winds, which blow 
 only 240 days out of 365, cause the equatorial current 
 to flow all through the year towards the north-west 
 without varying its velocity either to the force or to the 
 prevalence of the trade-winds ? * That the winds do 
 make currents in the sea, no one, he says, will have 
 the hardihood to deny ; but currents that are born of 
 the winds are as unstable as the winds; uncertain 
 as to time, place, and direction, they are sporadic and 
 ephemeral. 
 
 He then points to a fact which militates strongly 
 against the vast current-begetting power that has been 
 given by theory to the gentle trade-winds. In both 
 oceans, the Sargasso seas lie partly within the trade- 
 wind region ; but in neither do these winds give rise 
 to any current. The weeds are partly out of water, and 
 
THE GULF STREAM. 207 
 
 the wind has therefore more power upon them than it 
 has upon the water itself ; they tail to the wind. And 
 if the supreme power over the currents of the sea reside 
 in the winds, as Herschel would have it, then of all 
 places in the trade-wind region, we should here have 
 the strongest currents. Had there been currents here, 
 these weeds would have been borne away long ago ; but 
 so far from it, we know that they have been in the 
 Sargasso Sea of the Atlantic since the voyage of 
 Columbus. 
 
 In another argument, Maury certainly falls into an 
 error. He says, How can the north-easterly winds 
 cause the Grulf Stream to flow towards the north-east ? 
 But, as he himself points out, the trade-winds do not 
 blow over the Grulf Stream proper, and there can be no 
 doubt that, if the trade-winds sufficed to keep up a 
 continual equatorial current, finding a passage towards 
 the north after encountering the barrier opposed by the 
 American continent, this resulting northerly current 
 would assume a north-easterly course, for the very same 
 reason that the air-currents flowing from the equator 
 towards the north pole become south-westerly or counter 
 trade-winds. But he seems justified in asking how it 
 is possible that the impulse imparted by the gentle 
 trade-winds to the equatorial current could suffice to 
 generate a stream which eventually travels far towards 
 the north pole, if it do not even circle completely around 
 Greenland. When we inject water into a pool, he 
 says, be the force never so great, the jet is soon over 
 come, broken up, and made to disappear. In this 
 
208 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 illustration, the Gulf Stream may be likened to the jet, 
 and the Atlantic to the pool. We remember to have 
 observed, as children, how soon the mill-tail loses its 
 current in the pool below ; or we may now see at any 
 time, and on a larger scale, how soon the Niagara, cur 
 rent and all, is swallowed up in the lake below. 
 
 Franklin, who was the originator of the theory sup 
 ported by Herschel, had unnecessarily introduced the 
 supposition that the trade-winds maintain a c head of 
 water in the Gulf of Mexico, and that the Gulf Stream 
 flows downwards like a river from this head, as a 
 fountain or source. Maury rightly attacks this view, 
 which is undoubtedly a mistaken one ; but in doing so, 
 he falls into an error which exhibits his weakness in 
 the treatment of hydrodynamical problems. He points 
 out that, inasmuch as the Gulf Stream grows wider as 
 it crosses the Atlantic, it necessarily grows shallower, 
 so that the water-bed in which the stream flows has a 
 higher level under the shallow than under the deep 
 part of the current, and therefore, says Maury, the 
 current runs up hill. 9 Herschel terms this a strange 
 perversion of language, but perhaps it would be more 
 correct to speak of it as a strange blunder. The stream 
 could, of course, only be said to run up hill if its surface 
 were seeking a higher level, which does not and cannot 
 happen. That the spreading out of the water of the 
 current, so as to form a wider and shallower stream, 
 does not correspond to an upward flow, is evident from 
 this, that it happens often with rivers, which no one 
 will suspect of running up hill. 
 
THE GULF STREAM. 209 
 
 Herschel does not find an answer to the main objec 
 tions urged by Maury against the trade- wind theory. 
 Content with urging an apparently unanswerable objec 
 tion against his opponent s view, he leaves his own to 
 take care of itself. 
 
 In forming an opinion respecting the two theories, 
 one is struck with the immense superiority in the poiver 
 of Maury s agent. For, if we consider, we shall see that 
 almost the whole of the sun s action upon the ocean 
 goes to produce those variations in temperature and 
 saltness in which Maury sees the origin of the current- 
 system ; but a very moderate portion of the sun s action 
 is called into play in the production of the trade-winds. 
 Now it is very doubtful whether any large proportion 
 even of the force expended in producing the trade-winds, 
 ever acts on the water. For we know that the north 
 easterly and south-easterly air-currents of the northern 
 and southern hemispheres, do not wholly merge into 
 northern and southern currents meeting point-blank 
 near the equator, as Herschel s theory seems to imply. 
 On the contrary, there is a wide zone of calms at the 
 equator, and the two systems of trade-winds appear to 
 pass upwards above the calm air, without parting with 
 the whole of their easterly motion. When once they 
 begin to travel polewards, they lose their easterly motion 
 in the same way that they acquired it that is, through 
 the effects of the earth s rotation. And whatever portion 
 is lost in this way which, for aught we know, may be 
 a very considerable portion cannot be taken into 
 
 p 
 
2 i O LIGHT SCIENCE FOR LEISURE HO URS. 
 
 account as available to generate the easterly equatorial 
 current. 
 
 And now let us consider for a moment the relation 
 which holds between cause and effect in the case sup 
 posed by Herschel. We have more than a fourth part 
 of the Atlantic Ocean in a state of perpetual motion, 
 and it is assumed that the air immediately above the 
 ocean is responsible for this circulation. Now even if 
 we suppose that the whole of the vis viva in the aerial 
 circulation is imparted to the waters, and neglect all 
 consideration of the fact that for a large portion of the 
 year the winds do not act in the manner available for 
 the production of the currents we are considering, yet 
 even then, I apprehend that we shall find the vis viva 
 of the aerial very far below that of the aqueous circula 
 tion. The volume of moving water is, of course, far 
 less than that of the moving air, and the mean velocity 
 of the water-currents is less than that of the air-cur 
 rents ; but, on the other hand, the specific gravity of 
 water is some 830 or 840 times greater than that of 
 air, and this difference far more than counterbalances 
 the others. 
 
 But now, when we come to consider the forces called 
 into action in producing changes of temperature, etc., 
 we no longer find such a disproportion between cause 
 and effect. The sun s action on the equatorial and 
 tropical regions of the Atlantic not only produces a 
 great change in the density of the water, but also 
 raises immense masses by evaporation. Now the buoy 
 ancy caused by increase of temperature is partly 
 
THE GULF STREAM. 2 1 1 
 
 diminished through increase of saltness ; still it is an 
 important motive force. A large portion of the evapo 
 rated water is also precipitated over the equatorial 
 regions in the form of rain ; yet that a very large por 
 tion is carried away from equatorial and tropical to 
 temperate zones is beyond dispute. 
 
 But now, how are we to get over the arguments by 
 which Herschel seeks to show that the buoyant water 
 will not rapidly move off, and that the effect of evapo 
 ration is merely to produce opposing inrushes of water 
 which destroy each other s effect ? Easily, I take it, if 
 we remember that the buoyancy of the water does pro 
 duce a surface-flow from the equator, however slight, 
 and that this is sufficient to destroy the balance of 
 forces which might otherwise make it doubtful whether 
 the place of the evaporated water would be supplied 
 from below or from above. I apprehend that there is 
 a continual under-flow of cooler water, rushing in 
 towards the equator on both sides, to supply the place 
 of the water evaporated by the sun s heat. Now there 
 can be no question that under-currents arriving in this 
 manner, whether from the north or from the south, 
 would acquire a strong westerly motion (just as the 
 trade-winds do). Thus they would generate from below 
 the great equatorial westerly current. In this up-flow 
 of cool currents having a strong westerly motion, I find 
 the mainspring of the series of motions. The water 
 thus pouring in towards the equator is withdrawn from 
 beneath the temperate and arctic zones, so that room is 
 continually being made for that north-easterly surface- 
 
 p 2 
 
212 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 stream which is the necessary consequence of the con 
 tinual flow of the great western equatorial current 
 against the barrier formed by the American continent. 
 It would require much more space than I have at my 
 disposal to deal at length with the subject of my paper. 
 I therefore conclude by referring my readers to Maury s 
 interesting work on the Physical Geography of the 
 Sea, with the remark that his views seem to me only 
 to require the mainspring or starting force towards the 
 west which I have ventured to suggest, to supply a 
 complete, efficient, and natural explanation of the whole 
 series of phenomena presented by the great ocean- 
 currents. 
 
 The Student for July 1868. 
 
 OCEANIC CIRCULATION. 
 
 THERE are some questions, seemingly innocent enough, 
 which yet appear fated to rouse to unusual warmth all 
 who take part in their discussion. One cannot, for 
 instance, find anything obviously tending to warmth of 
 temper in the telescopic study of a planet ; yet the 
 elder Cassini was moved to passionate invective by cer 
 tain observations of Mars not perfectly according with 
 his own ; and Sir W. Herschel, usually so philosophic, 
 was roused by Schroter s recognition of mountains in 
 Venus to deliver himself of a criticism justly described 
 
OCEANIC CIRCULATION. 213 
 
 by Arago as c fort vive, et, en apparence du moms, 
 quelque peu passionnee. The question, again, whether 
 the Eozoon Canadense is a true Rhizopod, though 
 not altogether removed from the region of hard words, 
 might appear to be unlikely to excite warlike emotions ; 
 yet there has been some very pretty fighting over it. 
 The solar corona has in like manner given occasion for 
 rather strong writing; and if, on the one hand, the 
 supporters of a lately-abandoned theory said of their 
 opponents that they made themselves ridiculous, 
 these, in their turn, at times used a tone reminding 
 one of the scholar who said of a rival, May Grod con 
 found him for his theory of the Irregular Verbs : yet 
 the corona seems at a first view rather calculated to 
 produce a sedative effect than to excite unphilosophic 
 wrath. The subject of oceanic circulation would appear 
 to belong to the class of questions here considered. 
 
 The very name of the Grulf Stream is to some phy 
 sical geographers as a red cloth is to a bull. Even Sir 
 John Herschel, usually placidity itself, was moved when 
 he spoke on this point. But though he and Maury 
 grew warm enough in its discussion, their warmth wap 
 ice-cold compared with the fire of more recent dis 
 putants. We have before us the latest contribution to the 
 subject, a rather ponderous essay in one of our leading 
 quarterlies ; and herein we find pleasing references to 
 the stupidities of one set of opponents, the shallow 
 nonsense of a second, c the wrong-headedness of a 
 third, with other similar amenities. More than once 
 
214 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 during the progress of this controversy the gentle public 
 has been reminded of Bret Harte s remarks 
 
 about the row 
 That broke up the Society upon the Stanislow ; 
 
 and has been inclined to urge, with fc Truthful James, 
 that they 
 
 Hold it is not decent for a scientific gent 
 To say another is an ass, at least to all intent ; 
 Nor should the individual who happens to be meant, 
 Eeply by heaving rocks at him to any great extent. 
 
 The controversy has not, indeed, reached this last 
 stage of development, and we trust it never will ; but 
 it has gone so near to it as to suggest that the dis 
 putants have wished to demonstrate, by example, the 
 justice of Darwin s theory about the human snarling 
 muscles. ! 
 
 I propose to inquire into the subject which has been 
 thus warmly discussed, trusting not to be myself in 
 veigled by it into any warmth of expression. Indeed, 
 
 1 He who rejects with scorn the belief that the shape of his own 
 canine teeth, and their occasional great development in other men, are 
 due to our early progenitors having been provided with these formidable 
 weapons, will probably reveal, by sneering, the line of his own descent. 
 For though he no longer intends, nor has the power, to use these teeth 
 as weapons, he will unconsciously retract his " snarling muscles " (thus 
 named by Sir Charles Bell), so as to expose them ready for action, like 
 a dog prepared to fight/ Darwin s Descent of Man,* vol. i. p. 176. 
 We may mention, by the way, that an instance has recently occtirred, 
 in which the human teeth were used to some purpose against one of the 
 recognised masters in the art of biting. A man, proceeding in company 
 with several others through a wood, was attacked by a hyena (usually 
 one of the most cowardly of beasts). His companions fled, and having 
 no weapon he was reduced to the necessity of showing tooth for tooth, 
 and taking a good grip of the hyena s nose, he compelled that gentle 
 man to howl with anguish. On this, the man s companions returned 
 and presently beat the hyena to death. 
 
OCEANIC CIRCULATION. 
 
 but for the fate of others, I should feel no anxiety on 
 this point, though I have myself a favourite theory to 
 uphold respecting one branch of the subject. As it is, 
 I share something of the feeling of the Red Cross 
 Knight when he was approaching < Foul Error s den, 
 and his monitress said to him, The perils of this place 
 I better wot than thou ; therefore I rede, Beware. I 
 am not without hope, however, that I may be able to 
 keep my snarling muscles quiescent. 
 
 I shall direct attention chiefly to the Atlantic cur 
 rents, as being those whose real direction and extent 
 are best known, and those, moreover, whose character 
 istics are most important to European nations. 
 
 Let us begin with the surface currents, and though 
 the system of surface circulation can scarcely be said to 
 have a real beginning, let us start with the great equa 
 torial currents which flow westwards from the Ghilf of 
 Gruinea, 1 or more correctly from the Bight of Biafra. 
 We speak of the westwardly equatorial currents, because 
 not unfrequently there is an equatorial eastward current 
 running between two much more important tropical 
 westward currents. Yet ordinarily there is one great 
 westward current running in an unbroken stream from 
 equatorial Africa to the shores of Brazil, and even when 
 this great current is divided into two by an eastward 
 current this last is only to be regarded as a sort of 
 4 backwater. The water moving westwards is relatively 
 cold, more especially on the African side of the Atlantic. 
 
 1 Along the shores of the Gulf of Guinea there flows an easterly 
 current, several degrees warmer than the equatorial current. 
 
2l6 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 The accompanying map exhibits the nature of the 
 surface circulation of the North Atlantic. It is con 
 structed on one of the forms of equal-surface projection 
 described in my Essays on Astronomy, and has the 
 advantage over the ordinary Mercator s charts of ex 
 hibiting the true dimensions of the various currents. 
 I would, however, invite the student who wishes to fa 
 miliarise himself with the true nature of the Atlantic 
 currents to construct other maps ; for instance, a polar 
 map on the first method of equal-surface projection 
 described in that essay (see pp. 264, 265), and a map of 
 the whole Atlantic on the second plan, taking the 
 meridian 40 west of Greenwich as the central one. 
 
 Of the water carried westwards by the great equa 
 torial movement, the most important portion after 
 reaching Brazil is carried northwards towards the West 
 Indies. The reason of this is obviously to be found 
 in the fact that Cape San Eoque, forming the jutting 
 angle of Brazil, lies several degrees south of the equa 
 tor. The portion carried southward forms the Brazil 
 Current, and after travelling along the shores of South 
 America almost as far as the mouth of the La Plata, 
 acquires gradually an eastwardly motion which eventu 
 ally carries it back across the Atlantic towards the Cape 
 of Good Hope, there to pass northwards, and so again 
 to traverse the Bight of Biafra. The surface-circula 
 tion in the South Atlantic is thus seen to be com 
 paratively simple. 
 
 The larger portion of the equatorial current is carried 
 less quickly northward, because the northern shore-line 
 
2l8 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 of Brazil and Guiana is inclined at a much smaller 
 angle than the south-eastern to the westwardly course 
 of the great equatorial currents. Thus the water which 
 is carried towards the West Indies has time to acquire 
 under the tropical sun a much higher temperature than 
 it had possessed when traversing the Gulf of Guinea. 
 It is divided into two parts by the guasi-barrier which 
 the West Indian Islands (or rather the semi-submerged 
 mountains of which they form the crests) oppose to its 
 progress. A comparatively small portion finds its way 
 into the Caribbean Sea, and making the circuit of the 
 Gulf of Mexico, passes out eastwards round the penin 
 sula of Florida. We may fairly assume that this por 
 tion is comparatively small ; simply because this true 
 gulf stream, passing between Cuba and Florida on an 
 eastern course, would continue so to move for at least 
 some considerable distance, were it not in some way 
 deflected. But it actually turns almost due northwards 
 after passing through the Bahama Sea, traversing the 
 Bernini Narrows on this course, and so onwards towards 
 Hatteras. This would seem to imply that the true Gulf 
 Stream is pressed northwards by the arrival of a much 
 larger body of water which has travelled outside the 
 West Indies. It is true that the diversion of the Gulf 
 Stream northwards may be really caused by the great 
 Bahama Bank. But this would equally establish our 
 position ; for if the Bahama Bank is thus effective in 
 diverting the whole of this now swiftly moving current, 
 the Windward Isles may be assumed to be correspond 
 ingly effective in diverting the greater portion of the 
 
OCEANIC CIRCULATION. 219 
 
 sluggish equatorial current. Moreover, if we remember 
 how shoals commonly take their origin, we may con 
 sider that the very existence of the Bahama Bank is 
 probably due to the former encounter of the two im 
 portant branches of the equatorial current the part 
 which had circled the Gulf of Mexico and the part 
 which had travelled outside the West Indies. Thus, 
 the northerly course finally taken by the Gulf Stream 
 implies that the latter portion had prevailed over the 
 former, and therefore that it is the most considerable 
 portion, I must mention, however, that the Edinburgh 
 Eeviewer holds the part which enters the Caribbean Sea 
 to be the larger. 
 
 Be this as it may, the Gulf Stream proper has ac 
 quired, during its circuit, characteristics perfectly 
 distinct from those which it had had when entering the 
 Caribbean Sea, or from those possessed by the remain 
 ing portion when approaching the Bahamas. In the 
 first place, having traversed a much longer course under 
 the same intense tropical heat, the Gulf Stream has 
 become much warmer than the outer stream. In the 
 second place (probably from having traversed the outlets 
 of the Mississippi, and so carrying with it the finely- 
 divided matter brought down by that river), the Gulf 
 Stream has acquired a peculiar blue colour, somewhat 
 resembling that recognised in most of the Swiss lakes. 1 
 
 1 This explanation of the colour of the Gulf Stream seems the best 
 that has hitherto been offered. The Edinburgh Reviewer thus states the 
 matter : The remarkable blueness which distinguishes the water of the 
 Gulf Stream from the oceanic water through which it flows may be due 
 to its retaining in suspension the finest of the sedimentary particles 
 
220 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 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 its 
 outward passage through the Narrows of Bernini, takes 
 place at the rate of from 2J to 4 miles per hour. Its 
 width here is at the surface not more than about 25 
 miles, its maximum depth rather more than a quarter 
 of a mile (about two-fifths of the channel s maximum 
 depth), and its mean rate of flow probably about 50 
 miles per day. 
 
 I shall not follow the Edinburgh Reviewer in con 
 sidering the details of the progress of the Grulf Stream 
 from the Narrows of Bernini to Cape Hatteras, because, 
 though in themselves of the utmost interest and impor 
 tance, these details throw no special light on the general 
 subject of oceanic circulation. Suffice it that as far as 
 Hatteras the Gulf Stream remains distinctly recognis 
 able, and that even off Sandy Hook (New York) its 
 surface temperature is little reduced, and its velocity 
 still amounts to about one mile per hour. Off Nan- 
 brought down by that river, the coarser having been deposited near its 
 (the river s) mouth ; just as the intense blueness of the waters of Lake 
 Geneva depends on its retention of the finest sedimentary particles 
 brought down by the Rhone in the upper part of its course, while that 
 of the waters of the Mediterranean is due to its pervasion by the like 
 particles brought down by the river Rhone and other rivers, which dis 
 charge themselves into its western basin, and by the Nile into its eastern. 
 It will be remembered that Prof. Tyndall, by researches carried on 
 during the return of the Urgent from the eclipse expedition of 1870, 
 was enabled to throw considerable light on the cause of the colour and 
 shades of colour in water of greater or less depth. See also Dr. Car 
 penter s Report of Researches in the Mediterranean, in the Proceed 
 ings of the Royal Society, vol. xix. p. 200. 
 
OCEANIC CIRCULATION. 221 
 
 tucket the breadth of the current is about 410 miles, 
 its winter surface temperature only 10 below that 
 which it had in the Florida Channel, and its rate of 
 flow still nearly one mile per hour. It has at this part 
 of its course acquired a good deal of easting, a circum 
 stance which must (unquestionably, we conceive) be 
 ascribed to the fact that it brings from low latitudes 
 the more rapid easterly rotation movement of the earth. 
 The same would, of course, apply to the less character 
 istic but larger current which has arrived at the same 
 latitudes without circuiting the Gulf of Mexico. 
 
 Now here we approach a critical part of our subject. 
 It is admitted by all that off Newfoundland the Gulf 
 Stream loses its special characteristics. As Dr. Hayes 
 remarks, its strength diminishes ; the air of a higher 
 latitude brings its temperature down to that of the 
 North Atlantic generally (not, however, without 
 raising the temperature of the North Atlantic to some 
 extent) ; c the water loses all its Grulf Stream character 
 as to course, warmth, and flow (and as to colour also) ; 
 and it dies away into the sluggish Atlantic drift which 
 sets from a westerly to an easterly direction. It is not 
 so generally noticed, but will scarcely, I suppose, be 
 disputed, that the Gulf Stream water strengthens, and 
 that appreciably, this sluggish Atlantic drift. Then it 
 is reinforced by the portion which has travelled outside 
 the West Indian Islands ; and we may assume (without 
 giving rise to objections) that the general prevalence of 
 south-westerly winds will further strengthen the east 
 ward motion of the combined mass. At any rate, let 
 
222 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 the causes be what they may (and presently we shall 
 have a further cause to take into account), it is ad 
 mitted by all physical geographers that a great, though 
 slow current, or drift, does pass eastwards from the 
 neighbourhood of Newfoundland. Moreover, it is ad- 
 
 o 
 
 mitted by all that the southern part of this current 
 (which the Edinburgh Reviewer actually regards as 
 identifiable with the Gulf Stream 1 ) traverses the At 
 lantic until, nearing the Azores, it joins the southwardly 
 Guinea current ; while the northern part passes on a 
 north-easterly course, which carries it between Britain 
 and Iceland, between Sweden and Spitzbergen, onwards, 
 even as far as the very neighbourhood of Nova Zembla. 
 Lastly, it is admitted by all that, directly or indi 
 rectly, this great north-easterly current causes the 
 climate of Great Britain, and of the north-western 
 parts of Europe generally, to be milder than that of 
 North American regions in corresponding latitudes. 
 
 It might appear, then, that all these things being 
 admitted, no question of any importance remains, so 
 far as the actual facts of the oceanic surface-circulation 
 are in question. We shall presently see that a question 
 has arisen as to the cause of the observed facts ; but as 
 to their nature everything that seems worth discussing 
 at all appears to be satisfactorily disposed of. 
 
 Let those readers who in their simplicity have 
 adopted this notion hasten to dispossess themselves of 
 it by reading some remarks by Dr. Hayes, the American 
 
 1 He says that the great equatorial current is partly supplied by the 
 return of a portion of the Gulf Stream. 
 
OCEANIC CIRCULATION. 22$ 
 
 explorer, quoted with approval by the Edinburgh Ee- 
 viewer. The latter having repeated from 6 Lothair " a 
 sneer at the shallow nonsense which has been talked 
 about the Gulf Stream, and at the exaggerated esti 
 mates of its potency which have been put forward by 
 men (as well as women) who ought to have known 
 better " (these are the reviewer s words, not Mr. Dis 
 raeli s), proceeds as follows : As Dr. Hayes truly re 
 marks, " Weather predictors without end have launched 
 upon it their stupidities ; meteorologists have deluged 
 the world (sic) with their assumptions respecting it ; 
 theorists of all kinds have floated their notions upon it. 
 One whirls it away into the arctic regions, and opens a 
 passage to the pole with it ; another compels it to give 
 a climate to countries where otherwise there would be 
 no climate worth mentioning ; while still another spins 
 it round the Atlantic Ocean, and its wide-spread arms 
 close upon a stagnant sea. . . . Through means 
 such as these mankind has come to look upon the Gulf 
 Stream with a certain degree of awe. It is a breeder 
 of storms ; the giver of heat ; it might become the 
 father of pestilence. Will it always continue to do its 
 duty as hitherto ? or will it start off suddenly with some 
 new fancy, and pursuing some new course, upset the 
 physical and moral status of the world ? " 
 
 Now we have seen that the writer who thus endorses 
 Dr. Hayes diatribe, is among those who hold that a 
 southern offset from the Gulf Stream circles round 
 the Sargasso Sea to join the Guinea current. He says 
 farther on that he 6 entirely accords with the opinion 
 
224 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 of Buchan, the meteorologist, that the north-easterly 
 current above .referred to produces an afflux of warmth 
 brought to the British Isles by the water that laves our 
 western coasts. He proceeds : 6 There is ample evidence 
 that the cold of some parts of the north polar area is 
 greatly mitigated by an afflux of water bringing with 
 it the comparative warmth of temperate seas. It has 
 long been known that cocoa-nuts, tropical seeds, trunks 
 of tropical trees, timbers and spars of ships wrecked far 
 to the south, and sometimes portions of their cargo, 
 are found on the shores of the Western Hebrides, the 
 Orkney, Shetland, and Faroe Islands, the north of 
 Norway, and even Spitzbergen ; and since their trans- 
 sport has taken place just in the course of the Gulf 
 Stream if prolonged to the north-east, their arrival has 
 been accepted almost without question as evidence of 
 its agency. The evidence furnished by the surface tem 
 perature of that north-eastern portion of the Atlantic 
 Ocean which intervenes between Iceland and the North 
 Cape, and then stretches away to the eastward between 
 Spitzbergen and Nova Zembla, seems at first sight con 
 clusive to the like effect. A large amount of additional 
 thermometric evidence has been collected of late years; 
 and this has been most ably digested by the eminent 
 German geographer, Dr. Petermann, who has recently 
 put forward a series of maps for different periods of the 
 year, in which these observations are embodied, and 
 their results made obvious to the eye by the course of 
 the lines of equal temperature, which in the summer 
 pass between Iceland and the Shetland Islands, a little 
 
OCEANIC CIRCULATION. 22$ 
 
 to the east of north towards Spitzbergen, and thence 
 with more of an easterly bend even beyond the seventy- 
 fifth degree of north latitude. The existence of a warm 
 stream in this direction has been confirmed still more 
 recently by two adventurous officers Lieutenant Julius 
 Payer, of the Austrian army, and Lieutenant Wey- 
 precht, of the Grerman army who followed its path 
 last summer in a small sailing vessel hired by them 
 selves, and state that they found open water from 
 east longitude 42 to east longitude 60, even beyond 
 the seventy-eighth parallel of north latitude, the 
 highest point they reached being north latitude 79, in 
 east longitude 43. A Eussian expedition under Prince 
 Alexis Alexandrovitch, of which the distinguished 
 savant. Von Mildendorf, had the scientific charge, was 
 about the same time exploring the Polar Sea between 
 Nova Zembla and Iceland ; and Von Mildendorf has 
 stated to the Imperial Academy of St. Petersburg 
 that the corvette Wajag has proved the extension of 
 the Gulf Stream to the west coast of Nova Zembla, and 
 that we find it on the meridian of Banin Noss (in east 
 longitude 43J) still of a width equal to two degrees of 
 latitude, and of a temperature of fifty- four degrees 
 Fahrenheit, cooling down only four or six degrees at 
 depths of thirty and fifty fathoms. 
 
 As if to remove all question as to his real opinion 
 the reviewer immediately adds that he fully accepts, 
 not only the great body of facts so industriously cor 
 related by Dr. Petermann, but the inference Dr. Peter- 
 mann draws from them that an attempt to penetrate 
 
 Q 
 
226 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 the polar ice-wall to the north-east of Spitzbergen is; 
 more likely to be successful than the search for a pas 
 sage in any other direction. 
 
 So that (1) Dr. Petermann, regarded by our reviewer 
 as an eminent geographer ; (2) Von Mildendorf, whom 
 he regards as a distinguished savant ; and (3) the re 
 viewer himself, who no doubt does not regard himself 
 as either shallow or stupid, seem all agreed as to the 
 very points which the reviewer has spoken of as in 
 volving stupidities and shallow nonsense. Certainly 
 they all agree as to the only points which seem in the 
 least worthy of discussion. 
 
 What, then, the reader will ask, is the matter in 
 dispute? Over what momentous question have the 
 angry words quoted above been bandied ? 
 
 After diligent search for the apple of discord, the 
 student of the review will be led to the conclusion that 
 it is neither more nor less than the name Grulf Stream. 
 We have seen that Von Mildendorf calls the warm 
 current which passes by Nova Zembla the Grulf Stream. 
 In this, it appears, he has shown shallowness and stu 
 pidity. Dr. Petermann has equally committed himself, 
 or rather has committed a more serious offence. For 
 Von Mildendorf might have used the offensive epithet 
 only through inadvertence ; but Dr. Petermann not 
 only uses it, but has the hardihood (we might almost 
 say the cruelty) to maintain that it is a matter of no 
 consequence. Moreover, as our reviewer sadly admits, 
 other physical geographers agree with Dr. Petermann. 
 
 The reviewer is so grieved by the defection of the 
 
OCEANIC CIRCULATION. 227 
 
 < distinguished savant, the 4 eminent geographer, and 
 
 < the other physical geographers, that for a moment his 
 confidence deserts him, and instead of applying afresh 
 to them, directly, the lash which has indirectly reached 
 them, he proceeds thus mildly : c ln our belief, of which 
 we shall presently explain the grounds, the real Grulf 
 Stream has no more to do with the inflow into the 
 polar area than with the ripening of oranges at Naples, 
 or the maintenance of Catholicism at Eome, so that, 
 even if its current were to be entirely diverted by the 
 cutting of a wide channel through the Isthmus of Pan 
 ama, not only would the climate of the British Islands 
 suffer very little, but a north-easterly stream of warm 
 water . . . would still mollify the severity of polar 
 cold, and help to render Spitzbergen and Nova Zembla 
 accessible to arctic voyagers. This belief, in which I 
 cordially concur, would seem to afford excellent reason 
 for rejecting the name Grulf Stream whenever the course 
 of the stream shall thus have been diverted, but scarcely 
 seems to justify the disuse of the name under the actual 
 circumstances ; still less would it appear to afford good 
 grounds for using such hard words as shallow non 
 sense and stupidity. If the course of the Danube 
 were intercepted in Baden, it is tolerably certain that 
 a mighty river would continue to flow past Vienna, 
 Belgrade, and Ismail to the Black Sea ; nor would the 
 noble river which flows northward through Grermany be 
 much reduced though the Rhine were diverted in the 
 Orisons : yet geographers are satisfied to call these 
 rivers the Danube and the Rhine, not adopting new 
 
 Q 2 
 
228 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 names at every stage where some new influx changes 
 the size and character of either. And the title Grulf 
 Stream has, in like manner, advantages in point of 
 convenience, which are likely to prevent geographers 
 from rejecting it yet awhile. It may mislead some few 
 into supposing that the whole of the great north 
 easterly current has passed through the Grulf of Mexico, 
 just as we can conceive that some few students of geo 
 graphy might imagine all the water which flows past 
 Cologne or Coblentz to have come from the Grisons, or 
 all that flows past Nikopolis to have come from Baden. 
 Almost every convenient name, however, is open to 
 some such disadvantage ; and the student of oceanic 
 circulation who finds he has been to some degree misled 
 by a name must not mistake the detection of his error 
 for a great geographical discovery. 
 
 Majora canamus. 
 
 We have hitherto considered surface-currents only. 
 We have not, indeed, considered all the surface currents 
 which traverse the North Atlantic ; but the principal 
 streams have been indicated. We must now direct our 
 attention to submarine currents. 
 
 It is impossible to consider carefully the nature and 
 distribution of the surface circulation without recog 
 nising the fact that there must be currents beneath 
 the surface. It is true that one can conceive the 
 existence of a complete system of oceanic circulation 
 without any movement in the depths of the sea ; but 
 when we examine the actual surface currents we find 
 that either the commencement or the prolongation of 
 
OCEANIC CIRCULATION. 229 
 
 some currents must necessarily be submarine. For 
 instance, the quantity of water carried by the great 
 north-easterly drift into the Arctic Ocean is very much 
 greater than that which flows out of the Arctic Ocean, 
 by the so-called Arctic current, past Greenland. Ex 
 amining, indeed, the ordinary current charts, always 
 drawn on Mercator s projection (seemingly because this 
 projection is the very worst that could be devised for 
 the purpose), we might suppose that this arctic stream 
 was much more extensive than it really is. But what 
 can be expected of a projection which makes Green 
 land (whose real area is not much greater than that of 
 the Scandinavian peninsula) actually as large as South 
 America. The Arctic current, however, affords yet 
 better evidence of the occurrence of submarine streams, 
 for the extension which passes between the Gulf Stream 
 and the United States, is in places completely lost 
 sight of (the Gulf Stream touching the American 
 shores), and reappears farther on. It is clear that 
 it must have passed under the Gulf Stream in such 
 cases. 
 
 Now, the study of the submarine currents has of late 
 years thrown considerable light on the whole question 
 of oceanic circulation, and has supplied the solution of 
 some problems which had formerly appeared altogether 
 perplexing. 
 
 We owe to Drs. Carpenter and Wyville Thomson 
 some of the most important facts recently ascertained. 
 Others, however, have shared in the work. I would, 
 indeed, particularly invite attention to the fact that I 
 
230 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 do not here pretend to give anything like a complete 
 history of recent investigations into the subject. I 
 select only those facts which bear most significantly on 
 the wider relations the more marked features of 
 oceanic circulation. 
 
 In the first place, a result which had long perplexed 
 physical geographers has been shown to be erroneous. 
 It had been supposed that the temperature of sea-water 
 below a certain depth is in all latitudes constant, and 
 about seven degrees above the temperature at which 
 fresh water freezes. Sir John Herschel, in his Phy 
 sical Geography, adopted this supposed discovery as 
 well established- Now, let one consequence of such a 
 relation be carefully noted. The surface water in the 
 tropics is warmer than this supposed constant bottom- 
 temperature ; the surface water in arctic regions is 
 cooler ; at some intermediate latitude the surface water 
 has the same temperature as the water at the bottom. 
 Hence in this intermediate latitude the water is uni 
 formly warm (according to the supposed relation) from 
 the surface to the bottom. We may therefore regard 
 the water in this latitude as constituting, in effect, a 
 constant barrier between the tropical waters and the 
 arctic waters. Without regarding it as absolutely im 
 movable we should yet be compelled to regard it as so 
 far steadfast as to negative the theory of the existence 
 of submarine currents of an importance corresponding 
 to that of the surface currents. Accordingly, the theory 
 put forward by Humboldt and Pouillet to the effect 
 that there is an interchange of waters between polar 
 
OCEANIC CIRCULATION. 231 
 
 and equatorial regions was discredited by this supposed 
 discovery. 
 
 Drs. Carpenter and Wyville Thomson, however, have 
 been able to show that no such relation exists. There 
 are vast submarine regions of the Atlantic where the 
 temperature of the water is far lower than the constant 
 and uniform temperature believed in by Sir John Her- 
 schel. The temperature is, indeed, in places, as low, 
 or very nearly so, as the freezing-point of fresh water, 
 under a surface-temperature 20 degrees or so higher. 
 But in other regions having the same surface-tempera 
 ture the depths are 10, 12, or 14 degrees higher than 
 that of freezing fresh water. Moreover these relations 
 are constant, so far as the deep water is concerned. 
 
 Now, there can be only one interpretation of the cir 
 cumstances here mentioned. If the depths of the ocean 
 were unmoved by any process of submarine circulation 
 there can be no question that a general uniformity of 
 deep sea temperature would prevail in given latitudes. 
 We should not find the bottom water in one region 1 3 
 or 14 degrees warmer than the water in a closely adja 
 cent region. We have only to inquire what is the case 
 in inland seas where no great influx of water of alien 
 temperature can take place, to see that this must be so. 
 Take, for instance, the Mediterranean. Here we learn 
 from Dr. Carpenter s recent researches that the tem 
 perature at every depth beneath 100 fathoms is found 
 to be uniform, even down to a bottom of 1,900 fathoms ; 
 as had, indeed, been previously ascertained by Captain 
 Spratt, although his observations, made with thermo- 
 
232 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 meters not protected against pressure, set this uniform 
 temperature too high. In the western basin of the 
 Mediterranean, as shown by the Porcupine observations 
 of 1870, the uniform temperature is 54 or 55 degrees ; 
 being, in fact, the winter temperature of the entire 
 contents of the basin, from the surface downwards ; and 
 being also, it would appear, the mean temperature of 
 the crust of the earth in that region. We learn, then, 
 two things viz., first, that where extensive submarine 
 motions are impossible, a constant submarine tempera 
 ture may be expected to prevail in the same latitudes ; 
 and, secondly, that in the latitude of the Mediterranean 
 the submarine temperature is about 54 or 55 degrees 
 Fahr. Thus, it is clear, in the first place, that the 
 varieties of temperature observed in the depths of the 
 Atlantic must be due to the continual arrival of water 
 of the observed temperatures, at a rate great enough to 
 prevent the deep water from acquiring a constant tem 
 perature ; and in the second place it becomes possible 
 to tell whence the submarine currents are flowing. If 
 they are cooler than they should be supposing latitude 
 alone in question, then they are travelling from arctic 
 towards tropical regions, and vice versa. On this last 
 point no doubt remains. In a latitude corresponding 
 to that of the Mediterranean basin, the depths of the 
 Atlantic are far colder, even in their warmest por 
 tions, than they would be if latitude alone were in 
 question. In regard to surface-temperature, says 
 Dr. Carpenter, there is no indication of any essen 
 tial difference between the Mediterranean and the 
 
OCEANIC CIRCULATION. 233 
 
 Eastern Atlantic in the same latitudes ; and the 
 thickness of the stratum that undergoes superheating 
 during the summer is about the same. ... At 
 the depth of a hundred fathoms, in the Atlantic as in 
 the Mediterranean, the effect of the superheating seems 
 extinct, the thermometer standing at about 53 degrees ; 
 and beneath this (in the Atlantic only), there is a slow 
 and tolerably uniform reduction at the rate of about 
 two- thirds of a degree for every fathom, down to 
 700, at which depth the thermometer registers 49 
 degrees. But the rate of reduction then suddenly 
 changes in the most marked manner ; the thermometer 
 showing a fall of no less than nine degrees in the next 
 200 fathoms, so that at 900 fathoms it stands at 40 
 degrees. Beneath this depth the reduction again be 
 comes very gradual ; the temperatures shown at 1,500, 
 2,000, and 2,435 fathoms (the last being the deepest 
 reliable temperature sounding yet obtained) being, 
 respectively, 38, 37, and 36^ degrees. 
 
 Thus, there can be no possible question that the 
 depths of the Atlantic are occupied by a vast current 
 much colder than the deep sea temperature due to the 
 latitude, and, therefore necessarily flowing from the 
 arctic towards the tropical seas. 
 
 Such are the broad facts of the Atlantic circulation. 
 We have a surface circulation whose general features 
 are such as have been described, and are generally ad 
 mitted, though a dispute has arisen as to a question of 
 nomenclature ; and then we have a general submarine 
 set of water from the arctic regions towards the 
 
234 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 tropics, the existence of which is also generally ad 
 mitted. 
 
 But now we again approach a subject of controversy, 
 and one which is certainly better worthy of discussion 
 than that which we considered above. It relates, in 
 fact, to the question how this wonderful system of 
 oceanic circulation is brought about. 
 
 Passing over several crude theories which have long 
 since been disposed of, we come first to the theory that 
 the system of oceanic circulation is due to the action of 
 the trade-winds. This theory has been maintained by 
 Franklin and others in former times, by Sir John 
 Herschel in our own, and is warmly advocated in the 
 present day, by many whose opinions are not to be 
 rashly contradicted. 
 
 Against this theory it has been urged by Captain 
 Maury with singular wrongheadedness accordirg 
 to the Edinburgh Reviewer, but as it seems to me with 
 no small degree of reason that the trade-winds are 
 neither powerful enough nor persistent enough to ac 
 count for the great equatorial currents, or therefore for 
 the Grulf Stream. Maury says, with the view of ascer 
 taining the average number of days during the year 
 that the north-east trade-winds of the Atlantic operate 
 upon the water between the equator and 25 degrees 
 north latitude, log-books containing no less than 
 380,284 observations on the force and direction of the 
 wind in that ocean were examined. The data thus 
 afforded were carefully compared and discussed. The 
 results show that within these latitudes and on the 
 
OCEANIC CIRCULATION. 235 
 
 average the wind from the north-east is in excess of 
 the wind from the south-west only 111 days out of the 
 365. Now, can the north-east trades, by blowing for 
 less than one-third of the time cause the Gulf Stream 
 to run all the time, and without varying its velocity 
 either to their force or prevalence. Our reviewer 
 not only dwells on the wrongheadedness of this argu 
 ment wholly irresistible as it appears but asserts 
 that the trade-wind origin of the Gulf Stream is about 
 as certain as the rotundity of the earth. It could have 
 been wished that in place of abusing Captain Maury for 
 wrongheadedness, the reviewer would have devoted a 
 few lines to the demolition of Maury 1 s argument. 
 
 Maury himself advanced the relative lightness of the 
 equatorial water as the true reason of the oceanic circu 
 lation. But granting that the expansion of the equa 
 torial water under the sun s heat, as well as the resulting 
 buoyancy, would cause an overflow of equatorial water 
 polewards, this overflow would be an exceedingly slow 
 movement, and it would result in an eastwardly instead 
 of a westwardly flow, for the very same reason that the 
 counter trade- winds travelling polewards assume an 
 eastwardly direction. 
 
 In the Student for July 1868, I advanced another 
 explanation. I urged that the sun s action on the 
 equatorial and tropical regions of the Atlantic, raising 
 immense quantities of water by evaporation, causes an 
 influx of water from below. There can be no question, 
 I then wrote, that under-currents arriving in this 
 manner, whether from the north or from the south 
 
236 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 (that is from arctic or from antarctic regions) would 
 acquire a strong westerly motion (just as the trade- 
 winds do). Thus they would generate from below the 
 great equatorial westerly current. In this upflow of cool 
 currents having a strong westerly motion, we find the 
 mainspring of the series of motions. The water thus 
 pouring in towards the equator is withdrawn from 
 beneath the temperate and arctic zones, so that room is 
 continually being made for that north-easterly surface- 
 stream which is the necessary consequence of the con 
 tinual flow of the great westerly equatorial current 
 against the barrier formed by the American continent. 
 . . . . Captain Maury s views seem only to require 
 the mainspring or starting- force towards the west 
 which has been here suggested, to supply a complete, 
 efficient, and natural explanation of the whole series of 
 phenomena presented by the great ocean currents. 
 
 Four or five months later, and while the results on 
 which Dr. Carpenter subsequently based his theory of 
 the oceanic circulation were as yet unpublished, I drew 
 attention in the columns of the Daily Neivs to the 
 comparatively limited extent of the influences due to 
 polar cold. This cause, I pointed out, scarcely has 
 any influence in latitudes lower than the parallel of 50 
 degrees. 
 
 In the year 1869 Dr. Carpenter was first led to advo 
 cate the theory that the continual descent of cold water 
 in the Arctic Seas is the mainspring of the system of 
 oceanic circulation. He reasoned that the Arctic Seas 
 being exposed to great cold, the surface water descends 
 
OCEANIC CIRCULATION. 237 
 
 in virtue of its reduction in temperature and increase 
 of density, its place being taken, not by the rising up 
 of water from beneath, but by an inflow of water from 
 the neighbouring area; and since sea-water becomes 
 continually heavier in proportion to its reduction of 
 temperature, this cooling action will go on without the 
 check which is interposed in the case of fresh water. 1 
 Thus the water becoming denser and heavier will 
 descend, and there will be a continual tendency to 
 the flowing off of its deepest portion into the warmer 
 area by which the polar basin is surrounded ; producing 
 a reduction in the level of the polar area, which must 
 create a fresh indraught of surface-water from the warmer 
 area around to supply its place. This, in its turn, 
 being subjected to the same cooling action, will descend 
 and flow off at the bottom, producing a fresh reduction 
 of level and a renewed indraught at the surface. 
 
 Dr. Carpenter illustrated this theory, or rather the 
 combined action of polar cold and equatorial heat, by 
 an experiment, the plan of which had occurred also to 
 myself, and been described by me in conversation 
 somewhat earlier. A long narrow trough having glass 
 sides was filled with water, and a piece of ice was 
 wedged in at one end between its side plates just 
 beneath the top, whilst the surface of the water at the 
 other end was warmed by a piece of metal, of which a 
 part projected beyond the trough, and was heated by a 
 
 1 Fresh water expands with reduction of temperature, near the 
 freezing point, and hence, becoming lighter, the descending motion above 
 described is interfered with in the case of fresh water. 
 
238 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 spirit lamp placed beneath it : thus representing the 
 relative thermal conditions of the polar and equatorial 
 basins. A colouring liquid viscid enough to hold 
 together in the water, while mixing with it sufficiently 
 to move as its moves, being then introduced, the liquid 
 as it impinged on the ice was seen to sink rapidly to 
 the bottom, then .to flow slowly along the floor of the 
 trough towards the opposite extremity, then gradually 
 to rise beneath the heated plate, and then to flow slowly 
 along the surface towards the glacial end, repeating 
 the same movement until the ice had melted. 
 
 It will be observed that in this experiment the effect 
 of cold is not exerted alone, so that it by no means 
 proves that the arctic cold is the chief agent in pro 
 ducing the system of oceanic circulation. Moreover, 
 the conditions of the polar and equatorial basins are in 
 one respect not accurately (or even nearly) reproduced, 
 for the real arctic area is very much smaller, compared 
 with the real equatorial area, than in the case of the 
 experiment. Indeed it appears to me that Dr. Carpenter 
 paid far too little attention to the relative smallness of 
 the arctic area. This may have been partly due to the 
 erroneous ideas suggested by the ordinary maps on 
 Mercator s Projection, in which, as I have already 
 mentioned, the arctic regions are enormously exagger 
 ated. It is almost impossible to study such a map as 
 that which illustrates this paper (see page 2 17) without 
 feeling that the theory presented by Dr. Carpenter will 
 scarcely hold water, or rather if this way of presenting 
 the argument be permitted that the arctic area does 
 
OCEANIC CIRCULATION. 239 
 
 not hold water enough to produce the effects de 
 scribed by Dr. Carpenter. For in that map the whole 
 area of the Arctic Ocean is presented ; l and from out 
 of that area, be it noted, must come the northern supply 
 of descending water, not only for the Atlantic equatorial 
 current, but for the much larger equatorial current of 
 the Pacific, if Dr. Carpenter s theory be sound. 
 
 The following letter, written by Sir John Herschel 
 only a few weeks before his lamented decease, has been 
 very widely quoted in favour of Dr. Carpenter s theory ; 
 yet if carefully studied it will be found rather to set 
 forth the strength of the theory advocated a year earlier 
 by the present writer. In this letter, at least, Sir John 
 Herschel appears to be disposed, in so far as he con 
 cedes the efficiency of heat, cold, and evaporation, to 
 incline to the equatorial action as the most important. 
 Answering Dr. Carpenter, who had addressed a letter 
 to him on the subject, he says : After well considering- 
 all you say, as well as the common-sense of the matter, 
 and the experience of our hot-water circulation pipes 
 in our green-houses, &c., there is no refusing to admit 
 that an oceanic circulation of some sort must arise from 
 mere heat, cold, and evaporation, as verce causce ; and 
 you have brought forward with singular emphasis 2 the 
 more powerful action of the polar cold, or rather, the 
 
 1 The bounding lines drawn from the pole on the right and loft of 
 the white space represent one and the same meridian. 
 
 2 In Sir John Herschel s letters one can often recognise slight touches 
 we will not say of sarcasm (for he was incapable of saying aught that 
 could be considered bitter or unpleasant), but of what may be described 
 as a humorous suggestiveness. 
 
240 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 more intense action, as its maximum effect is limited 
 to a much smaller area than that of the maximum of 
 equatorial heat. The action of the trade and counter 
 trade winds, in like manner, cannot be ignored ; and 
 henceforward the question of ocean-currents will have 
 to be considered under a twofold point of view. 
 
 It appears to me that not only is the equatorial or 
 rather tropical action much wider in range, but it is 
 also more intense than the polar action. For, let us 
 consider what happens during the heat of the day over 
 the tropical Atlantic. Here, over an area enormously 
 exceeding the whole arctic basin (we are considering, 
 be it understood, only the northern part of the system 
 of circulation) a process of evaporation is taking place 
 at so rapid a rate as to furnish almost the whole of 
 that rain-supply whence the rivers of Europe and North 
 America (east of the Eocky Mountains) take their 
 origin. There is thus produced a continual defect of 
 pressure, not merely along an equatorial strip, but so 
 far as 20 or even 30 degrees of north latitude, while 
 the downfall of rain which, taking one part with an 
 other of the temperate and sub-arctic Atlantic, may be 
 regarded as incessant, continually adds to the pressure 
 in these last-mentioned regions. That on the whole 
 there must result a most effective excess of pressure 
 over the temperate zone of the Atlantic, as compared 
 with the tropical and equatorial portion, seems to me 
 indisputable. Now, if we compare this with the effects 
 of refrigeration over the relatively insignificant arctic 
 area, which as I have said has to supply the North 
 
OCEANIC CIRCULATION. 241 
 
 Pacific submarine circulation (if Dr. Carpenter s theory 
 be true), as well as that of the North Atlantic, we can 
 scarcely doubt, as it seems to me, which cause is the 
 more effective. I would venture to predict that if 
 Dr. Carpenter s experiment were tried first with the 
 ice alone to produce circulation, and secondly with the 
 heat alone, the superior efficiency of the latter cause 
 would be at once recognised ; but I much more confi 
 dently predict that if, as in the experiment I myself 
 proposed, the relative areas of the equatorial and arctic 
 basins were represented, there would be found to be 
 scarcely any comparison between the effects of arctic 
 cold and equatorial heat, so largely would the latter 
 predominate. 
 
 It is necessary to mention, however, that the prin 
 ciple itself of the experiment has been objected to, on 
 the ground that the gradation of temperature must 
 always be much more rapid in such an experiment 
 than in the actual case of the Atlantic Ocean. This 
 objection, however, is, in reality, based on a misappre 
 hension. It is sufficient that the difference of tem 
 perature at the two ends of the trough should corre 
 spond to the difference between the temperature of the 
 arctic and equatorial seas ; and it is a matter of no im 
 portance whatever that the real rate of gradation should 
 be represented. The case may be compared to the 
 illustration of the descent of water to form springs or 
 the like. Here an experiment would be valid in which 
 the outflow of the illustrative spring was obtained by 
 causing the vent to be so much below the level of the 
 
 n 
 
242 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 reservoir, though the slope from the reservoir to the 
 vent were very much greater than in the case of any 
 natural spring. Just as in the case of a spring it is 
 the difference of level, and not the rate of slope, which 
 is effective in causing the rate of outflow, so in the 
 case of the oceanic vertical circulation, it is the actual 
 difference of temperature, and not the rate of grada 
 tion, which produces the activity of the circulation. 
 
 Another objection has been urged against the heat 
 and cold theory by a very skilful mathematician, Mr. 
 Croll. He reasons on this wise : Since the water which 
 is carried from the equator to the latitude of England 1 
 (say) must have partaken, when at the equator, of the 
 earth s rotation there, which has a velocity of more 
 than 1,000 miles per hour eastwards, whereas, when it 
 reaches our latitudes, it partakes of a rotation-move 
 ment reduced to about 620 miles per hour, it follows 
 that, neglecting the drift motions as relatively quite 
 insignificant, friction has deprived the water which has 
 thus travelled from the equator to our latitudes of a 
 velocity amounting to no less than 380 miles per hour. 
 If friction is thus effective, how utterly inconceivable 
 is it, says Mr. Croll, that the descending currents of 
 Dr. Carpenter s theory (or the ascending currents of 
 the evaporation theory) should maintain their motion. 
 Hence, Mr. Croll is an earnest advocate of the trade- 
 wind theory. 
 
 The worst of this reasoning is that it proves too 
 
 1 I present the general nature of Mr. Croll s reasoning, without fol 
 lowing him in details. 
 
OCEANIC CIRCULATION. 243 
 
 muck If friction is so effective, then when the trade- 
 winds flag, as we have seen that they do, the ocean 
 currents ought to be brought to a standstill. More 
 over, the submarine currents exist, and the wind theory 
 leaves them unexplained. The fact really is that Mr. 
 Croll s reasoning has no application to a system of fluid 
 circulation, where the advance of one part of the fluid 
 is always made room for, so to speak, by the removal 
 of that which it replaces. We might equally well 
 apply Mr. Croll s reasoning to prove that a river cannot 
 flow because of the friction along its banks, as to show 
 that ocean currents cannot flow within their liquid 
 banks. Indeed, many of the points in dispute in this 
 matter of oceanic circulation may be excellently illus 
 trated by considering the case of a river. I propose to 
 draw this paper to a conclusion by setting forth such 
 an illustration. My readers will not fail to recognise 
 the opinions here severally parodied, so to speak, and 
 so to infer the theory which I regard as affording, 
 on the whole, the best explanation of the observed 
 relations. 
 
 Shallow persons, might one say, have launched 
 all sorts of stupidities upon the Mississippi Eiver. 
 Physical geographers have deluged the world with their 
 assumptions respecting it; theorists of all kinds have 
 floated their notions upon it. One says that it brings 
 down, past Baton Eouge and New Orleans, the drainage 
 of half the United States ; others ascribe to it the 
 detritus around the delta of that great river which 
 flows into the Gulf of Mexico ; yet others consider that 
 
 R 2 
 
244 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 it breeds the fogs infesting the path of the great stream 
 which flows from Vicksburg to Placquemines. All 
 this is utter nonsense. The Mississippi has no more 
 to do with the great stream flowing through Louisiana 
 than with the Thames at London. The real Mississippi 
 is a stream of singular purity, and presents other charac 
 teristics clearly recognisable as far as its junction with 
 the Missouri ; but in the stream which runs past St. 
 Louis none of the characteristics of the Mississippi can 
 be traced. Here, to all intents and purposes, the Mis 
 sissippi comes to an end. As for the cause of the 
 motion of the great stream itself there can be little 
 question. Some have urged that it is due to the gra 
 dual slope of the land ; but in all the experimental 
 illustrations of the effects of such slope which we have 
 yet seen, the inclination has been monstrously exag 
 gerated. If slope were the cause of the river s flow, 
 then unquestionably the effective part of the action 
 must reside in the Rocky Mountains, and not in the 
 great reaches of the river. We admit that the chief 
 bulk of the river lies in the great reaches ; but this 
 fact has no bearing, we assert, on the question at issue. 
 However, it is demonstrable that no cause of this sort 
 can be in question. For let the following reasoning 
 be carefully marked. In Wisconsin, in 40 north 
 latitude, the river partakes of the earth s rotation 
 motion, there equal in rate to about 800 miles per 
 hour; in Louisiana, in 30 north latitude, the river 
 still partakes of the earth s rotation movement, here 
 equal to about 900 miles per hour. Hence, were it 
 
OCEANIC CIRCULATION. 245 
 
 not for the friction exerted by the banks, the water of 
 the river in Louisiana would be flowing at the rate of 
 100 miles per hour westwards. If, then, friction de 
 prives the river of this enormous velocity as it obvi 
 ously does how much more must it deprive the river 
 of the minute velocity of four or five miles per hour 
 due to slope or inclination. It is certain, therefore, 
 that the flow of the stream is due to the prevalent 
 northerly winds of the so-called Mississippi valley. 
 There are not wanting those, indeed, who assert that 
 this cannot be the case, because northerly winds are 
 not prevalent in this region. But the singular wrong- 
 headedness of this reasoning renders reply unnecessary. 
 That the flow of the great stream is caused by these 
 winds is as certain as the rotundity of the earth. 
 
 From English Mechanic for July and Aupust 1872. 
 
 ADDENDUM. 1 
 
 IT is impossible but that on a subject so difficult and 
 complicated as that of oceanic circulation, different 
 views should be entertained by students of science. 
 And it is clear that in the present stage of the inquiry 
 no useful purpose could be fulfilled by making the 
 problem a matter for controversy. Dr. Carpenter him 
 self has shown that much more is to be gained by 
 
 1 This paper was written in reply to comments by Dr. Carpenter on 
 the former paper. The nature of these comments will be inferred from 
 my reply ; in fact I quote the most important passages. 
 
246 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 observation than by reasoning on imperfect knowledge. 
 If I venture to remark that his deep-sea researches 
 have led to the most important contribution which has 
 been added for many years to our information respect 
 ing oceanic circulation, he will not, I trust, consider 
 that I am passing beyond the bounds of controversial 
 courtesy. But I am, indeed, not anxious to treat the 
 matter as one for controversy in any sense. It will be 
 perceived by those who have read my remarks on the 
 subject, that I have rather put them forward as sug 
 gestions than as indicating theories which can be 
 maintained with any degree of assurance, far less with 
 conviction. Nor does it seem to me likely that one 
 explanation can suffice to account for all the pheno 
 mena recognised in oceanic circulation. This is a case, 
 if ever such case were, in which more causes are in 
 operation than one ; so that it may very well happen 
 that excellent arguments can be adduced in main 
 tenance of different views. If, therefore, I enter on 
 the defence of what I have already written on this 
 subject, it is not with the wish to show that one parti 
 cular explanation of oceanic circulation is correct, and 
 all others erroneous. If I am desirous of dealing with 
 the considerations urged by Dr. Carpenter, it is not 
 because they seem to him to militate against the views 
 I have to some extent advocated. What I wish to 
 show is that I have not addressed your readers on 
 the subject of oceanic circulation without making 
 myself familiar with the facts which bear upon that 
 subject, and at the very least, with those compara- 
 
OCEANIC CIRCULATION. 247 
 
 tively fundamental facts to which attention has been 
 invited. 
 
 And here I would remark that one who writes so 
 much and so often as I have had occasion to do on this 
 and kindred subjects, is placed to some degree at a 
 disadvantage. He cannot, on the one hand, assume 
 that the readers of any particular essay have also read 
 all that he has written on the subject ; yet, on the 
 other, he cannot assume that none have done so, and 
 that he is therefore free to repeat (in a more or less 
 modified form) much that he has formerly urged. I 
 was, perhaps, somewhat too careful in writing for your 
 pages to avoid touching at any length on any parts of 
 the subject which I had more particularly dealt with 
 elsewhere ; and accordingly I have laid myself open to 
 a method of attack, which in reality involves the sug 
 gestion that I have written without clue consideration 
 even of the elements of my subject. I have no doubt 
 that Dr. Carpenter has no wish to imply this directly, 
 yet indirectly it is implied in every paragraph of his 
 reply. I shall be able to show, however, that every one 
 of the points touched on by Dr. Carpenter had been 
 fully considered by me and, for the most part, several 
 months before he had turned his attention to this 
 subject. 
 
 First, there is the remark that I have left out of 
 view the circumstance that if there is excess of evapo 
 ration in the intertropical area, the excess ought to 
 show itself, as in the Mediterranean, in an increase of 
 specific gravity, whereas the specific gravity of the 
 
248 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 equatorial water is lower than that of tropical water. 
 Now, it is unquestionably true that the effect of evapo 
 ration is to increase the specific gravity of sea water ; 
 but it is equally true that the effect of the heat which 
 causes the evaporation is to diminish the specific 
 gravity. The point is considered in my essay entitled 
 Is the Gulf Stream a Myth? in the first series of 
 Light Science for Leisure Hours. We recognise, I 
 there say, two contrary effects as the immediate results 
 of the sun s action. In the first place, by warming the 
 equatorial waters it tends to make them lighter ; in 
 the second place, by causing evaporation it renders them 
 salter, and so tends to make them heavier. And I 
 proceed to inquire which cause is likely to be the more 
 effective, arriving at the conclusion that the water is 
 made lighter. The case, indeed, appears to me to be 
 altogether different from that of the Mediterranean 
 Sea cited by Dr. Carpenter. In the Mediterranean we 
 have the same heating action as on the Atlantic in the 
 same latitudes, but not the same relatively enormous 
 quantity of water freely communicating with the region 
 so heated. We have, then, in the Mediterranean 
 evaporation as everywhere else, and evaporation to the 
 same degree, appreciably, as elsewhere in similar lati 
 tudes ; but evaporation not compensated as in the open 
 Atlantic by the effects of free communication with 
 surrounding water. Hence we have in the Mediter 
 ranean an increase of saltness ; in other words, an in 
 crease of specific gravity. And precisely because this 
 increase takes place in the Mediterranean, whereas the 
 
OCEANIC CIRCULATION. 249 
 
 water of the Atlantic in the same latitudes, exposed to 
 the same average degree of heat, is not rendered heavier, 
 it may be maintained not unreasonably that the 
 water of the equatorial Atlantic being unconfined, 
 will in like manner not be rendered heavier by 
 evaporation. It seems to me that we have here a 
 positive argument of great weight in favour of my 
 views. But independently of this I would ask whether 
 it can be questioned that enormous evaporation does 
 take place over the equatorial area. This is what I 
 contend for, and I should have imagined that few would 
 undertake to deny the proposition. 
 
 In passing, I must remark that I do not adopt the 
 distinction between equatorial and tropical water which 
 Dr. Carpenter appears to recognise. I have in view 
 the evaporation over an enormously larger area than he 
 considers no less an area, in fact, than the whole 
 ocean between latitudes 40 north and south of the 
 equator (at the equinoxes, and varying according to 
 the season). It by no means follows that because the 
 equatorial current does not cover this enormous area, 
 therefore the relation which I have suggested as 
 the mainspring of oceanic circulation has not that 
 extent. On the contrary, while it is on the one hand 
 certain that there is an excess of heat over this 
 enormous area, it is on the other almost a necessity 
 of my theory that the resulting current should be found 
 running along the middle only of the great region of 
 evaporation. 
 
 This brings me to Dr. Carpenter s second objection, 
 
250 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 that if the removal of equatorial water draws in polar 
 water from the bottom, the whole intermediate stratum 
 should first rise towards the surface. I do not hold 
 the view thus demolished, but simply that the inflow 
 is from below. The question whether the inflow would 
 be from above or below was dealt with by me in a 
 paper on Oceanic Circulation in the Student for 
 July 1868. I do not urge this as a proof that Dr. 
 Carpenter s objection is invalid. My reasoning may 
 admit of being refuted. But I wish to show that the 
 objection is not a new one to me. The inflow may be 
 from below without being from the bottom. If it were 
 from the bottom it would not have the effects I have 
 ascribed to it, that is, it would not result in a west- 
 wardly-flowing current. What I conceive is that since 
 the whole tropical and equatorial area is a region of 
 excessive evaporation (as surely no physicist will deny), 
 there is over the whole region a depression of the ocean 
 level. This depression may be, or rather must be, 
 exceedingly minute ; but the total quantity of water 
 thus, as it were, wanting, must be enormous. The 
 difference must by the laws of fluid equilibrium be 
 supplied, and though the immediate supply in equa 
 torial regions may come from tropical regions, the 
 actual source of the total supply must be sought for in 
 higher latitudes. That the water drawn in under these 
 circumstances would traverse the surface of the Atlan 
 tic, is by no means proved by the fact that the eminent 
 mathematicians cited by Dr. Carpenter consider that 
 an in-draught to replace water swept off from the 
 
OCEANIC CIRCULATION. 251 
 
 surface, by trade-wind action would be a surface cur 
 rent. The two cases are wholly dissimilar. I must, 
 however, admit that my case is one of extreme diffi 
 culty regarded as a problem in hydrodynamics. It is 
 so difficult that I do not believe it can be solved even 
 after the very imperfect fashion in which hydrodyna- 
 mical problems have hitherto perforce been dealt with. 
 When the physics of hydrodynamics have been treated 
 by mathematicians like the physics of astronomy, or 
 rather when they can be so treated, it may be possible 
 to deal with this problem. Unless I greatly mistake, 
 however, in such a then, we shall find a never. 
 
 I do not see how the action of the cause I have 
 considered is affected by the circumstance that the 
 equatorial heat does not show any effects below 200 
 fathoms ; for the cause is in its very nature a surface 
 one. But I would remark that so far as continuity of 
 action is concerned, the equatorial heat seema at least 
 on a par with the polar cold. For as the aqueous vapour 
 rises it finds its way to regions where the atmospheric 
 circulation is at work to carry it away (it is only the 
 surplus quantity which is condensed into clouds, and 
 even these are in great part carried away) ; and thus 
 the process of evaporation can hardly be exhausted. 
 Even at night, though in a modified manner, the eva 
 poration must continue. But the action of the polar 
 cold, though it is continuous in the sense that the 
 increase of cold extends to great depths, yet has this 
 great difficulty to contend with, that the descending 
 water must perforce wait until room is made for it by 
 
252 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 the slow removal, the creeping aivay, as it were, of that 
 which it replaces. That this cause, per se, can ever 
 become one of sufficient activity 1 to generate a complete 
 system of vertical oceanic circulation seems at the least 
 open to grave question. It appears to me also that 
 when applied to the North Pacific this theory fails. 
 Very little water can pass through Behring s Straits, and 
 beyond Behring s Straits there is an island-locked and 
 shallow sea of enormous area, altogether unlike the deep 
 North Atlantic. 
 
 I would further point out that the interesting fact 
 above mentioned, namely that the equatorial heat exerts 
 no perceptible effect at a depth exceeding 200 fathoms, 
 is in reality almost a necessity for my theory. For if 
 the whole of the equatorial ocean were heated, and, 
 therefore, of reduced specific gravity, the water arriving 
 from higher latitudes would flow to the bottom, and so 
 have to force up the intervening strata, in order to pro 
 duce the observed effects ; and this may be regarded as 
 impossible. As it is, such colder and heavier water 
 would be in dynamical equilibrium within a very short 
 distance of the surface. 
 
 Next, as to the question of rainfall. Dr. Carpenter 
 considers that I have overlooked the considerations (1) 
 that the rainfall of Europe and North America may be 
 accounted for by the evaporation in the Mid-Atlantic, 
 
 1 In passing I may notice that I did not suppose Sir J. Herschel to 
 be humorous in reference to the intensity of the polar action, but in his 
 use of the word emphasis. I should not have touched on the point, 
 did I not thoroughly sympathise with the emphatic utterance of specula- 
 tire or theoretical opinions. 
 
OCEANIC CIRCULATION. 253 
 
 beyond the region of the trade-winds^ say between 20 
 and 40 north latitude ; and (2) that there is an enor 
 mous rainfall in the region of equatorial calms, which 
 Sir John Herschel attributes to the deposit of waters 
 taken up by the N.E. and S.E. trades. To this I must 
 reply that in my essay on Eain in the Intellectual 
 Observer for December 1867, I have weighed the 
 whole question of rainfall at least with great care, and 
 with constant reference to the best sources of informa 
 tion. One circumstance I there note which seems at a 
 first view (or rather viewed as Dr. Carpenter appears to 
 consider the matter) much more fatal as an objection 
 to my theory than either of those noted by Dr. Car 
 penter ; viz., that according to the observations of 
 Humboldt and others, the annual rainfall is at a maxi 
 mum at the equator, and diminishes with increase of 
 latitude. But the whole question is, where does all 
 this rain come from ? If it comes from tropical and 
 equatorial evaporation it will surely not be argued that 
 what falls in or near the place of evaporation itself, 
 represents the total amount of such evaporation. It is 
 unquestionable, I conceive, that the rainfall is only the 
 excess of the aqueous vapour poured so copiously into 
 the air from the whole of this region. It is the quan 
 tity which the air, as it were, rejects. It is a matter of 
 little importance where the rainfall of higher latitudes 
 comes from, though it should be noticed that the views 
 of Dove, Kaemtz, and other leading meteorologists re 
 specting the winds and rains of high and low latitudes, 
 support my remark about the great rivers. 
 
254 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 Now we have in the phenomena of the zone of calms 
 a crucial test of Sir J. Herschel s theory as to the origin 
 of the equatorial rains. It appears to me that this test 
 altogether negatives Herschel s theory. If the moisture 
 to which these equatorial rains are due came from the 
 trade-wind regions, we should certainly not expect the 
 fall of these rains to be associated in any marked degree 
 with the progress of the equatorial day ; or, if at all, 
 then the cooler parts of the day, when the point of 
 saturation is lower, would be the time of precipitation. 
 With the mid-day heat would come a cessation of pre 
 cipitation. As a matter of fact the contrary is the 
 case. The sun (we are told by Dove, Kaemtz, Hum- 
 boldt, Maury, Buchan, and many more) rises commonly 
 in a clear sky in equatorial regions. As the day proceeds 
 clouds form, and towards mid-day they grow dense. It 
 is at noon that heavy showers fall, and towards evening 
 the skies again become clear. Now, any one who has 
 noticed what happens on calm summer days in any well- 
 watered region can see that the equatorial phenomena 
 represent the same processes on a greatly enlarged scale. 
 On a summer s day in such regions we see how scattered 
 cumulus clouds begin to form in early morning, become 
 larger and more numerous as the day proceeds,, and in 
 the afternoon begin to be transformed into cumulo- 
 stratus. The explanation is simple. The sun s heat 
 has caused aqueous vapour to rise into the air, until 
 there is so much that not very far above the earth s 
 level the saturation point is reached. The further rise 
 of the vapour is followed by the process of condensation 
 
OCEANIC CIRCULATION. 255 
 
 into clouds, much heat being given out in the process, 
 causing the air to expand in the neighbourhood of the 
 clouds so formed, and thus giving to these clouds their 
 peculiar rounded tops. (At least this feature seems 
 better explained thus than by De Saussure s theory.) 
 Now suppose the conditions changed to those existing 
 at the equator. The supply of vapour is very much 
 greater, the saturation point is very much higher near 
 the sea-surface, and the contrast between the conditions 
 prevailing there and in the region where condensation 
 begins is very much more marked. The air above the 
 equatorial and tropical seas contains, in the form of 
 invisible aqueous vapour, an enormous quantity of water ; 
 this vapour rises and extends itself, its place being con 
 tinually supplied by fresh evaporation. What must 
 happen when the process has continued for several 
 hours, but precisely what is observed to happen ? There 
 is an overflow, so to speak, resembling, only much more 
 marked, that which causes the formation of our summer 
 clouds. Enormous cloud-masses are formed, which 
 cannot be carried away by the atmospheric circulation 
 (very high above the calm zone), so fast as they are 
 formed. Hence follows excessive accumulation, pre 
 sently resulting in precipitation, accompanied by re 
 markable electrical phenomena. 
 
 But to suppose that the whole quantity of water 
 evaporated at the equator and in tropical regions, is 
 precipitated there in the form of rain, corresponds to 
 such a supposition as that the water overflowing a dam 
 includes all that has risen to the level of the dam. 
 
256 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 I should not be greatly concerned if the result of the 
 experiments I spoke of should not accord with my predic 
 tion. But merely to put ice in water capable of melting 
 it, is not in any sense to represent the conditions of the 
 actual case. The addition of water from the ice as it 
 melts is not in accordance with these conditions. It 
 cannot surely be maintained that the oceanic circulation 
 depends on the addition of water from the melting of 
 ice ; and yet I apprehend that the melting of ice is no 
 unimportant feature of Dr. Carpenter s experiment. At 
 any rate, the ice does melt, and the movement comes to 
 an end when all the ice has melted away. Let the ice 
 be packed outside the arctic end of the canal, so as 
 merely to produce a refrigeration corresponding to what 
 actually takes place with water carried into arctic lati 
 tudes, and I conceive that a very feeble circulation 
 would result. Under the actual circumstances, the 
 melting of the ice produces effects much more nearly 
 corresponding to those due to rainfall than to the mere 
 effects of arctic cold. The very activity of the circu 
 lation shows that the water which moves towards the 
 ice does not undergo refrigeration. Water does not 
 cool quite so quickly. It is the melted ice-water which 
 descends ; and nothing takes place in the arctic regions 
 which corresponds to this continued addition of water 
 to that already circulating. Otherwise, the arctic ice 
 would be continually diminishing, which, of course, is 
 not the case. 
 
 It will be gathered that I agree entirely with the 
 opinion which Sir W. Thomson expressed, as to the 
 
OCEANIC CIRCULATION. 257 
 
 reason why heat is necessary for Dr. Carpenter s experi 
 ment. Heat is necessary, because the ice must be 
 melted to make the experiment succeed. But comparing 
 the effects of heat and refrigeration (not of heat and 
 the continual inflow of ice-cold water), I conceive that 
 heat would be found altogether the more effective. 
 
 Lastly, as to the wind theory of the Grulf Stream, Dr. 
 Carpenter remarks that, so far as he knows, I am the 
 only man of science in this country agreeing with Capt. 
 Maury in attributing the Grulf Stream to some other 
 cause than the impelling force of the trade winds. He 
 must be aware that there are not half a dozen students 
 of science in this country who have expressed definite 
 opinions on the subject after a thorough and independent 
 inquiry into the evidence. Amongst those who main 
 tain the wind theory there is not one, so far as I know, 
 with whom Dr. Carpenter is in agreement. Mr. Laugh ton 
 disputes the very principle of Dr. Carpenter s reasoning, 
 holding that the change of temperature from equator to 
 poles proceeds too slowly mile for mile to produce the 
 effects which Dr. Carpenter indicates. Mr. Croll, in 
 like manner, has expressed his complete dissent from 
 Dr. Carpenter s reasoning. So also has Mr. Findlay. I 
 believe these gentlemen to be mistaken, and I conceive 
 that I have been able to put my finger on the precise 
 point where their respective lines of reasoning fail. 
 But, if Dr. Carpenter is to take general consent as an 
 argument, and to maintain that I am wrong because he 
 knows of no one who agrees with me, I may as well 
 point out that he is entering into a very questionable 
 
258 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 alliance, so far as his special views are concerned. So 
 far as I know, all the continental students of science 
 who share our common views as to vertical circulation, 
 reject the wind theory as solely sufficing to account for 
 the Gulf Stream. Again, he sets Sir J. Hersehel s 
 opinion (thirty years ago) that the Gulf Stream is 
 entirely due to the trade winds as almost conclusive 
 against me. It is, at least, not new to me, since it 
 is cited in every paper I have written on the subject. 
 But is there no evidence to show that Sir J. Herschel 
 abandoned the view he formerly entertained ? I would 
 ask what Sir John Herschel implies when, in his letter 
 to Dr. Carpenter, he writes, The action of the trade 
 and counter-trade winds, in like manner, cannot be 
 ignored ; and henceforward the question of ocean cur 
 rents will have to be considered under a twofold point 
 of view. The word henceforward implies very dis 
 tinctly that Sir J. Herschel was entertaining a new 
 opinion that is, an opinion new to him ; and I think 
 Dr. Carpenter would find it dimcult to demonstrate that 
 this new opinion would not have enforced the omission 
 of the word entirely from the sentence quoted by Dr. 
 Carpenter. 
 
 I need hardly say that I do not agree with Captain 
 Maury, whose theory of oceanic circulation appears to 
 me to be wholly untenable. Nor do I for a moment 
 assert that the winds play no part in producing oceanic 
 circulation. I may have been mistaken in attaching 
 so much weight as I have to Maury s evidence as to the 
 trade wind zones, though it is known that science owes 
 more to him than to any man for our present knowledge 
 
OCEANIC CIRCULATION. 259 
 
 of the winds prevalent in certain regions ; and when I 
 first wrote on the Gulf Stream there was no evidence on 
 the subject even approaching Maury s (or that collected 
 by Maury) in accuracy and completeness. But there is 
 one argument which those who have adopted the trade 
 winds as the primary cause of the Gulf Stream appear 
 to me to have overlooked, and it is on this argument 
 that my own view has been chiefly based. The trade 
 wind zone of the northern hemisphere is not constant 
 in position ; but travels northwards and southwards 
 with the northerly and southerly motion of the sun in 
 declination. The change in the position of the zone of 
 calms is not, indeed, so great as is stated in Buchan s 
 meteorology, where it is said to travel from 25 north to 
 25 south of the equator ; but it is considerably greater 
 than was supposed by Dove, Kaemtz, and others. If we 
 set the extreme shift of the northern trade-zone at ten 
 degrees we are certainly not over-rating it. Taking 
 this zone as extending in spring or autumn from 10 to 
 25 north latitude, we should have it in winter extending 
 from 5 to 20, and in summer from 15 to 30, the 
 only part common to these two ranges being that from 
 15 to 20 that is to say, the northern five degrees 
 of the winter zone, and the southern five degrees of the 
 summer zone, each zone being 15 wide. Now, if any 
 one will mark these zones on the North Atlantic, he 
 will find that while the zone of winter trades would 
 produce a current flowing into the southern half of the 
 Gulf of Mexico, the zone of summer trades would pro 
 duce a current flowing into the northern half. The 
 
 s 2 
 
260 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 former would produce a current flowing as the Grulf 
 Stream actually flows ; the latter would produce a cur 
 rent flowing precisely in the opposite direction. This 
 being the case, I do not find the evidence for the trade 
 winds as the sole or even the main cause of the Grulf 
 Stream altogether convincing. The case does not, for 
 instance, seem quite c as clear as the rotation of the 
 earth. It seems, also, not undesirable to mention that 
 the equatorial current and the Gulf Stream are not 
 mere drift-currents, and that on a careful estimation of 
 the frictional action of such winds as the trades on the 
 surface of the ocean, the action will be found quite 
 unequal to the propulsion of so vast a body of water as is 
 actually carried westwards (not, by the way, before these 
 winds). Until difficulties such as these have been 
 removed from the trade wind theory as solely sufficient 
 to account for the Grulf Stream, I think I would rather 
 be the only student of science opposing that theory, 
 than one of a phalanx, however large, maintaining it. 
 There is, however, no such phalanx ; the subject being 
 regarded by nearly all students of science as a very 
 open one. 
 
 English Mechanic, Aug. 30, 1872. 
 
 THE CLIMATE OF GREAT BRITAIN. 
 
 IF there is one feature in the material relations of a 
 country which may be considered as characteristic as 
 of itself sufficient to define the qualities of the inhabit 
 ants, and the position they are fitted to occupy in the, 
 
THE CLIMATE OF GREAT BRITAIN. 261 
 
 world s history it is climate. It includes, says Hum- 
 boldt, all those modifications of the atmosphere by 
 which our organs are affected such as temperature, 
 humidity, variations of barometric pressure, its tran 
 quillity or subjection to foreign winds, its purity or 
 admixture with gaseous exhalations, and its ordinary 
 transparency that clearness of sky so important through 
 its influence, not only on the radiation of heat from the 
 soil, the development of organic tissue and the ripening 
 of fruits, but also on the outflow of moral sentiments in 
 the different races I do not propose, however, to deal 
 with the constitution of the climate of Great Britain 
 under this general view. To do so, indeed, would require 
 somewhat more space than can in this volume be con 
 veniently allotted to a single subject. I wish chiefly 
 to consider the subject of temperature (mean annual 
 and extreme winter or summer temperature) ; though 
 I shall have a few words to say respecting that feature 
 of our climate which most foreigners consider to be its 
 chief defect the want of transparency or clearness in 
 our skies as compared with those of some other European 
 countries. 
 
 The mean annual temperature of a country is less 
 important to the welfare of the inhabitants than the 
 extreme range of temperature exhibited in the course 
 of the year. Of two countries which have the same 
 mean annual temperature, one may have a climate 
 most 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 
 
262 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 inhabitants resemble the unfortunates described by 
 Dante, doomed 
 
 a sofFrir tormenti e caldi e geli. 
 
 However, I shall deal first with this feature mean 
 annual temperature as affording a starting-point from 
 which to proceed to other considerations. 
 
 If the surface of the earth were perfectly uniform, or 
 symmetrically distributed into districts of land and 
 water arranged in zones along latitude-parallels, and if 
 the strata of the soil were throughout of like density, 
 radiating power, and elevation, the lines of equal mean 
 temperature would be parallels of latitude. This hypo 
 thetical condition of things is, we know, very far from 
 representing the true condition of the earth s surface. 
 Land and water are distributed in a manner which 
 hardly presents the semblance of law ; elevations and 
 depressions, not merely of areas of limited extent, but 
 of whole 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-ordination of climates 
 under latitude-parallels. 
 
 It is to Humboldt that we owe the valuable propo 
 sition that maps of the world should exhibit parallels 
 of heat, as well as latitude-parallels ; and no atlas is 
 now considered complete without maps in which iso 
 therms, or lines of equal mean annual temperature, 
 isochimenals or lines of equal winter heat, and iso- 
 therals or lines of equal heat in summer, are exhibited. 
 
THE CLIMATE OF GREAT BRITAIN. 263 
 
 These lines are usually presented in maps on Mercator s 
 projection, an arrangement which has some advantages, 
 but is not, on the whole, very well suited to exhibit the 
 true conformation of the isothermal lines the study of 
 which, ifr has been well remarked, constitutes the basis 
 of all climatology. 
 
 In Figs. 1 and 2, the northern hemisphere of the 
 earth is presented on a projection (the equal surface) 
 which has been discussed in my 6 Essays on Astronomy. 
 The smallness of the scale would not readily permit of the 
 introduction of the system of isothermal lines usually 
 presented, therefore I have only introduced the isotherm 
 which passes through London. In both figures this 
 isotherm is represented by a dotted closed curve passing 
 across the south of England, thence across the Atlantic 
 in a south-westerly direction, and across the continent 
 of America nearly on the latitude of New York. After 
 it has entered the Pacific Ocean, the isotherm passes 
 somewhat northwards, but trends southwards again as 
 it nears the Asiatic continent, reaching its greatest 
 southerly range in the sea of Japan, traversing Asia 
 nearly on the latitude of the Aral Sea, and thence passing 
 somewhat northwards through the Crimea, Vienna, and 
 Brussels to London. Along its whole extent the iso 
 therm nowhere has a higher latitude than where it 
 crosses the British Isles; in other words, the mean 
 annual temperature of Great Britain is higher than 
 that of any country lying between the same latitude- 
 parallels. The advantage of this arrangement is second 
 only in importance to that which England will be seen 
 
264 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 to possess when we come to consider the extreme 
 range of temperature during the year. In fact, Eng- 
 
 FIG. i. 
 
 Northern hemisphere on an equal-surface projection, showing curves of 
 mean annual and midwinter temperature through London. 
 
 land is thus brought to the centre of the true temperate 
 zone of the northern hemisphere ; and the considera 
 tion of Figs. 1 and 2 will show that the isotherm of 
 London approaches as near to the tropic of Cancer 
 in one part of its course, 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 CLIMATE OF GREAT BRITAIN. 265 
 
 climate of Great Britain, but geographically interesting. 
 In examining the polar presentation of the London 
 
 FIG. 2. 
 
 Northern hemisphere on an equal-surface projection, showing curves of 
 mean annual and midsummer temperature through London. 
 
 isotherm, we see that in two parts of its course it 
 exhibits a tendency to travel northwards, and becomes, 
 in fact, convex towards the pole. If we laid down iso 
 therms of greater mean temperature that is, nearer 
 the equator we should find this peculiarity gradually 
 diminishing. But if we laid down isotherms of lower 
 mean temperature, we should find the convexities 
 gradually becoming sharper and more defined, approach- 
 
266 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 ing each other more and more nearly, until finally they 
 would meet, and the isothermal curve be divided into 
 two irregular ovals. Proceeding to trace out curves of 
 still lower temperature, we should find the two ovals 
 closing in towards two poles of cold. These are indi 
 cated in Figs. 1 and 2 by two black spots, one north 
 of the American, the other north of the Asiatic con 
 tinent. It is to be noted, however, that at the American 
 pole the mean annual temperature is not quite so low 
 as at the Asiatic pole, the former temperature being 
 3i, the latter 1 Fahrenheit. 
 
 Eeturning to our subject, let us consider the all- 
 important question of range of climate. The effects 
 of climate, unimportant to the stronger inhabitants of 
 a country, but largely influencing the health and com 
 fort of the majority, are chiefly felt through the changes 
 that occur during the year. Now, we have seen that 
 the line of mean annual temperature of England departs 
 in a very marked manner from coincidence with a 
 latitude-parallel ; but we shall find the lines indicating 
 the extreme temperatures of the year much more 
 irregular ; and the peculiarity of climate, which their 
 conformation illustrates, much more important. 
 
 In Fig. 1 the isochimenal, or the line of equal winter 
 heat, through London, is indicated by a strongly marked 
 closed curve. Its form is remarkable. It passes nearly 
 in a north and south direction, along the length of 
 England and Scotland, approaches singularly near 
 to Iceland, but turns sharply southwards and travels 
 across the Atlantic in a direction which brings it to 
 
THE CLIMATE OF GREAT BRITAIN. 267 
 
 the American continent near Washington. Still ap 
 proaching the tropics, it travels through the northern 
 parts of Texas, where it reaches its greatest southerly 
 range. Passing gradually northwards to the neigh 
 bourhood of the Aleutian Islands, it thence trends 
 southwards again, passes through the Corea, traverses 
 the Asiatic continent nearly on the latitude-parallel 
 of Nankin ; thence travelling slightly northwards, it 
 crosses the southern part of the Caspian Sea, the Black 
 Sea, and the north of Turkey, passing through Venice 
 and Paris to London. On the continents the isochi- 
 menal falls outside (that is, south of) the annual iso 
 therm, while on the oceans the reverse holds. The 
 projection of the isochimenal thus appears as an 
 irregular oval, whose greatest length lies on the con 
 tinents. 
 
 We see here, again, the indication of a tendency to 
 form two curves, and thus of the presence of two poles 
 of extreme winter cold in the northern hemisphere. 
 The isochimenals of greatest cold hitherto traced in 
 the two continents are shown by two broken curves 
 in Fig. 1. The cold of the Asiatic curve is very much 
 greater than that of the Ameri can, the former curve 
 marking a winter cold of 40 Fahrenheit (72 below 
 freezing), the latter a winter cold of 26 5 , only if 
 one may apply such an adverb to a cold of 58 5 below 
 freezing. Professor Nichol remarks that, if a polar 
 projection were made of these regions for January, it 
 would be found that the two coldest spaces of these 
 continents form a continuous band passing across the 
 
268 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 pole of the earth. I cannot but think that this is a 
 mistake. I believe that if the isotherms traced, in 
 part, in Fig. 1 could be completed, they would be 
 found to form two ovals. The American oval would 
 enclose the American pole of mean temperature, but 
 very eccentrically, showing that the pole of extreme 
 winter temperature lay westwards and southwards, pro 
 bably near Victoria Land. The Asiatic oval would 
 not probably enclose the Asiatic pole of mean tempera 
 ture ; and the position indicated for the Asiatic pole 
 of extreme winter cold lies on or near the Arctic circle, 
 where it is crossed by the river Lena. At the true 
 pole of the earth the extreme winter cold is probably 
 not nearly so intense as the cold at either of the points 
 here 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 the 
 British Isles, while Cornwall and the south-westerly 
 parts of Ireland enjoy the mildest winter climates. 
 In fact, winter in Cornwall is not more severe than in 
 Constantinople ; and in south-west Ireland the winter 
 is still milder, approaching, in this respect, to the 
 winter climate of Teheran. 
 
 The contrast, when we turn to the isotheral of Lon 
 don, is remarkable. Instead of travelling nearly north 
 wards, this curve passes in a south-westerly direction, 
 reaching its greatest southerly range in the central 
 part of the Atlantic Ocean; thence it travels with a 
 northerly sweep through Nova Scotia and Canada, till 
 
THE CLIMATE OF GREAT BRITAIN. 269 
 
 it reaches its greatest northerly range near the Eocky 
 Mountains. 1 Thence it turns sharply southwards, 
 crosses Vancouver s Island, sweeps nearly to latitude 
 45 in the central part of the Pacific, whence passing 
 slightly northwards it crosses the southern part of 
 Saghalien Island. Here it turns sharply northwards, 
 crosses that very district of Siberia which, in Fig. 1, 
 is occupied by the isochimenal of intensest winter cold, 
 traverses Siberia, and passes near St. Petersburg, through 
 Berlin and Amsterdam to London. 
 
 The relations thus presented by the isotheral of 
 London are precisely the reverse of those exhibited 
 by the isochimenal. The isotheral forms a closed 
 irregular oval, whose greatest length lies on the two 
 oceans : here it falls outside the line of mean annual 
 heat, while on the continent it falls far within this 
 line. 
 
 In another respect the isotheral presents a note 
 worthy contrast to the isochimenal. While the latter 
 encloses an area largely exceeding the area enclosed by 
 the mean annual line, the isotheral encloses an area 
 noticeably smaller. 2 
 
 A tendency to break up into two curves is exhibited 
 in the isotheral, even more markedly than in the two 
 other curves. But singularly enough, here, where one 
 
 1 It is noteworthy that the minimum distance of the isotheral from 
 the North Pole here attained is exactly equal to the minimum distance 
 of the isochimenal from the equator. 
 
 2 Here an important advantage of the isographic projection is ex 
 hibited. The relation pointed out is altogether obliterated in Mercator s 
 projection, and could only be roughly inferred from any but an isographic 
 projection. 
 
270 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 would expect more certain information of the existence 
 of poles of cold, since so much more of the northern 
 hemisphere can be traversed in summer than in winter, 
 we have no satisfactory evidence. In fact, the irregular 
 curve marked in near the pole in Fig. 2 is the most 
 northerly isotheral yet determined. The temperature 
 corresponding to this isotheral is 36 Fahrenheit, or 
 four degrees above freezing. From a consideration of 
 the form-variations of the isotherals as they travel 
 northwards, I have been led to the opinion that there 
 exist three poles of summer cold, and that these fall 
 not very far from the positions indicated by the small 
 dark circles in Fig. 2. 
 
 From the direction of the isotheral line through 
 London, it is evident that along the south-eastern coast 
 of England the heat of summer is greater than in any 
 other part of the British Isles. On the other hand, 
 the northern parts of Scotland, which we have seen 
 enjoy a winter climate fully as warm as that of London, 
 have a much cooler summer climate. The south 
 western parts of Ireland exhibit a change even more 
 remarkable For whereas the winter climate in these 
 parts is the same as that of Persia, the summer 
 climate is the same as that of the very portion of 
 Siberia in which (most probably) the greatest cold 
 ever observable in our northern hemisphere is ex 
 perienced in winter. The summer of the Orkney 
 Islands, again, is no warmer than that of the southern 
 parts of Iceland. 
 
THE CLIMATE OF GREAT BRITAIN. 271 
 
 It appears, then, that the inhabitants of England 
 enjoy three notable advantages as respects climate. 
 First, a higher mean annual temperature than that of 
 any other country so far from the equator ; secondly, 
 a moderate degree of cold in winter; and lastly, a 
 moderate degree of heat in summer. The last two 
 advantages resolve themselves into one, viz., small 
 range 6f temperature throughout the year. Our range 
 of climate is from about 36 in winter to 62^ in 
 summer, or in all, 26^ Fahrenheit. Compare with 
 this the climate of the country near Lake Winnipeg, 
 with a winter cold of 4 below zero, and a summer heat 
 scarcely inferior to that of London ; so that the range 
 of climate is no less than 65. Yet more remarkable 
 is the variation of climate in parts of Siberia, near 
 Yakutsk ; here the range is from 40 in winter to 
 62 in summer a variation of 102, or four times the 
 variation of our London climate. Other parts of the 
 British Isles have, however, a yet smaller range even 
 than that of London. Thus in the south-western parts 
 of Ireland, and in the Orkney Isles, the variation is less 
 than 19. 
 
 Nor is it difficult to assign sufficient reasons for the 
 mildness of the British climate for our warm winters 
 and cold summers. It will appear, on examination, 
 that nearly all the constant causes affecting the tem 
 perature of a climate operate to raise the mean tem 
 perature of our year, while, of variable causes those 
 which tend to generate increased heat operate in 
 
272 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 winter, while those which have a contrary effect operate 
 in summer. 
 
 Humboldt enumerates among the causes tending to 
 exalt temperature the following non-variables : The 
 vicinity of a west coast in the northern temperate zone ; 
 the configuration of a country cut up by numerous deep 
 bays and far-penetrating arms of the sea; the right 
 position of a portion of the dry land, i.e. its relation to 
 an ocean free of ice, extending beyond the polar circle 
 or to a continent of considerable extent which lies 
 beyond the same meridional lines under the equator, or 
 at least in part within the tropics; the rarity of swamps 
 which continue covered with ice through the spring, or 
 even into summer; the absence of forests on a dry, 
 sandy soil ; and the neighbourhood of an ocean-current 
 of a higher temperature than that of the surrounding 
 sea. 
 
 All these causes, it will be observed except the 
 neighbourhood of a tropical continent on the same 
 meridian tend to increase the mean heat of the climate 
 in England. The great Grulf Stream probably exer 
 cises a more important influence than any of the others. 
 Its position is indicated in Figs. 1 and 2. Humboldt 
 attaches a high importance to the presence of a tropical 
 continent on the same meridian ; and he considers that 
 the climate of Europe is warmer than that 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, how 
 ever, to the reasoning he adopts. In the first place, if 
 
THE CLIMATE OF GREAT BRITAIN. 273 
 
 the heat-radiating power of a continent really influenced 
 the country lying to the north, it should tend to lower 
 rather than raise the temperature, for the ascending 
 currents of air would strengthen the currents of colder 
 air pouring in from the north, and these currents 
 on Humboldt s assumption that the country directly to 
 the north is that affected would lower the mean 
 annual temperature. It would only be exceptionally 
 that the warmer returning currents would descend, and 
 thus exalt the temperature. It seems clear, however, 
 that Asia is the continent chiefly affected by the heat- 
 radiating power of Africa ; since the cold currents from 
 the north travel eastwards, while the warm return- 
 current has a westerly motion. We should thus attri 
 bute the milder climate of Europe rather to the 
 influence of the tropical parts of the Atlantic Ocean, 
 than to the cause assigned by Humboldt, and we 
 should invert the effects he attributes to oceans and 
 continents respectively. With this change somewhat 
 a bold one, I confess 1 we may say that all the non- 
 variable causes tending to exalt temperature operate in 
 England s favour. 
 
 The constant causes tending to lower temperature 
 are simply the converse of those above considered. 
 
 1 Not unsupported, however, by good authority. Thus Professor 
 Nichol, speaking of the climate of Europe, writes : The air that rises 
 in Africa blows rather over Asia than Europe. The cradle of our 
 winds is not in Sahara but in America. Again, Kaemtz notices, that 
 if the effects of oceans and continents were those assigned by Humboldt, 
 we should find in the western parts of America a colder climate than 
 in the eastern parts ; the reverse, however, is the case. 
 
274 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 Of variable causes increasing temperature, the 
 principal are a serene sky in summer, and a cloudy 
 sky in winter. It may appear, at first sight, para 
 doxical to assign opposite effects to a cloudy sky. It 
 must be remembered, however, that clouds considered 
 with reference to temperature, have two functions : 
 they partially prevent the access of heat to the earth, 
 and they partially prevent the escape of heat from the 
 earth. Now, in summer the first-named influence is 
 more important than the second: the days are longer 
 than the nights ; that is. the earth is receiving heat 
 during the greater part of the time in summer. A 
 cause, therefore, which affects the receipt of heat is 
 more important than a cause affecting the escape of 
 heat. On the other hand, in winter the nights are 
 shorter than the days, and the influence of clouds in 
 preventing the escape of heat becomes more important 
 than their effect on the receipt of heat. 1 In fact, we 
 may compare the influence of clouds to the effects of 
 certain kinds of clothing ; flannel, for instance, is as 
 suitable an article of dress for the cricketer as for the 
 skater. 
 
 Xow the climate of England is remarkably humid 
 both in winter and summer. And this humidity is 
 shown, not so much by the quantity of rain which falls, 
 
 1 Gilbert White noticed long ago apparently without understanding 
 the influence of a clouded sky on the temperature. We have often 
 observed, he says, that cold seems to descend from above ; for, when a 
 thermometer hangs abroad on a frosty night, the intervention of a cloud 
 shall immediately raise the mercury ten degrees ; and a clear sky shall 
 again compel it to descend to its former gauge. 
 
THE CLIMATE OF GREAT BRITAIN. 27 ~ 
 
 as by the frequent presence of large quantities of 
 aqueous vapour in the atmosphere. Skies, even, which 
 we in England consider clear, are overcast compared 
 with the deep-blue skies of France or Italy. What the 
 influence of these humid palls may be on the out-flow 
 of moral sentiments which Humboldt considered to be 
 so favourably influenced by transparent skies, I shall 
 not here pause to inquire. It is clear, however, that 
 the influence of our cloudy skies tends to modify the 
 severity both of our winter and our summer seasons ; 
 and these benefits are so great that we may cheerfully 
 accept them as more than a counterpoise for hypo 
 thetical injurious effects on the outflow of our moral 
 sentiments (whatever that may mean). 
 
 I proceed to consider the actual variations presented 
 in the course of a year in England. As some selection 
 must be made, I shall select a series of observations 
 which have been made at Greenwich during the present 
 century. It will be gathered from the preceding 
 pages that the range of temperature at Greenwich is at 
 least not less than the average range of the British 
 Isles. Greenwich, also, from its neighbourhood to 
 London, and from the number and accuracy of the 
 observations made there, is obviously the best selection 
 that could be made. It must not be forgotten, how 
 ever, that the climate of Greenwich is not the climate 
 of the British Isles, and that careful observations made 
 in other places have sufficiently indicated the existence 
 of local peculiarities, which, therefore, it may fairly be 
 assumed, characterise also the Greenwich indications. 
 
 T 2 
 
276 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 In Fig. 3 the annual variations of mean diurnal 
 temperature are represented graphically. The figure 
 was formed in the following manner : A rectangle 
 having been drawn, each of the longer sides was 
 divided into 365 parts, and a series of parallel lines 
 joining every tenth of these divisions was pencilled in. 
 The spaces separating these lines represented successive 
 intervals of ten days throughout the year. The shorter 
 sides were divided into thirty-three parts and parallel 
 lines drawn, joining the points of division. Of these 
 longer parallels the lowest was taken to represent a 
 temperature of 32 Fahrenheit (i.e., the freezing point) 
 and the others, in order, successive degrees of heat up 
 to 65. Then, from the Greenwich tables, which have 
 been formed from the observations 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 365 points were marked in, and 
 these being joined by a connected line, presented the 
 curve exhibited in Fig. 3. The lines bounding the 
 months, and the lines indicating 35, 40, &c., Fahren 
 heit, were then inked in and the figure completed. 
 
 The resulting curve is remarkable in many respects. 
 In the first place, it was to have been expected that a 
 curve representing the average of so many years of 
 observation would be uniform ; that is, would only 
 exhibit variations in its rate of rise and fall, not such 
 a multiplicity of alternations as are observed in Fig. 3, 
 And this irregularity will appear the more remarkable 
 when it is remembered that the temperatures used as 
 
Its 
 
278 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 the Greenwich means are not the true average tem 
 peratures. They were obtained by constructing a curve 
 from the true averages, and taking a curved line (the 
 curve of Fig. 3, in fact) in such a way as to take off 
 the most marked irregularities of the true curve of 
 averages ; or to use the words of the meteorologist who 
 constructed the Greenwich table of means, Mr. Glaisher, 
 a curved line was drawn which passed through or near 
 all the points determining the true curve of averages, 
 4 and in such a way that the area of the space above 
 the adopted line of mean temperature was equal to that 
 below the line. Despite this process, the curve exhibits 
 no less than fourteen distinctly marked maxima of 
 elevation, and a much larger number of variations of 
 flexure. The sudden variations of temperature at the 
 beginning of February, early in April, and early in 
 May are very remarkable ; they have their counterparts 
 in the three variations which take place between the 
 latter part of November and the end of the year, only 
 these occur in much more rapid succession. The 
 nature of the curve between June and August is also 
 remarkable, as are the three convexities which are ex 
 hibited in the September, October, and November 
 portions of the curve. 
 
 If we follow our leading meteorologists in taking the 
 curve of Fig. 3 as representing the true annual climate 
 of London, how are we to assign physical causes for the 
 remarkable variations above indicated ? Not easily, I 
 take it. It were, indeed, as easy as inviting to specu 
 late on cosmical causes ; to follow Ertel, for instance. 
 
THE CLIMATE OF GREAT BRITAIN. 
 
 279 
 
 in assigning effects to those zones of meteorities 
 which are known to. intersect the earth s orbit, and 
 others which may fairly be assumed to fall within or 
 without that orbit. It may be, perhaps, that the 
 recognised shooting-star periods have, some of them, 
 their counterparts in heat-changes ; but certainly the 
 time has not yet come to pronounce a consistent theory 
 of such effects. The evidence afforded by the Greenwich 
 curve on this point is unsatisfactory, to say the lease. 
 The elevation at the beginning of January, and the 
 marked irregularity in February, correspond to Ertel s 
 views ; so also the fact that large aerolites have fre 
 quently fallen in the first week in April, about the 
 20th of April, about the 18th of May, early in August, 1 
 about the 19th of October, and early in December, 
 seems to correspond to elevations in the curve ; while 
 depression opposite the 12th of May, might be referred 
 to the intervention of the zone of meteors, which causes 
 the now celebrated November shower. But the nega 
 tive evidence is almost equally strong. Where, for 
 instance, is the elevation which one would expect, on 
 Ertel s theory, in November ? Also, if the cause of the 
 observed irregularities were that suggested by Ertel, 
 the curves for other countries in the northern hemi 
 sphere should exhibit similar irregularities on corre 
 sponding dates, which does not appear to be the case. 
 In fact, if there really exist effects due to cosmical 
 causes, these are not likely to be educed from observa- 
 
 1 Reference is not made here to the August shooting-star shower, 
 which takes place a week later than the epoch alluaed to. 
 
280 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 tions of the variation of mean diurnal temperature, 
 since it is clear that a cause of variation due to objects 
 external to the earth could affect only the temperature 
 of certain hours of one day or of several days. A cluster 
 of meteors between the earth and the sun might 
 diminish the mid-day heat ; one external to the earth s 
 orbit might increase the nocturnal temperature ; and 
 though in either case the mean diurnal temperature 
 would be affected, yet it is obvious that the effect 
 would be masked in taking the mean, or even that two 
 or more opposing influences might cancel each other. 
 If it could be shown that the curve for mid-day, or for 
 midnight heat corresponded to the curve of mean heat, 
 Ertel s theory would be overthrown at once ; since, for 
 its support it is necessary to show that depressions in 
 the mean curve are due to mid-day loss of heat, and 
 elevations to midnight gain of heat. 
 
 There are, however, terrestrial causes to which the 
 irregularities of our curve (which irregularities, be it 
 remembered, represent regularly recurring irregulari 
 ties of heat) may be ascribed. For instance, there can 
 be no doubt that our climate is considerably affected by 
 the changes which take place in the Polar seas ; and it 
 may not unfairly be assumed that the processes by 
 which different regions of Polar ice are successively 
 set adrift (to be carried southward by the strong under 
 current known to exist in the northern Atlantic Ocean), 
 take place at epochs which recur with tolerable regu 
 larity. And it may be that the irregularity of the 
 rising as compared with the falling of the heat-curve 
 
THE CLIMATE OF GREAT BRITAIN. 281 
 
 is due to this cause ; since the breaking-up of ice-fields 
 and their rapid transport southwards would clearly 
 produce sudden changes, having no counterpart in the 
 effects due to the gradual process of freezing. 1 
 
 It may well be, however, that the observations of 
 forty-three years are not sufficient to afford the true 
 mean diurnal temperature for a climate so variable as 
 ours. Indeed, if the curves given by Kaemtz for 
 continental climates be as accurately indicative of 
 observed changes as that of Fig. 3, we must either 
 accept such an hypothesis, or else assume that the 
 English climate is marked by regularly recurring 
 variations altogether wanting in continental climates ; 
 and it is to be noted that the regular recurrence of 
 changes is a peculiarity wholly distinct from variability 
 of climate, properly so termed, and seems even incon 
 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 figure ; and 
 that by comparing the observations of the eighty or 
 ninety years, which would then be available, many, or 
 all, of the irregularities exhibited in Fig. 3 might be 
 removed. In this case we might expect our climate- 
 curve to assume the form indicated by the light line 
 taken through the irregularities of Fig. 3. It will be 
 observed that this modified curve exhibits but one 
 maximum and one minimum. It is not wholly free, 
 
 1 Icebergs have been seen travelling southwards against a strong 
 northward surf ace- current, and even forcing their way through field-ice 
 in so travelling. 
 
282 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 however, from variations of flexure. It presents, indeed, 
 six well-marked convexities, and as many concavities ; 
 in other words, no less than twelve points of inflexion. 
 The most remarkable irregularity of this sort is that 
 exhibited near the end of November ; and it is note 
 worthy that this irregularity is presented by continental 
 climate-curves also. It has been ascribed by Ertel to 
 the effect of the meteor-zone which causes the 
 November shower. But as it is exhibited by the curves 
 of horary as well as of diurnal means, while the 
 meteor-zone cannot by any possibility affect the tem 
 perature of the earth s following hemisphere, and as, 
 further, it does not correspond to the true date of the 
 shower, this view may be looked upon as doubtful. 
 The August curve occurring near the maximum 
 elevation where slow change was to be expected, is 
 also well worthy of notice ; as are the January and 
 May flexures. 
 
 It will be noticed that nothing has been said of 
 extreme heat or cold occasionally experienced in 
 England. As such visits generally last but for a short 
 time, their effects are not very injurious, save on the 
 very weak, the aged, or the invalid. Corresponding to 
 the passage of an immense heat-wave or cold-wave, 
 there invariably occurs a sudden rise in the mortality- 
 returns ; but almost as invariably the rise is followed 
 by a nearly equivalent, but less sudden fall ; showing 
 conclusively that many of the deaths which marked the 
 epoch of severest weather occurred a few weeks only 
 before their natural time. 
 
THE CLIMATE OF GREAT BRITAIN. 283 
 
 The weather during a part of the late winter was 
 somewhat severer than our average English winter- 
 weather. The thermometer, however, at no time 
 descended below zero, as it did on January 3, 1854; 
 and the diurnal mean did not descend at any time so 
 low as 10 7 , as it did on January 20, 1838. There is 
 no foundation for the opinion, sometimes expressed, 
 that our winter weather is changing. An examination 
 of the columns in the Greenwich meteorological tables, 
 show that the successive recurrence of several mild 
 winters is not peculiar to the last decade or two. The 
 observations of Gilbert White, imperfect as they are 
 compared with modern observations, point the same 
 way. 
 
 Among severe, but short, intervals of cold weather may 
 be noted that which occurred in January 1768. The 
 frost was so intense, says Gilbert White, that horses 
 fell sick with an epidemic distemper which injured the 
 winds of many and killed some ; meat was so hard 
 frozen that it could not be spitted, nor secured but in 
 cellars ; and bays, laurustines, and laurels were killed. 
 White s account of the summer of 1783 will fitly 
 close our sketch of British weather-changes. This 
 summer, he says, was an amazing and portentous one, 
 and full of horrible phenomena ; for besides the alarm 
 ing meteors and tremendous thunder-storms that 
 affrighted and distressed the different counties of this 
 kingdom, the peculiar haze or smoky fog that prevailed 
 for many weeks in this island, and in every part of 
 Europe, and even beyond its limits, was a most ex- 
 
284 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 traordinary appearance, unlike anything known within 
 the memory of man. By my journal, I find that I had 
 noticed this strange occurrence from June 23 to July 
 20, inclusive, during which period the wind varied to 
 every quarter, without making any alteration in the air. 
 The sun at noon looked as blank, and ferruginous as a 
 clouded moon, and shed a rust-coloured ferruginous 
 light on the ground and floors of rooms, but was par 
 ticularly lurid and blood-coloured at rising and setting. 
 All the time the heat was so intense that butchers meat 
 could hardly be eaten the day after it was killed ; and 
 the flies swarmed so in the lanes and hedges, that they 
 rendered the horses half frantic, and riding irksome. 
 The country people began to look with a superstitious 
 awe at the red, louring aspect of the sun. Milton s 
 noble simile, in his first book of " Paradise Lost," fre 
 quently occurred to my mind ; and it is, indeed, par 
 ticularly applicable, because, towards the end, it alludes 
 to a superstitious kind of dread, with which the minds 
 of men are always impressed by such strange and un 
 usual phenomena : 
 
 As when the sun new risen, 
 Looks through the horizontal misty air, 
 Shorn of his beams ; or, from behind the moon, 
 In dim eclipse, disastrous twilight sheds 
 On half the nations, and with fear of change 
 Perplexes monarchs. 
 
 Intellectual Observer, March 1867. 
 
 
LOW BAROMETER OF ANTARCTIC ZONE. 285 
 
 THE LOW BAROMETER OF THE ANTARCTIC 
 TEMPERATE ZONE. 
 
 THE great difficulty presented by the science of 
 meteorology lies in the intricate combination of causes 
 producing atmospheric variations, and the impossi 
 bility of determining by experiment the relative 
 efficiency even of the most important agents of change. 
 As Sir W. Herschel well observed, we are in the 
 position of a man who hears at intervals a few frag 
 ments of a long history narrated in a prosy, un 
 methodical manner. A host of circumstances omitted 
 or forgotten, and the want of connection between the 
 parts, prevent the hearer from obtaining possession of 
 the entire history. Were he allowed to interrupt the 
 narrator, and ask him to explain the apparent contra 
 dictions, or to clear up doubts at obscure points, he 
 might hope to arrive at a general view. The questions 
 that we would address to Nature, are the very experi 
 ments of which we are deprived in the science of 
 meteorology. l 
 
 It is, therefore, but seldom in the study of this 
 science that we meet with phenomena to which we can 
 assign a definite cause, or which we can explain on 
 simple principles. Even those marked phenomena, 
 which appear most easily referable to simple agencies, 
 
 1 Kaemtz s Meteoro j ogy. 
 
286 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 present difficulties on a close investigation which com 
 pel us at once to recognise the efficiency of more causes 
 than one. For instance, the phenomenon of the trade- 
 winds, as explained by Halley, appears at first sight 
 easily intelligible ; but when we look on this phenome 
 non as a part merely as indeed it is of the marvel 
 lously complex circulation of the earth s atmosphere 
 when we come to inquire why these winds blow so 
 many days in one latitude, and so many in another, or 
 why they do not blow continually in any latitude 
 when we consider the character of these winds as 
 respects moisture and temperature, the variation of the 
 velocity with which they blow, and of the quantity of 
 air they transfer from latitude to latitude we 
 encounter difficulties which require for their elucida 
 tion the comparison of thousands of observations, or 
 which baffle all attempts at elucidation. 
 
 There is, however, one atmospheric phenomenon 
 that which I have selected for the subject of this paper 
 which presents a grand simplicity, rendering the 
 attempt at a simple solution somewhat more hopeful 
 than is usually the case with meteorological phenomena. 
 The discovery of this phenomenon formed one of the 
 most interesting results of Captain Sir J. C. Eoss s 
 celebrated expedition to the Antarctic Ocean. He 
 found, as the result of observations conducted during 
 three years, that the mean barometric pressure varied 
 in the following manner at the latitudes and places 
 specified : 
 
LOW BAROMETER OF ANTARCTIC ZONE. 287 
 
 South latitude Height of the barometer 
 
 Place 
 
 
 
 
 
 29-974 in. 
 
 13 
 
 
 
 30-016 
 
 22 
 
 17 
 
 30-085 
 
 34 
 
 48 
 
 30-023 
 
 42 
 
 53 
 
 29-950 
 
 45 
 
 
 
 29-664 
 
 49 
 
 8 
 
 29-469 
 
 51 
 
 33 
 
 29-497 
 
 54 
 
 26 
 
 29-347 
 
 55 
 
 52 
 
 29-360 
 
 60 
 
 
 
 29-114 
 
 66 
 
 
 
 29-078 
 
 74 
 
 
 
 28-928 
 
 At sea 
 
 Cape of Good Hope and Sydney 
 
 Tasmania 
 
 At sea 
 
 Kerguelen and Auckland Isles 
 
 Falkland Isles 
 
 At sea 
 
 Cape Horn 
 
 At sea 
 
 We see here a gradual increase of barometric pressure, 
 from the equator to about 30 south latitude, and from 
 this point at first a gradual diminution so that in 
 
 * 2(0" for 3(0 ^als4tol6 OVforfflO^nj 
 
288 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 about 40 south latitude we find the same pressure as 
 at the equator, and thence a more rapid diminution. 
 The rate of change is illustrated graphically in Fig. 1, 
 which represents the height of the barometer above 
 28J inches for different southern latitudes. In the 
 northern hemisphere there is a similar increase of 
 pressure as we leave the equator, a maximum is there 
 also attained in about latitude 30 ; but from this 
 point towards the poles there is a marked difference in 
 the rate of diminution of pressure in the two hemi 
 spheres. The following table by Schow is sufficient to 
 indicate this : 
 
 North latitude Barometric pressure 
 
 29-853 in. 
 
 10 30-002 
 
 20 30-004 
 
 30 30-069 
 
 40 30-006 
 
 45 30-011 
 
 50 29943 
 
 55 29-960 
 
 60 29-835 
 
 65 29-623 
 
 70 29-722 
 
 75 29-863 
 
 There are minor irregularities in this table, due, 
 doubtless, to local peculiarities, the arrangement of 
 land and water being so much more complicated in the 
 northern than in the southern hemisphere. Neglecting 
 these (as in Fig. 2, which represents for the northern 
 hemisphere the relations corresponding to those exhibited 
 for the southern hemisphere in Fig. 1), we see that 
 there is a much greater resemblance between the rise 
 
LOW BAROMETER OF ANTARCTIC ZONE. 289 
 
 and fall of barometric pressure as we proceed north 
 wards than as we proceed southwards. In fact, the 
 curve is almost exactly symmetrical on either side of 
 30 north latitude to the equator on one side, and to 
 latitude 60 on the other. From 60 the pressure con 
 tinues to diminish for awhile, but appears to attain a 
 minimum in about latitude 73, and thence to increase. 
 In the southern hemisphere, if there is any correspond 
 ing minimum, it must lie in a latitude nearer the south 
 pole than any yet attained. 
 
 The most marked feature in the comparison of the 
 two hemispheres is the difference of pressure over the 
 southern and northern zones, between latitudes 45 and 
 75. This is a peculiarity so remarkable, that for a 
 long time many meteorologists considered that the 
 observations of Captain Eoss were insufficient to warrant 
 our concluding that so important a difference really 
 exists between the two hemispheres. But not only has 
 Captain Maury from a comparison of 7, 000 observations 
 confirmed the results obtained by Ross, but, in meteor 
 ological tables published by the Board of Trade, the 
 same conclusions are drawn from 115,000 observations, 
 taken during a period of no less than 13,000 days. In 
 fact, it is now shown that the difference is yet greater 
 than it had been supposed to be from the observations 
 of Captain Eoss. From a comparison of observations 
 made in the Antarctic Seas with those of Captain Sir 
 Leopold McClintock, it appears that the average dif 
 ference of barometric height in the northern and southern 
 zones, between latitudes 40 and 60, is about one inch. 
 
2QO LIGHT SCIENCE FOR LEISURE HOURS. 
 
 Figs. 1 and 2 exhibit a relation midway between these 
 later results and those tabulated above. 
 
 Assuming an average difference of only three-quarters 
 of an inch in the northern and southern zones, between 
 latitudes 40 and 60, let us consider what is the dif 
 ference of pressure on these two zones of the earth s 
 surface. The area of either zone is 21,974,260*5 square 
 miles, and the pressure on a square mile due to a 
 barometric height of three-quarters of an inch is about 
 670,000 tons, therefore the pressure on the northern 
 zone, between the latitudes named, exceeds the pressure 
 on the southern zone by no less than 14,500,000,000,000 
 tons. Including all latitudes within which there has 
 been ascertained to be a difference of barometric pressure 
 in the two hemispheres, we shall probably be within the 
 mark if we say, that the atmospherical pressure on the 
 northern hemisphere is 20,000,000,000,000 tons greater 
 than the atmospherical pressure on the southern hemi 
 sphere. 
 
 Such a peculiarity as this may almost deserve to be 
 spoken of in the terms applied by Sir J. Herschel to 
 the distribution of land and water upon our earth, it is 
 massive enough to call for mention as an astronomical 
 feature. I propose to examine two theories which have 
 been suggested in explanation of this feature of the 
 earth s envelope. These theories are founded on local 
 peculiarities, and the feature considered appears as a 
 dynamical one that is, as a peculiarity resulting from 
 states of motion in the aerial envelope. I shall endea 
 vour to establish a theory founded on a consideration 
 
LOW BAROMETER OF ANTARCTIC ZONE. 291 
 
 of the earth s mass as a whole, and presenting the 
 atmospheric feature in question as a statical one. 
 
 The first theory I have to notice is one founded on 
 the configuration of land and water upon the northern 
 and southern hemispheres of the earth s globe. In the 
 northern hemisphere, and more especially in that part 
 of the northern hemisphere in which barometrical 
 observations have been most persistently and systemati 
 cally conducted, there is much more land than in the 
 southern hemisphere. Now barometrical observations 
 are referred to the sea-level, and observations made in 
 Europe and America may be considered as referred to 
 the level of the northern parts of the Atlantic Ocean. 
 It is argued that the North Atlantic, compared with 
 southern oceans, is little more than a large lake, having 
 elevated banks east and west. Practically, the air 
 there is a portion of the solid globe, so that the uncon- 
 fined air will rest upon and rise above the former, as if 
 it were solid and a portion of the earth ; so that the 
 altitude of the air over the North Atlantic will be in 
 creased some hundreds of feet, and the barometer at 
 the sea-level will be pressed upon, not only by the free 
 air clear of the earth s banks, but also by the air con 
 fined in the basin, much as if the air were at the bottom 
 of a mine. ! 
 
 Presented in the above form, the theory that the 
 higher northern barometer is due to the contour of the 
 northern hemisphere scarcely deserves serious comment. 
 
 1 From a letter addressed to the editor of the Athenceum by Dr. H. 
 Muirhead. 
 
 u 2 
 
2Q2 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 To speak of the confined air of the North Atlantic 
 Ocean is surely unreasonable. An ocean 2,000 miles 
 across, swept by more frequent storms than are experi 
 enced in any other part of the globe, cannot be very 
 aptly compared to the bottom of a mine. An inelastic 
 fluid flowing steadily over a rugged surface shows no 
 trace, or but the slightest trace, of the nature of that 
 surface, by any variations of its own level. But it is 
 still less conceivable that an elastic fluid should be in 
 fluenced in the manner suggested. In fact, if this 
 happened, we should no longer be enabled to determine 
 the heights of mountains by barometric observations ; 
 for according to the theory the air should extend to a 
 greater height above mountains than above plains ; and 
 as regards comparison between a barometer at the foot 
 of a mountain and one at the summit, we might argue 
 that the barometer in the valley, compared with a 
 barometer at the same level in a plane district, is 
 pressed upon, not only by the air clear of the mountain 
 tops, but also by the air confined within the valley, so 
 that the altitude over the valley is greater by some 
 hundreds of feet than the altitude over a plain at the 
 same level as the valley ; and thus, before we could 
 determine the height of the mountain above the level 
 of the plain, we should have to determine the exact 
 effects due to the confinement of the air in the valley. 
 We know that, on the contrary, the average barometric 
 pressure in the most confined valley does not differ 
 appreciably from the average pressure over the most 
 widely extended plain at the same level. 
 
LOW BAROMETER OF ANTARCTIC ZONE. 293 
 
 We may, however, reasonably inquire whether the 
 presence of continents in the northern hemisphere 
 might not operate in another manner. If we place 
 any mass within a vessel containing fluid, it is clear 
 that we increase the fluid pressure over every point 
 of the vessel s bottom, because this pressure depends 
 wholly on the depth of the bottom below the level of 
 the fluid, and the level rises when any solid substance 
 is placed within the vessel. Now if we suppose a globe 
 covered all over by water to be surrounded by a 
 perfectly uniform atmospheric envelope, the mean 
 pressure of this envelope at the water-level would 
 certainly be increased if continents were supposed to be 
 raised in any manner above the surface of the water ; 
 and if the atmosphere over one half of such a globe 
 were supposed to be prevented in any way from mixing 
 freely with the atmosphere over the other half, then it 
 is clear that the mean pressure at the water-level would 
 be greatest on that half-globe over which the most 
 extensive and highest continents had been raised. On 
 the assumption, then, of some such arrangement over 
 our own earth an arrangement, that is, which should 
 prevent the northern air from mixing with the southern 
 one might see in the northern continents a true 
 
 O 
 
 cause of increased barometric pressure at the sea-level 
 of the northern hemisphere. 
 
 We have, however, not only no evidence that such 
 an arrangement exists, but very strong evidence of an 
 atmospheric circulation which carries air from hemi 
 sphere to hemisphere, and mixes in the most intimate 
 
294 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 manner the whole mass of gases which form the earth s 
 atmospheric envelope. The whole question of the 
 circulation of the air is investigated in Maury s in 
 teresting work on the Physical Geography of the Sea, 
 and he appears to establish in the most convincing 
 manner the interchange of air between the northern 
 and southern hemispheres. 
 
 And even if we could assume that the atmospheric 
 covering of any portion of the earth s surface was in 
 any way prevented from passing freely to other 
 regions, yet the cause assigned would be inadequate 
 to account for the difference of barometric pressure 
 actually existing between the two hemispheres. All 
 the land above the sea-level in the northern hemisphere, 
 if uniformly distributed over the surface of that 
 hemisphere, would be raised to a height of less than 
 200 feet above the present sea-level, and the actual 
 difference of level corresponding to the observed dif 
 ference of barometric pressure is more than four times 
 as great. 
 
 Passing over this theory as neither consistent with 
 the known laws regulating the motions of elastic fluids, 
 nor sufficient even if the consideration of those laws 
 were neglected, we come to the theory suggested by 
 Captain Maury a theory deserving of much more 
 attentive consideration. I shall quote his own words, 
 as the fairest method of presenting his theory; after 
 stating the observed difference of barometric pressure 
 in the two hemispheres, and mentioning the expulsion of 
 air from the northern hemisphere as the cause of this 
 
LOW BAROMETER OF ANTARCTIC ZONE. 295 
 
 difference, lie writes: To explain the great and 
 grand phenomena of nature, by illustrations drawn from 
 the puny contrivances of human device, is often a feeble 
 resort, but nevertheless we may, in order to explain 
 this expulsion of air from the watery south, where all 
 is sea, be pardoned for the homely reference. We all 
 know, that, as the steam or vapour begins to form in 
 the tea-kettle, it expels air thence, and itself occupies 
 the space which the air occupied. If still more heat be 
 applied , as to the boiler of a steam-engine, the air will be 
 entirely expelled, and we have nothing but steam above 
 the water in the boiler. Now at the south over this 
 great waste of circumfluent waters, we do not have as 
 much heat for evaporation as in the boiler or the tea 
 kettle ; but, as far as it goes, it forms vapour, which 
 has proportionately precisely the same tendency that 
 the vapour in the tea-kettle has to drive off the air 
 above, and occupy the space it held. Nor is this all. 
 This austral vapour, rising up, is cooled and condensed. 
 Thus a vast amount of heat is liberated in the upper 
 regions, which goes to heat the air there, expand it, and 
 thus, by altering the level, causes it to flow off. 
 
 The theory thus divides itself into two parts : we 
 have first the expulsive effects due to the vapour 
 raised from southern oceans ; and, secondly, the expan 
 sive effects due to the liberation of heat as the vapour 
 is condensed. Now I would, in the first place, submit 
 that we cannot assign to the second cause the effects 
 here considered. The amount of heat liberated as the 
 vapours rising from southern ocean are condensed is 
 
296 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 undoubtedly great, but it cannot be more than the 
 equivalent of the amount of heat rendered latent as the 
 vapours are formed, and therefore the expansive effects 
 due to the liberation of heat cannot be greater than the 
 contrary effects due to the prior imprisonment of heat. 
 It is quite true, and has been accepted as the undoubted 
 explanation of many climatic effects, that if vapour be 
 raised in one place and condensed over another, then 
 the temperature of the air over the latter place is raised. 
 But when we have to consider a phenomenon extending 
 over a zone twenty or thirty degrees in width, we can 
 not argue in this manner. Nay, it is necessary to the 
 force of Maury s second cause that the condensation of 
 vapour should take place over the very zone in which 
 the vaporisation is proceeding. To assign similar effects 
 to 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 constant 
 evaporation going on in the southern hemisphere, must 
 not be derived from changes of temperature. So far 
 as these are effective at all, they must depend on the 
 excess of evaporation over condensation (since the 
 excess cannot possibly lie the other way), and therefore 
 represent diminution of heat or increase of pressure, the 
 contrary effect to that we have to account for. We 
 have, therefore, only to consider the first cause men 
 tioned by Maury ; that is the expulsive effects due to 
 the formation of aqueous vapour. , 
 
 At first sight, this process of expulsion appears simple 
 
LOW BAROMETER OF ANTARCTIC ZONE. 297 
 
 enough, and seems further to coincide with many well- 
 known phenomena. The theory supposes that over a 
 wide zone of the southern hemisphere aqueous vapour 
 is continually rising; that as it rises it displaces in 
 part the heavier air over these regions; and that 
 equilibrium being thus disturbed, the excess of air flows 
 off continually towards the- equator. Now we know 
 that the prevailing surface-winds over that zone of the 
 southern hemisphere in which the barometer exhibits 
 the peculiarity we are considering, blow from the 
 equator ; that is, they tend to sweep the lower strata of 
 the atmosphere towards the south pole. They therefore 
 tend to increase the quantity of humid air in high 
 southern latitudes. We know also that the prevailing 
 upper currents over the southern zone we are considering 
 blow towards the equator. They tend, therefore, to 
 carry the drier portion of the air towards the equator. 
 All this seems in accordance with Maury s theory, and 
 indeed if the prevailing upper and lower currents flowed 
 in directions contrary to those indicated, the theory 
 would fall at once. 
 
 Again, although we find no evidence in barometric 
 pressure over the south tropical zone of that increase 
 which Maury s theory would lead us to expect (since 
 the surplus air is carried first to this zone), yet it might 
 be argued that the surplus is so distributed as to appear 
 in another way. It is evident that if the atmospheric 
 envelope normally appertaining to the southern hemi 
 sphere were, through the effects of the causes assigned 
 by Maury, increased in extent, this increase might show 
 
298 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 itself, not in an increase of pressure over the south 
 tropical zone that is, not in an increase of height there 
 but in the extension of the surplus atmosphere into 
 the northern hemisphere. This would be shown by the 
 extension of the southern trade-winds to or beyond the 
 equator, so that the (so-called) equatorial zone of calms 
 should lie north of the equator. As this is really the 
 position occupied by the belt of calms, Maury s theory 
 appears to gain additional force by the coincidence. 
 
 Another argument may be drawn from the analogy 
 of the low barometer in moist weather. In fact, it is 
 well known that Deluc explained this phenomenon in a 
 manner precisely accordant with the views expressed 
 by Maury. 
 
 Despite the apparent force of these arguments, and 
 others that might be adduced, it will not be difficult, 
 I think, to show that neither is Maury s theory con 
 sistent with known physical laws, nor (passing over this 
 objection) is the theory sufficient to accouot for the 
 grand phenomenon tinder consideration. 
 
 It is quite true that a volume of aqueous vapour 
 weighs less than an equal volume of air ; it is equally true 
 that a volume of moist air weighs less than an equal 
 volume of dry air at the same tension.- But water, 
 quietly evaporating in the open air, does not displace 
 the air, but penetrates into its interstices, according 
 to the well-established law regulating the mixture 
 of vapours. The aqueous vapour which thus intimately 
 mixes itself with the air produces no effect whatever, 
 either by its weight or by its elasticity, on the move- 
 
LOW BAROMETER OF ANTARCTIC ZONE. 299 
 
 ments of the atmosphere. The experiments of Gray- 
 Lussac, Dalton, and others, have long since proved that 
 the actual effects of the quiet evaporation of water are 
 those here described. It is on this account that Deluc s 
 hypothesis in explanation of the fall of the barometer 
 when the air is moist is now no longer accepted. It 
 has been shown that the observed fall is not due to the 
 moistness of the air, but to increase of temperature. 
 Hot winds bring (in Europe) moist air, and thus moist 
 air and a low barometer are found to be coexistent 
 phenomena. But they are not in the relation of cause 
 and effect. In fact, in New Holland, where hot winds 
 bring dry air, we find the barometer low when the air 
 is dry. 
 
 It follows from what has just been said of the manner 
 in which aqueous vapour associates itself with air, that 
 atmospheric pressure is increased instead of diminished 
 by the process of quiet evaporation, since the weight 
 of the vapour is added to that of the air. Therefore, 
 all things being equal, we should expect to find the 
 barometer higher in the southern or watery hemisphere 
 than in the northern. 
 
 It might seem unnecessary to consider Maury s theory 
 further, but as some doubts may still remain whether 
 some process of the kind conceived by him may not 
 take place, 1 I proceed to consider the efficiency of such 
 
 1 In fact, Sir J. Herschel, in his work on Meteorology, assigns as a 
 cause of the low barometric pressure near the equator, compared with 
 that near the tropics, a process similar to that conceived by Maury, only 
 depending on the excess of heat near the equator. I cannot but agree 
 
300 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 a process to account for the great phenomenon we are 
 dealing with. 
 
 It must be remembered, in the first place, that the 
 theory requires that there should be a greater volume 
 of mixed air and vapour over the southern temperate 
 zone than there is in the corresponding northern zone, 
 otherwise there would not be that continual overflow 
 towards the equator which is required by the theory. 
 So far as it goes, this increment of volume implies an 
 increment of weight. The increase of volume is more 
 than compensated (in theory) by diminution of specific 
 gravity, but it must be held in mind that the increase 
 of volume has to be accounted for by the theory as well 
 as the difference in barometric pressure. 
 
 Again, the theory requires that the upper regions of 
 air should be dry, for it is the upper air that is carried 
 towards the equator ; and if this air were moist, we 
 should no longer have the different proportions of moist 
 and dry air which are required by the theory. We must 
 have an aggregation of moist air in high southern lati 
 tudes, and of dry air towards the equator. 
 
 Again, we must call to mind that one-half of the 
 northern hemisphere is covered by water, and a part of 
 the southern hemisphere is not so covered, so that the 
 effects suggested by Maury are (1) not peculiar to the 
 
 with those metereologists who consider that the notion of any appreci 
 able uplifting of the air by the rising vapour of water is a mistaken one. 
 But whether it be so or not, it is evident that Herschel s view would re 
 quire a regular increase of pressure from the equator to the antarctic 
 pole, and therefore is opposed to Maury s explanation. 
 
LOW BAROMETER OF ANTARCTIC ZONE. 301 
 
 southern hemisphere, nor (2) do they prevail over the 
 whole of that hemisphere. 
 
 Lastly, we must remember that the process conceived 
 by Maury must be wholly or principally a diurnal pro 
 cess, and so can only take place (on an average) over 
 one half of the southern zone at any one time. 
 
 All these considerations tend to diminish very im 
 portantly the efficiency of the cause assigned by Maury, 
 Let us, however, consider what is the maximum value 
 that efficiency could have if all these circumstances were 
 neglected. We shall see that even in this case, which 
 assigns an efficiency at least three or four times as great 
 as would be consistent with actual facts, we shall still 
 find the cause assigned by Maury inadequate to the 
 production of the phenomenon under consideration. 
 
 The greatest weight of aqueous vapour which is ever 
 present in a given volume of air is equivalent to about 
 one-sixtieth part of the weight of the air. Now, if we 
 suppose the barometer at thirty inches, and the whole 
 column of air above the barometer to be impregnated 
 with air in the above-named proportion a view very 
 favourable to the theory, since the cold of the upper 
 regions of air largely diminishes the proportionate 
 weight of aqueous vapour it is clear that one-sixtieth 
 part or half an inch of the barometer s height is due 
 to the presence of aqueous vapour. Now, at mean 
 tensions the specific gravity of aqueous vapour is about 
 three-fifths of the specific gravity of air, so that the 
 proportion of one-sixtieth part of weight corresponds 
 to a proportion of one-thirty-sixth part of volume ; in 
 
302 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 other words, our column of air owes one-thirty-sixth 
 part 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 in such a manner that 
 one complete thirty-sixth part of the volume of the 
 column should pass off then, instead of standing at a 
 height of thirty inches, the barometer would stand at a 
 height of 29 inches, less by only one-third of an inch 
 than the height of 29J inches due to the dry air alone. 
 Now we cannot, in accordance with Maury s theory, 
 legitimately add the five-sixths of an inch of barometric 
 pressure to the height of the barometer under a neigh 
 bouring column. For we have no evidence to show 
 that the air assumed to be expelled from the southern 
 temperate zone is heaped over the southern tropical 
 zone ; on the contrary, we have a barometer in the 
 latter zone not quite so high even as the barometer in 
 the corresponding northern zone. Therefore if air is 
 expelled in the manner supposed by Maury, it must be 
 distributed over a very much greater portion of the 
 globe s surface than it had been expelled from. Hence, 
 returning to our imaginary column of air, but a small 
 fraction of the five-sixths of an inch due to overflow 
 must be added to the barometer under a neighbouring 
 air-column. The latter barometer originally at 29 J 
 may be fairly assumed to rise at most to about 29-J 
 inches. We have, then, a difference of 29f 29^- 
 inches, or two-thirds of an inch ; so that despite all the 
 opposing considerations we have neglected, we still 
 have a difference less by one-third than that for which 
 
LOW BAROMETER OF ANTARCTIC ZONE. 303 
 
 we have to account ; and. indeed, so far as the com 
 parison between the northern and southern temperate 
 zones is concerned (and this is the true question at 
 issue), we are only entitled to consider the third part 
 of an inch lost by overflow, as the true measure of the 
 efficiency of this cause. 
 
 So far as I am aware, the theory I am about to pre 
 sent in explanation of the phenomenon of a low antarctic 
 barometer is original. It is sufficiently simple; 
 perhaps, if we remember how very seldom physical 
 phenomena admit of a simple explanation, one may 
 say that the theory labours under the disadvantage of 
 simplicity. 
 
 It is obvious that the centre of gravity of the solid 
 portion of the earth s globe lies somewhat to the south 
 of the centre of figure. This arrangement has long 
 been accepted as the explanation of two remarkable 
 geographical features the prevalence of water over 
 the southern hemisphere, and the configuration of 
 nearly all the peninsulas over the whole globe. Whether 
 or not those parts within the antarctic regions which 
 have not yet been explored, are occupied by land 
 (chiefly) is a question which has little more bearing on 
 our views respecting this point than has the counter 
 question whether the unexplored north-polar regions 
 are or are not occupied by a north-polar ocean. 1 Sup- 
 
 1 Captain Maury holds the affirmative on both points. I have already 
 had occasion to discuss in these pages his theory of a tidal north-polar 
 ocean, and I think the theory cannot be maintained. But the theory of 
 a polar ocean communicating with the Atlantic and Pacific is a sufficiently 
 
304 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 posing these arrangements to exist, it is evident that 
 they form mere local peculiarities. The general tend 
 ency of water towards the southern hemisphere is very 
 obvious, and, so far as I am aware, no other explanation 
 of the peculiarity has ever been offered than that founded 
 on a slight displacement southwards of the earth s 
 centre of gravity. If, then, C is the centre of the black 
 circle in Fig. 3, representing the solid part of the 
 earth, the centre of gravity of this part lies (in the Fig.) 
 slightly below C between C 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 is clear 
 that there would be a tendency to some such arrange 
 ment as is represented (on a greatly exaggerated scale) 
 in Fig. 3, in which the shaded part represents the sea 
 that is, a shell of water, thicker towards the south, 
 
 probable one. The theory of an antarctic continent is hardly in the 
 same position, since antarctic explorations have given us but faint indi 
 cations, here and there, of the habitudes of the south-polar regions. 
 But I may note, in passing, a very singular argument used by Captain 
 Maury in favour of the existence of such a continent. He states it as 
 a physical law that land is scarcely ever antipodal to land ; therefore, 
 he says, since the north-polar regions are probably occupied by a vast 
 ocean, the south-polar regions are probably occupied by a vast con 
 tinent. He seems to forget that it by no means follows that because 
 land is seldom antipodal to land, water should seldom be antipodal to 
 water. Since the extent of water is nearly three times that of land, it 
 is absolutely necessary that nearly two-thirds of the water should be 
 antipodal to water. The supposed peculiarity that nearly all the land 
 is antipodal to water (one twenty-seventh only being antipodal to land), 
 is in reality no peculiarity at all. It would have been far more singular 
 if any large proportion of the land (which occupies little more than one- 
 fourth of the globe) had been antipodal to land. 
 
LOW BAROMETER OF ANTARCTIC ZONE. 305 
 
 would surround the solid earth. For our present pur 
 poses it is sufficient to consider this supposed arrange 
 
 ment, as minor inequalities of the earth s surface- 
 contour have clearly nothing whatever to do with the 
 phenomenon* we are considering. . 
 
 Let C be the centre of the spheroid which bounds 
 the earth s fluid envelope. Then it is very clear that 
 if this envelope were of the same specific gravity as the 
 solid portion of the earth, the centre of gravity of the 
 entire mass would lie very near C , but slightly soutli 
 of that point, on account of the slight southerly dis- 
 
306 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 placement of the centre of gravity of the solid portion. 
 But when we consider that the specific gravity of the 
 fluid envelope is less than one-fifth of that of the solid 
 globe, it is perfectly clear that the centre of gravity of 
 the entire mass will not be so far south as C . For, of 
 the entire mass, the northern half is the heavier, and 
 therefore 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 than to C . 
 
 Thus, the centre of gravity of the solid globe, and 
 that of the entire mass, solid and fluid, both lie be 
 tween C and C . Now it is evident that the central 
 point, about which the earth s atmospheric envelope 
 tends to form itself as a spherical or spheroidal shell, is 
 the centre of gravity of the entire solid and fluid 
 terrestrial globe that is, is a point north of C . There 
 fore, precisely as the effect of the fluid envelope collect 
 ing itself centrally about a point south of C is to cause 
 the mean depth of water to be greatest in the southern 
 hemisphere, so the fact that the atmospheric envelope 
 collects itself centrally about a point north of C should 
 result in giving a greater mean depth of air (referred 
 to the sea-level) over the northern hemisphere. This 
 arrangement is represented in Fig. 3, in which the un 
 shaded part is supposed to represent the atmosphere. 
 
 I have endeavoured to make the above explanation 
 of my theory in explanation of the low antarctic baro 
 meter as complete and exact as possible ; but there is 
 another way of presenting the theory, which, though 
 less complete, may appear clearer to some minds : 
 
LOW BAROMETER OF ANTARCTIC ZONE, 307 
 
 Variation of mean barometric pressure, as we pro 
 ceed from one place to another, may be due either to a 
 variation of circumstances of heat, moisture, and other 
 like relations, or to difference of level. Maury s ex 
 planation assigns to the low antarctic barometer a cause 
 or causes falling under the former category. My theory 
 amounts to the supposition that the low barometer is 
 due to an absolute difference of level. I say that the 
 sea-level, to which we refer barometric pressure, is not 
 a just level of reference when atmospheric pressure over 
 the whole globe is the subject of inquiry, because the 
 southern seas stand out to a greater distance than the 
 northern seas from the true centre of gravity of the 
 earth s solid and fluid mass. 
 
 Assuming my theory to be correct, we have a means 
 rough, it may be, but not uninstructive of deter 
 mining the displacement of the centre of gravity of the 
 earth s solid mass from the centre of figure. For. accept 
 ing one inch as the difference of barometric height at 
 the two poles, it is easily calculated that this difference 
 amounts to a difference of level of about 850 feet. In other 
 words, the surface of the water at the south pole lies 
 farther than the surface of the water at the north pole 
 from the centre of gravity of the entire fluid and solid 
 globe, by about 850 feet. Hence this centre of gravity 
 must lie about 425 feet north of C (which is the centre 
 of the bounding surface of the water). Now, it is 
 evident that both the centre of gravity of the entire 
 fluid and solid mass, and that of the solid mass, must 
 lie much nearer to C than to C . Hence both these 
 
308 LIGHT SCIENCE FOR LEISURE HOURS. 
 
 centres of gravity lie considerably within 400 feet of C, 
 and C lies considerably within 825 feet of C. Thus 
 the centres of figure and the centres of gravity of the 
 earth s solid mass, and of the entire fluid and solid mass 
 are collected within a space less than one-eighth of 
 mile in length a distance almost evanescent in com 
 parison with the dimensions of the earth s globe, 
 
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INDEX. 
 
 Acton s Modern Cookery 39 
 
 4 ird s Blackstone Economised 39 
 
 Upine Club Map of Switzerland 
 
 Upine Guide (The) oo 
 
 4 moss Jurisprudence ] 
 
 Primer of the Constitution 10 
 
 Andersons Strength of Materials 20 
 
 4 rmstrong s Organic Chemistry 20 
 
 Arnold s (Dr.) Christian Life 29 
 
 Lectures on Modern History 2 
 
 Miscellaneous Works 12 
 
 School Sermons 29 
 
 Sermons 
 
 (T.) Manual of English Literature 
 
 Arnould s Life of Lord Denman 
 
 Atherstone Priory 
 
 Autumn Holidays of a Country Parson ... 
 Ay res Treasury of Bible Knowledge 
 
 Bacons Essays, by Whately 
 
 Life and Letters, by Spedduig 
 
 . Works 
 
 Bain s Mental and Moral Science 
 
 on the Senses and Intellect 
 
 Bakers Two Works on Ceylon 
 
 Ball s Guide to the Central Alps 
 
 Guide to the Western Alps 
 
 Guide to the Eastern Alps 
 
 Beckers Charicles and Gallus. 
 
 Black s Treatise on Brewing 
 
 Blackley s German- English Dictionary 
 
 Elaine s Rural Sports 
 
 Bloxams Metals 
 
 Boultbee on 39 Articles 
 
 Bournes Catechism of the Steam Engine . 
 
 . Handbook of Steam Engine 
 
 . Treatise on the Steam Engine ... 
 
 . -Improvements in the same 
 
 Bawdier s Family Shakspeare............. 
 
 Bramley-Moore s Six Sisters of the Valley . 
 
 Brande s Dictionary of Science, Literature, 
 and Art 
 
 Bray s Manual of Anthropology 
 
 Philosophy of Necessity 
 
 Brinkleys Astronomy ; 
 
 Browne s Exposition of the 39 Articles 
 
 Brunei s Life of Brunei 
 
 Buckle s History of Civilisation 
 
 Posthumous Remains 
 
 Bull s Hints to Mothers 
 
 Maternal Management of Children , 
 
 33 
 38 
 34 
 39 
 IS 
 36 
 20 
 28 
 27 
 27 
 27 
 27 
 35 
 39 
 
 22 
 22 
 
 I 
 
 17 
 2 
 
 Burkes Rise of Great Families . 
 
 Vicissitudes of Families . 
 
 Busk s Folk-lore of Rome 
 
 Valleys of Tirol 
 
 Cabinet Lawyer 
 
 Campbell s Norway 
 
 Gates s Biographical Dictionary 
 
 and Woodward s Encyclopaedia 
 
 Changed Aspects of Unchanged Truths . 
 
 Chesney s Indian Polity 
 
 Modern Military Biography.... 
 
 Waterloo Campaign 
 
 CloiigKs Lives from Plutarch 
 
 Colenso on Moabite Stone &c 
 
 s Pentateuch and Book of Joshua. 
 
 Speaker s Bible Commentary ... 
 
 Collins s Mineralogy of Cornwall 
 
 Perspective 
 
 Commonplace Philosopher in Town and 
 
 Country, by A. K. H. B 
 
 Comte s Positive Polity 
 
 Comyns Elena 
 
 Congreve s Essays 
 
 Politics of Aristotle 
 
 Coningtons Translation of Virgil s 
 Miscellaneous Writings 
 
 Contanseau s Two French Dictionaries ... 
 Conybeare and Howson s Life and Epistles 
 
 of St. Paul 
 
 Cotton s Memoir and Correspondence ...... 
 
 Counsel and Comfort from a City Pulpit.., 
 
 Cox s (G. W.) Aryan Mythology 
 
 . Crusades 
 
 History of Greece 
 
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