J ixn. i, 1880] 



NATURE 



213 



In atmospheric conditions, indeed, we find great ap- 

 proach to identity ; yet even here there are discrepancies ; 

 the polar snows of the earth would probably not be dis- 

 tinguishable from the upper surfaces of terrestrial clouds 

 floating in any latitude, while on Mars such peculiar 

 whiteness, though sometimes vividly brought out in 

 certain localities, is by no means universally concurrent 

 with the local indistinctness and confusion that so often 

 puzzle the areographer. The action of solar heat on the 

 polar deposits seems identical, and yet it may be a ques- 

 tion whether our Arctic snows are marked out by as 

 regular a contour as those of Mars, and still less would 

 they show what has often been observed there — a strongly- 

 marked border of darkness. And however striking and 

 suggestive may be the fact that in either globe the 

 thermal axis is not that of rotation, we have the dis- 

 crepancy that on Mars the glaciation is reduced in a 

 much greater ratio, so that the pole, according to Schia- 

 parelli, was, in 1877, entirely free. This observer, who is 

 fully impressed with the terrestrial theory, admits that the 

 vertical sunlight, instead of producing clouds, as on the 

 earth, appears to clear the sky of Mars, and thinks the 

 atmospheric changes there of a more simple nature. 

 That the southern hemisphere would be subject to greater 

 extremes of temperature than the opposite, as shown by 

 the variation in size of the white caps, might have been 

 expected as a direct consequence of the elliptical form of 

 its orbit greatly surpassing our own. 



A passing reference will be sufficient to the brighter 

 zone, which, according to some observers, distinguishes 

 the edge of the disk, but which others, including myself, 

 have never detected ; or to the bluish or greenish 

 patches sometimes noticed on the limb. Such appear- 

 ances maj- be mere results of contrast ; at any rate they 

 may be left on one side as not directly affecting our 

 present comparison. But there is one consideration 

 which cannot be thus disposed of, and which, obvious as 

 it is, seems to have been taken little into account — the 

 very different amount of solar radiation on the two 

 planets. The heat derived from the sun on Mars is only 

 from § to \ of that received by ourselves. And thus we 

 seem reduced to the alternative of either abandoning to a 

 considerable extent the supposed closeness of resemblance 

 in material and constitution, or of maintaining it by the 

 hypothesis of a supply of heat on Mars derived in 

 some other way. No ice such as ours would be so 

 reduced by the unaided action of that distant sun — 

 no terrestrial continents could remain so long unclothed 

 with snow. The dilemma is a curious one. It may 

 not be incapable of explanation, but it certainly requires 

 more special and careful consideration than it has yet 

 received. 



We have been looking at the subject much as though 

 a supposed view of the earth at a suitable distance might 

 be fairly paralleled with a corresponding representation 

 of Mars as drawn by the best observers. But it must be 

 added, with much regret, that such is not yet the case. 

 As to certain main features of that planet, there is indeed 

 a very satisfactory agreement ; but with regard to others, 

 and as to details in general, we feel, as a first impression, 

 some extent of disappointment. It may be fairly admitted 

 that the disk is after all not large, and its markings often 

 feeble ; and there is great diversity in instruments, and 

 eyes, and hands, and aptitude for the work. Vet still an 

 exhaustive survey, of which we cannot even indicate the 

 materials in this place, but which we trust will be carried 

 on, as it has been most ably commenced, by Dr. Terby of 

 Louvain, would show much unexplained, and some things 

 unsatisfactory. Mailer laid the foundation of definite areo- 

 graphy ; but his successors, while enlarging, have not always 

 confirmed his results, and, to say nothing of others who 

 havebestowed much pains upon the subject with more or less 

 mutual agreement, our own keen-eyed and accomplished 

 Dawes — atleastasrepresentedby Proctor — isfoundto differ 



in some parts materially from Lockyer, Kaiser, and Secchi. 

 At the last opposition in 1877, the subject was taken in 

 hand with especial zeal and perseverance by Schiaparelli 

 at Milan with an exquisitely sharp Merz object-glass of 

 7"IS inches aperture and 10 feet 8 inches focus, and by 

 Green, who went out purposely to Madeira with a 13-inch 

 mirror by With, the perfect polish and critical definition 

 of which are sufficiently guaranteed by the maker's name. 

 Each did his best ; each was far in advance of the other 

 observers of the season ; and yet at first sight there is 

 more apparent difference in their results than might have 

 been expected. It is not surprising that in the case of 

 minute details each should have caught something pecu- 

 liarly his own ; but there is a general want of resemblance 

 not easily explained, till, on careful comparison, we find 

 that much may be due to the different mode of viewing 

 the same objects, to the different training of the observers, 

 and to the different principles on which the delineation 

 was undertaken. Green, an accomplished master of form 

 and colour, has given a portraiture, the resemblance of 

 which as a whole, commends itself to every eye familiar 

 with the original. The Italian professor, on the other 

 hand, inconvenienced by colour-blindness, but of micro- 

 metric vision, commenced by actual measurement of 

 sixty-two fundamental points, and carrying on his work 

 with most commendable pertinacity, has plotted a sharply- 

 outlined chart, which, whatever may be its fidelity, no one 

 would at first imagine to be intended as a representation 

 of Mars. His style is as unpleasantly conventional as 

 that of Green indicates the pencil of an artist ; the one 

 has produced a picture, the other a plan. The discord- 

 ance arising from such opposite modes of treatment 

 would naturally be less real than apparent ; still, a good 

 deal remains that it is not easy to harmonise. Let us 

 hope that during the present favourable opportunity, much 

 may be effected towards clearing up the obscurities that 

 still rest upon the study of Mars. Every contribution 

 may prove of use, provided it is the result of that con- 

 scientious spirit that will show only what it sees, and take 

 care to show it well. 



A suggestion may be permitted that observations in 

 the twilight might obviate the unpleasant glare arising 

 from the vivid light of the disk, or that a screen-glass 

 might be advantageously employed for the same purpose 

 at a later hour. 



Meanwhile the nomenclature of the spots — a point of 

 increasing importance for identification — is in a state of 

 pitiable confusion. This ought to be remedied at once ; 

 and its revision could be more suitably intrusted to no 

 one than to Dr. Terby, who so thoroughly knows its 

 difficulties, and is so competent to decide upon some 

 system that may be adopted with the general concurrence 

 of observers. 



With regard to the satellites, we have entered into so 

 much detatl about the primary, that little space remains 

 for them. Yet we must express our hope that, once 

 discovered, they may be more easily caught in our larger 

 instruments, and that the magnificent reflector of Mr. 

 Common may, as is very possible, increase their recog- 

 nised number. Those already discovered are certainly 

 among the most wonderful objects in the whole solar 

 system. So disproportionally minute, according to our 

 limited ideas of proportion ; so speedy in their revolution 

 that the innermost rises in the west and sets in the east, 

 and compasses the whole heavens more than three times 

 in a Martial day ; so close that the same attendant ranges 

 at less than 4,000 miles from the surface of his primary ; 

 so much of their time invisible in total eclipse ; so power- 

 less to influence any fluid mass beneath them ; one might 

 call them exceptions, while yet they are among the 

 strongest illustrations of the great principle of identity of 

 character combined with the extremest variety in detail, 

 in the inscrutable work of the Creator. 



T. W. Webb 



