NATURE 



{Jan. i, 1 1 



ent jamais la region zdnithale qu'ils n'atteignent meme pas. 

 Cette seule circonstancc distingue profondement les cumu- 

 lus des fracto-cumulus." Truly a profound distinction ! 

 We had supposed that in regard to clouds, as in some 

 other matters, " one man's horizon is another man's 

 zenith." Are cloud-classifiers really driven to such ex- 

 tremities ? What would be thought of the botanist who 

 appended to his description of the UlmaeeaXiiB statement 

 that " the trees belonging to this family are objects seen 

 upon the horizon," and then proceeded to meet the 

 reasonable objection of the surprised reader by the remark 

 that certainly the elm trees around his (the botanist's) resi- 

 dence, were all seen near the horizon ? Yet this is how 

 (p. 24) the author handles his descriptions of cumulus. 

 (The fact of course is that the characteristic form of 

 cumulus is not readily discernible when the cloud is near 

 the zenith.) A somewhat similar remark, made by the 

 author in explanation of the fact that a belt of cirrus, 

 clearly visible towards opposite points of the horizon, is 

 frequently invisible, or nearly so, overhead, a fact of 

 which the optical explanation is obvious, is so strange 

 that we must quote it. " Nous l'attribuons a 1' extreme 

 degre" de froid que nous avons toujours observe' dans la 

 region zenithale, relativement aux autres azimuts. Sous 

 cette basse temperature et cette extreme sdcheresse, la 

 vapeur d'eau se maintient a l'etat e'lastique, et se pre'eipite 

 difficilement sous la forme de filaments extremement 

 dele's. C'est pour cela que les cirrus sont plus rares, 

 moins denses et passagers vers la region zenithale," 

 (p. 69). 



It is with reluctance that we notice statements of this 

 kind in a work the general idea of which we admire, and 

 in the aim of which we cordially sympathise. 



W. Clement Ley 



THE PLANETS OF THE SEASON 

 Mars 



IF the two great leaders of the planetary system have 

 filled us with astonishment at their magnitude and 

 velocity, and with perplexity in the contemplation of 

 arrangements so incomprehensibly unlike our own, they 

 have not exhausted all the resources of the season. There 

 yet remains a much nearer and more intelligible neigh- 

 bour, who possesses a peculiar interest for an opposite 

 reason — his similarity to ourselves. This especial cha- 

 racter of the ruddy planet has long been known to 

 astronomers, and will naturally make him an object of 

 careful study before we leave him too far behind ; and 

 though the opposition of this year does not diminish his 

 distance so much as that of 1S77, yet his almost startling 

 brilliancy has been alone enough to prove it among the 

 favourable ones ; for English astronomers, at least, it is 

 far more propitious than the last, from his greatly- 

 increased elevation. Much had been expected at that 

 last opposition from the broad expansion of his disk, but 

 the indistinctness of detail was a general source of disap- 

 pointment here, though the success of Schiaparelli at 

 Milan and Green at Madeira showed that the fault lay 

 chiefly — perhaps not exclusively — in the English sky. My 

 own impression certainly then was that, besides the want 

 of clear outline inseparable from so low an altitude, there 

 was a deficiency in decidedness of form and strength of 

 tone as compared with previous observations, the cause of 

 which may have lain in the atmosphere of the planet, 

 affected possibly by especial proximity to the sun in an 

 orbit of considerable excentricity. At any rate, we may 

 reasonably hope to find the present season more favour- 

 able for exploration than the last ; for though at nearest 

 approach we have only had 23" of disk instead of 2o/''4 

 in 1877, success depends, with equal instrumental sharp- 

 ness, much more upon altitude and steadiness of air than 

 on increase of visible surface. Schiaparelli was enabled 



to obtain his most valuable results after opposition, when 

 the diameter had decreased to 20" or even 16", and he 

 asserts that he was able to continue his researches with 

 advantage even till it came down to less than 6". 



We have alluded to the special interest of this planet 

 arising from its supposed close correspondence with the 

 earth, and it may not be out of place on this occasion if 

 we bestow a little pains in examining the ground of that 

 supposition. This we may conveniently do by imagining 

 what would be the telescopic aspect of our own globe at 

 a distance not equal to that of Mars, as we should then 

 appear about twice as large, but such as to reduce our 

 apparent diameter to equality with his in a favourable 

 opposition. 



There is every reason to believe that our surface would 

 then appear mapped out by a distinct separation into 

 oceans and continents, the fluid being darker than the 

 solid masses, and preserving their bluish-green tinge but 

 little affected by distance. Except in very shallow parts, 

 their darkness would be uniform from the rapid absorp- 

 tion of incident light, and their contour would be sharply 

 defined. The general hue of the land would be lighter ; 

 and at a distance where its variegated patches of colour 

 would be separately undistinguishable, the result would 

 be a grey resulting from the mixture of many tints, ex- 

 cept where tracts such as the great deserts or prairies 

 might subtend a sufficient angle to preserve their natural 

 hue, or where extensive forests might rival seas in depth 

 of tone. In many places, too, brilliant streaks and 

 patches would show where mountain masses were capped 

 with dense clouds, or surpassed the level of perpetual 

 snow ; but our largest rivers, except possibly at some 

 great embouchure, would be totally imperceptible. 



Such, in its general lineaments, would be the distant 

 aspect of our globe, if the whole lay at once distinctly 

 before the eye. But this would never be the case. The 

 formation and transference of masses of vapour would 

 produce incessant and most uncertain changes. In some 

 regions and at certain times of year there would be un- 

 broken clearness ; in other tracts the outlines and colour- 

 ing of land and sea would be indistinct, or concealed, at 

 times for short, but occasionally for very lengthened, 

 periods. And the interposition would doubtless be 

 always of a white aspect, since such is the character of our 

 clouds wherever they are illuminated by the sun. Towards 

 our polar regions this whiteness would be permanent in 

 the form of great spots, excentric as regards the axis of 

 rotation, increasing through and after the winter, with a 

 corresponding diminution after the summer solstice. 

 There would always be, however, a large unmelted area, 

 even at the warmest period, and its outlines would pro- 

 bably be often irregular and extended from the presence 

 of great masses of frozen clouds. Now, if these would 

 be the probable features of the earth, presented to 

 us at a distance of seventy or eighty millions of miles, 

 in what respects shall we be able to trace the resem- 

 blance on Mars? We are soon brought to the conclusion 

 that, according to the general rule already referred to, 

 there is more analogical than identical correspondence : 

 the inclination of axis, the excentricity of orbit, the 

 duration of day and night, the respective length of the 

 seasons — from the relative similarity but not identity in 

 these particulars, we are prepared to meet with the same 

 kind of proportion throughout. As far as aspect goes, a 

 solid and fluid condition may be thought to divide each 

 superficies ; but if so, the land there is in a much larger 

 ratio to the water; and if the colour of our oceans is 

 repeated on Mars, we have little to correspond with the 

 orange-yellow tinge which, since it leaves unaffected the 

 polar snows, cannot arise from atmospheric absorption. 

 The so-called seas, too, though in some places apparently 

 deep and dark, frequently shoal off and show subaqueous 

 markings in a way that perhaps would be scarcely paral- 

 leled in our own. 



