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Show 160 ISLAND I,Ili'E. (PART I. - bserved durinO' tbe In the year 1830 the southern snow was 0 'd. 0 . . f M d' . . h to half its former Iamcter In midsummer o ars, to. ImllliS h h ena on Ma rs b em. ('l' a fortnight (the duratwn of sue .P enom ~: lfth f ~ reckoned in Martian months equivalent to oone:t.we . o a . ) Th J 23rd it was 11 30 m diameter, Martian year . us on une . . . J I 9 h h d d . · · hed to 5o 46' after wh10h It rapidly and on u y t a Imlms ' . . d . I 1837 the same cap was observed near 1ts mcrease agam. n ~o · d' . . . t d was found to be about 3<.> In mmeter. maximum In wm er, an . In the same year the northern snow-cap was observed dunng its summer, all<l was found to vary as follows :- J\Iay 4Lh. Juno 4th. " 17th. July 4th. " 12th. " 20th. Diameter of spot " " " , " " " ,, " " 310 24 28° 0' 22° 5-i:' 18° 2-l-~ ] 5° 20' 18) 0' We thus see that Mars has two permanent snow-caps, of nearly equal size in winter but diminishing very unequally .in summ: r, when the southern cap is reduced to nearly one-th1rd the s1ze of the northern; and this fact is held by Mr. Carpenter, a,s it was by the late Mr. Belt, to be opposed to the view of the hemisphere which has winter in aphelion (as the southern now has both in the Earth and Mars), having been alone glaciat ed during periods of high excentricity.1 Before, however, we can draw any conclusion from the case of Mars, we must carefully scrutinise the facts, anJ the conditions they imply. In the first place, there is evidently this radical differeace between the state of Mars now and of the Earth during a glacial period-tha,t Mars has no great iccsheets spreading over her temperate zone, as the Earth undoubtedly had. This we know from the fact of the 1·apid 1 In an article in Nattl1'e of Jan. 1, 1880, the Rev. T. vV. Webb states th at in 1877 the pole of Mars(? the south pole) was, according to~ Schia parelli, entirely free of snow. He remarks also on the regular contour of the supposed snows of Mars as offering a great contrast to ours, and al so the strongly marked dark border which has often been observed. On the whole Mr. Webb seems to be of opinion that there can be no really close resemblance between the physical condition of the Earth and Mars, and that any arguments founded on such snpposed similarity are therefore untrustworthy· CHAP. VITI.] THE CAUSES OF GLACIAL EPOCHS. 161 di.~<tp~earanco of the white patches over a belt three deo-rees WI 0 m a fortnight (equal to a width of about 100 miles of our measure), and in the northern hemisphere of eight deO'rees wide (about 280 miles) between May 4th and July 12th. Even with our much more powerful sun, which gives us more than twice as much heat a~ Mars receives, no such diminution of an icesheet, or of glaciers of even moderate thickness, could possibly occur; but the phenomenon is on the contrary exactly analoo-ous to what actually takes place on the plains of Siberia in sum~er These, as I am informed by l\Ir. Seebohm, are covered with sno; during winter and spring to a depth of six or eio-ht feet which d~minishes ;ery l~ttle even under the hot suns of 0 May, tiil warm wmds combi~e ~Ith the sun in June, when in about a fortnight the whole of It chsappears, and a little later the whole of Northern Asia is free from its winter covering. As, however, the sun of Mars is so much less powerfu] than oursr we may be sure that the snow (if it is real snow) is much less thick-a mere surfacecoating in fact, such as occurs in parts of Russia where the precipitation is less, and the snow accordingiy does not exceed two or three feet in thickness . . W ~ now ~ee the reason why the so1.dlLern pole of Mars parts with Its white covering so much quicker and to so much greater ~n extent than the northe1·n, for the south pole during summer IS nearest the sun, and, owing to the great excentricity of Mars, would have about one-third more heat than during the summer of th~ northern hemisphere ; and this greater heat would cause the wmds from the equator to be both warmer and more powerful, and able to produce the same effects on the scanty Martian snows as they produce on our northern plains. The reason w by both poles of Mars are almost equally snow-covered in winter is not di~c~lt to understand. Owing to the greater obliquity of the echptw, and the much greater length of the year, the polar regions will be subject to winter darkness fully twice as lonO' as with us, and the fact that one pole is nearer the sun durin()' this period than the other at a corresponding period, will there~ fore make no perceptible difference. It is also probable that the two poles of Mars are approximately alike as rco-ards their geographical features, and that neither of them is ;urrounded M |