166 ISLAND LIFE 



in summer, when the southern cap is reduced to nearly 

 one-third the size of the northern ; and this fact is held 

 by Mr. Carpenter, as 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 glaciated during periods of high 

 excentricity.^ 



Before, however, we can draw any conclusion from the 

 case of Mars, we must carefully scrutinise the facts, and 

 the conditions they imply. In the first place, there is 

 evidently this radical difference between the state of Mars 

 now and of the Earth during a glacial period — that Mars 

 has no great ice-sheets spreading over its temperate zone, 

 as the Earth undoubtedly had. This we know from the 

 fact of the rapid disappearance of the white patches over 

 a belt three degrees wide in a fortnight (equal to a width 

 of about 100 miles of our measure), and in the northern 

 hemisphere of eight degrees wide (about 280 miles) be- 

 tween May 4th and July 12th. Even with our much 

 more powerful sun, which gives us more than twice as 

 much heat as Mars receives, no such diminution of an ice- 

 sheet, or of glaciers of even moderate thickness, could 

 possibly occur ; but the phenomenon is on the contrary 

 exactly analogous to what actually takes place on the plains 

 of Siberia in summer. These, as I am informed by Mr. 

 Seebohm, are covered with snow during winter and spring 

 to a depth of six or eight feet, which diminishes very little 

 even under the hot suns of May, till warm winds combine 

 with the sun in June, when in about a fortnight the whole 

 of it disappears, 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 powerful than ours, we may be 



1 In an article in Nature of Jan. 1, 1880, the Rev. T. W. Webb states that 

 in 1877 the pole of Mars (? the south pole) was, according to Schiaparelli, 

 entirely free of snow. He remarks also on the regular contour of the sup- 

 posed snows of Mars as offering a great contrast to ours, and also 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 resem- 

 blance between the physical condition of the Earth and Mars, and that 

 any arguments founded on such supposed similarity are therefore untrust- 

 worthy. 



