CHAP. VIII.] THE CAUSES OF GLACIAL EPOCHS. 



161 



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) between 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 snovv^ 

 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 sure that 

 the snow (if it is real snow) is much less thick— a mere surface- 

 coating in fact, such as occurs in parts of Russia where the 

 precipitation is less, and the snow accordingly does not exceed 

 two or three feet in thickness. 



We now see the reason why the southern pole of Mars parts 

 with its white covering so much quicker and to so much greater 

 an extent than the northern, 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 the northern hemisphere ; and this greater heat would cause 

 the winds from the equator to be both warmer and more power- 

 ful, and able to produce the same effects on the scanty Martian 

 snows as they produce on our northern plains. The reason why 

 both poles of Mars are almost equally snow-covered in winter is 

 not difficult to understand. Owing to the greater obliquity of 

 the ecliptic, and the much greater length of the year, the polar 

 regions will be subject to winter darkness fully twice as long as 

 with \is, and the fact that one pole is nearer the sun during 

 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 regards their 

 geographical features, and that neither of them is surrounded 



M 



