CHAP. VIII THE CAUSES OF QLACIAL EPOCHS 167 



sure that the snow (if it is real snow) is much less thick 

 — a mere surface-coating in fact, such as occurs in parts 

 cf 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 more quickly 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 hemi- 

 sphere ; and this greater heat would cause the winds 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 snow-plains. The reason 

 why both poles of Mars are almost equally snow-covered in 

 winter is not difficult to understand. Owingf to the sreater 

 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 us, and the fact that 

 . one pole is nearer the sun during this period than the 

 other at a corresponding period, will therefore 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 sur- 

 rounded by very high land on which ice may accumulate. 

 With us at the present time, on the other hand, geograph- 

 ical conditions completely mask and even reverse the 

 influence of excentricity, and that of winter in perihelion 

 in the northern, and summer in 2^(^'i^ihelion in the southern, 

 hemisphere. In the north we have a preponderance of 

 sea within the Arctic circle, and of lowlands in the temperate 

 zone. In the south exactly opposite conditions prevail, 

 for there we have a preponderance of land (and much of it 

 high land) within the Antarctic circle, and of sea in the 

 temperate zone. Ice, therefore, accumulates in the south, 

 while a thin coating of snow, easily melted in summer, is 

 the prevalent feature in the north ; and these contrasts 

 react upon climate to such an extent, that in the southern 

 ocean, islands in the latitude of Ireland have glaciers 

 descending to the level of the sea, and constant snowstorms 



