chap, vin.] 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 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 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 us, 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 
