(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 {56} 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 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 hemisphere; 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. 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 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 surrounded by very high land on which ice may
accumulate. With us at the present time, on the other hand, geographical
conditions completely mask and even reverse the influence of excentricity,
and that of winter in _perihelion_ in the northern, and summer in
_perihelion_ in the southern, h