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, 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 {57} in the height of summer, although the sun is then actually nearer the earth than it is during our northern summer!