Page:Popular Science Monthly Volume 31.djvu/73

Rh advances more and more northward, till all the low lands of our hemisphere are (so far as we know) freed from their covering.

This advance is not continuous, but proceeds, as we might say, by leaps and bounds. Warm winds coming from the south, or from the sea, further it, but cold winds arrest it, and sometimes reduce the temperature, where the thaw has already begun, to below the freezing-point. The disappearance of the snow in all the plains of the northern hemisphere is due to the geographical conditions of the hemisphere, or because all its known parts are reached by warm, melting winds. Moreover, in some countries in high latitudes but little snow falls, so that there is not much to melt. This, however, is not always the case, for snow may be seen to cover the ground, and the temperature of the freezing-point to prevail even during the summer. The fact is not only possible, but is a reality in very high northern latitudes. We learn from observations by Sir James Ross, that on the shores of the Antarctic lands the mean temperature, even in the height of summer, is considerably below the freezing-point, and never rises above it. This is explained by geographical conditions. The shores in question are at least 20° away from all other land, and can be influenced only by the seas north of them, while it has been observed that the temperature of these seas, down to 68° of south latitude remains below the freezing-point all through the summer. The Antarctic lands, therefore, do not receive from any quarter winds which can cause a thawing of the snow; and as this remains during the summer, the rays of the sun, notwithstanding its greater nearness to the earth at that time, can not raise the temperature above this point.

The existence of a very extensive bed of snow produces another important effect which has not received the attention it deserves: it keeps the temperature at but a little distance from the freezing-point, and below it. The mean temperature in February is the same at Bogoslowsk, at the eastern base of the Ural Mountains, and at Barnaul, on the upper Obi, at the foot of the Altai Mountains. But toward the southwest, not far from the Altais, in the Kirghiz steppes, there is usually but little snow. The mean and maximum of February are therefore higher. The same difference is observed between Ustsinolsk, in the government of Vologda, and Irgirs, in the Kirghiz steppes; and analogous differences may be remarked in other places. Wherever the snow-bed is less regular, the mean and extreme maxima of temperature are higher; and the difference goes on augmenting toward the south. It is especially considerable between Mitau, near the Baltic, and Nukurs, on the lower Amou (Oxus). In February the mean temperatures of these places differ by only 0·7° C, but the mean maximum is 10·2° and the absolute maximum ll·7°. The last result is all the more striking, for it is deduced from only six years of observations, while the observations at Mitau include a space of more than forty years. An analogous result is presented in