INFLUENCE OF SNOW-MASSES ON CLIMATE. 63 



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 ques- 

 tion are at least 20 away from all other land, and can be influ- 

 enced only by the seas north of them, while it has been observed 

 that the temperature of these seas, down to 68 of south latitude re- 

 mains below the freezing-point all through the summer. The Ant- 

 arctic lands, therefore, do not receive from any quarter winds which 

 can cause a thawing of the snow ; and as this remains during the sum- 

 mer, 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 Bar- 

 naul, 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 Febru- 

 ary 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 maxi- 

 ma of temperature are higher ; and the difference goes on augment- 

 ing toward the south. It is especially considerable between Mitau, 

 near the Baltic, and Nukurs, on the lower Amou (Oxus). In Febru- 

 ary 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 De- 



