222 DISCUSSIONS IN CLIMATOLOGY. 



greater extent than the air. The probability is that 

 the mean annual temperature of the surface is as low 

 as, if not lower than, that of the air over it. And 

 although the mean temperature of the regions around 

 the South Pole has not been ascertained by direct 

 observation, yet it certainly cannot be much higher 

 than that of those around the North Pole, which we 

 know is but a few degrees above zero F. 



Now, if the mean annual temperature of the air 

 over the Antarctic ice-sheet be not very much above 

 zero F., then that of the surface of the sheet cannot be 

 much higher ; and if this be so, it follows, from what 

 has been already advanced, that the temperature of the 

 great mass of the ice down to near the bottom of the 

 sheet must be considerably below the freezing-point. 



Temperature of the Ice in some Regions determined 

 by Pressure. — In regions such as Switzerland, where 

 the mean temperature is above the freezing-point, the 

 temperature of the ice in the interior of a glacier is not 

 determined by the air above. The tendency of the air 

 in this case is to keep the entire mass of the glacier at 

 the melting-point. But as the temperature of the 

 melting-point depends upon the pressure, the tempera- 

 ture of the glacier at any depth from the surface will 

 depend upon the pressure to which the ice at that 

 depth is subjected. At and near the surface, where 

 the pressure is small, the temperature of the ice will 

 be 32°. At the depth of a quarter of a mile, where the 

 pressure would be equal to about 36 atmospheres, the 

 temperature would be, not 32°, as at the surface, but 

 31°' 5. And if the glacier were half a mile thick the 

 temperature at the bottom would be 31°, and so on in 

 proportion to the thickness of the glacier. This 

 lowering of the melting-point could not, however, go 

 on without limit, for in a country like Switzerland a 



