220 DISCUSSIONS IN CLIMATOLOGY. 



course, in the present case, is equal to half the thick- 

 ness of the sheet, viz., 700 feet. If 1400 feet, as Sir 

 Wyville Thomson supposes, be the thickness of the 

 Antarctic ice-cap, 700 foot-pounds per pound is the 

 utmost quantity of work that gravity can have per- 

 formed on the ice. Supposing the whole of this work 

 had been employed in heating the ice by compression, 

 or by the friction of the particles of the ice on one 

 another, or on the rocky floor of the sheet, the heat 

 generated would not have amounted to one thermal 

 unit per pound of ice. The specific heat of ice being 

 about one-half that of water, the total work of com- 

 pression — assuming that it had all been converted into 

 heat, and the heat equally distributed through the 

 entire mass of the cap — would not have raised the 

 temperature of the ice by 2°. 



The foregoing considerations do not afford a means 

 of determining what the actual temperature of the 

 great mass of the ice below the surface is. They show, 

 however, that whatever that temperature may be, it is 

 not very materially affected either by the heat of 

 compression, or by undergound heat, or by that trans- 

 mitted from the surface either by conduction or by 

 melted ice. 



On what, then, does the temperature of the ice 

 mainly depend ? 



Temperature of the Ice Determined by the Tempera- 

 ture of the Surface. — The temperature of the great 

 mass of the ice is mainly determined by the mean 

 temperature of the upper surface of the sheet. All 

 the ice down to the bottom of the sheet originally 

 came from the surface. It once existed at the surface 

 in the form of a coating of snow, which, becoming 

 consolidated into ice, was afterwards covered over 

 with fresh layers of snow, while these in turn, passing 



