CONDITIONS OF CONTINENTAL ICE. 219 



the work performed, and the amount of work done by 

 pressure is proportionate to the space through which 

 the pressure continues to act. When the pressure is 

 gravity, the work is measured by the distance that 

 the body is allowed to descend. A pound weight 

 descending 1 foot performs 1 foot-pound of work ; 

 descending 2 feet it performs 2 foot-pounds ; and so 

 on in proportion to the number of feet of descent. In 

 estimating the total amount of work which gravity 

 can perform in the descent of a glacier down the side 

 of a mountain, we measure the work by the vertical 

 distance the glacier descends ; but in the case of the 

 Antarctic ice -cap, the slope of the ground does not 

 enter as an element into our calculations, for the 

 ground is assumed to have no slope, the continent 

 being regarded as flat. The surface no doubt may 

 have great irregularities, such as hills and mountain- 

 ridges ; those irregularities, however, do not assist 

 gravity, but rather act as obstructions to the general 

 flow of the ice. 



Nevertheless, just as in the case of a glacier, the 

 amount of work that gravity can perform is determined 

 by the distance the ice can descend ; and this distance 

 is determined not by the slope of the ground, but by 

 the thickness of the sheet. If the Antarctic ice-sheet 

 be 1400 feet in thickness, the greatest distance to 

 which a pound of ice can descend is of course 1400 

 feet. Gravity acting on this pound of ice can, there- 

 fore, perform only 1400 foot-pounds of work. But, 

 in order that gravity may do so, the pound must 

 descend the whole distance from the surface to the 

 bottom of the sheet. In estimating the total amount 

 of heat which could possibly have been conferred on 

 the ice by gravity, we must find the mean vertical 

 distance to which the ice has descended. This, of 



