1379-3 
13 
Relations to that of the Glacial Epoch . 
descends ; but in the case of the Antarctic ice-cap, the slope 
of the ground does not enter as an element into our calcula- 
tions, 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 adt 
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 "deter- 
mined 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 therefore 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 course, in the present 
case, is equal to half the thickness 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 
performed on the ice. Supposing the whole of this work 
had been employed in heating the ice by compression, or 
by the fridfion 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 compression — assuming that it 
had all been converted into heat, and the heat equallv dis- 
tributed through the entire mass of the cap — would not 
have raised the temperature of the ice by 2 0 . 
The foregoing considerations do not afford a means of 
determining what the adfual 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 
affedted either by the heat of compression, or by underground 
heat, or by that transmitted 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 Temperature of the 
Surface . — The temperature of the great mass of the ice is 
