i879-1 
Relations to that of the Glacial Epoch. 
*5 
Now if the mean annual temperature of the air over the 
Antarctic ice-sheet be about 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 temperature 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 0 . 
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 0 , as at the surface, but 3i°.5. And if the 
glacier were half a mile thick the temperature at the bottom 
would be 31 0 , and so on in proportion to the thickness of the 
glacier. This lowering of the melting-point could net, how- 
ever, go on without limit, for in a country like Switzerland 
a point would soon be reached where the ice could no 
longer retain the solid form. If, for example, owing to the 
heat of the climate, we could not have ice at a lower 
temperature than say 30°, then a glacier over 1 mile in 
thickness would be an impossibility, for the bottom of a 
glacier of greater thickness would not remain solid at that 
temperature. 
Having considered the various circumstances affeefting 
the temperature of the Antarctic ice, and the sources from 
which it derives its heat, we have found that the temper- 
ature of the ice must be considerably under the freezing- 
point. We are now prepared to examine the reasons which 
have been adduced for concluding that about 1400 feet is 
the probable limit to the thickness of the Antarctic ice- 
sheet. 
