12 
Thickness of the Antarctic Ice , and its [January, 
this means carried down below the surface, but not so the 
cold of winter. 
The melting of the ice on the Antarctic continent will 
be greatly retarded, however, by the coldness of the air, 
the temperature of which, even during summer, is consider- 
ably below the freezing-point. A wind, a few degrees below 
the freezing-point, blowing on the icy surface would probably 
re-freeze the ice as rapidly as the sun’s rays could melt it. 
These conditions differ entirely from those that obtain 
in the Ardtic regions. In the latter the air in summer 
is above the freezing-point, and consequently assists the 
sun in melting the ice, whereas in the Antarctic regions 
it is below the freezing-point, and tends to prevent the sun 
from melting the ice. This circumstance explains the fadt, 
which so much surprised Sir James Ross, that no streams 
of water How off the Antarctic ice, similar to those that 
escape from the great ice-fields of Greenland. 
Such water as formed on the surface could not penetrate 
to any considerable depth, for the ice, as we shall presently 
see, being much below the freezing-point, the water would 
be re-frozen before it could descend to any great depth. 
It therefore follows that the great mass of ice, up to 
within a short distance of the surface, can be very little 
affedted by heat transmitted either by conduction, or by 
radiation, or by water from ice melted at the surface. 
Heat derived from Work of Compression and Friction . — I 
shall now consider the third and last source from which the 
ice can obtain heat, viz., Work of Compression and Friction. 
We are fortunately able to come to a pretty definite conclu- 
sion in regard to the total amount of heat derivable from 
this source. The force employed is Gravity, and we can 
thus determine with certainty the greatest amount of 
work which that can possibly perform on the ice. Mere 
pressure, however great, cannot of course generate heat 
unless it perform work, and the heat thus generated is not 
proportionate to the pressure, but to the work performed, 
and the amount of work done by pressure is proportionate 
to the space through which the pressure continues to adt. 
When the pressure is gravity, the work is measured 
by the distance that the body is allowed to descend. A 
pound weight descending i foot performs i 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 esti- 
mating 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 
