10 
Thickness of the Antarctic Ice, and its rjanuaty, 
is internal heat from below carried away by the horizontal 
flow of the ice, but the upward motion of the heat is 
checked by a downward flow of the ice from above ; and the 
ice is, in all probability, moving downwards more rapidly 
than the heat is travelling upwards. We must therefore 
conclude that underground heat is confined to a very thin 
layer of the cap at the bottom, and that its effects, either in 
melting the ice or in raising its temperature, are so trifling 
that they may be practically disregarded in our present 
inquiry. 
It must further be observed that when it is stated that 
underground heat will maintain at the melting-point the ice 
in contact with the ground, it is not meant that it will main- 
tain it at the temperature of 32 0 , for, as Prof. James 
Thomson discovered, the temperature at which the ice melts 
is lowered by pressure at the rate of about 0*0137° F* f° r 
every atmosphere of pressure. In the present case the 
pressure depends upon the thickness of the ice : so that, if 
the sheet be 1400 feet deep, the melting-point will be 31*5°; 
if half a mile deep, it will be 31 0 ; if 1 mile deep, 30° ; and 
so on.* 
Heat derived through the Upper Surface . — It follows, from 
what has already been shown, that the greater part, if not 
nearly all, the heat possessed by the ice must have been 
received through the upper, and not the under, surface 
of the sheet. But what we are at present concerned 
with is not so much the amount of heat received by the ice 
as the temperature at which the heat can maintain the ice. 
* The melting-point does not, however, vary uniformly with the pressure ; 
for Mousson (Ann Chim. et Phys., 3rd series, lvi., p. 257, 1859) found that it 
required a piessure of 13,000 atmospheres to lower the melting-point to zero, F., 
whereas if the melting-point had decreased in proportion to the increase of 
pressure, a pressure of 2337 atmospheres would have been sufficient. Bous- 
singault succeeded in lowering the melting-point n° below zero, F., but the 
amount of pressure employed was not determined (Ann. Chim. et Phys., xxvi., 
p. 544, 1872). 
'1 he fadt that the melting-point of ice would be lowered by pressure, or 
rather that pressure would prevent freezing, was suggested nearly a century 
ago by Dr. Charles Hutton, Professor of Mathematics in the Military Academy 
of Woolwich. From certain experiments on the expansive force of ice, made 
in Canada by Major W'lliams, in the year 1784-85, Dr. Hutton makes the fol- 
lowing remarks : 
“From these ingenious experiments we may draw several conclusions: — 
First. We hence observe the amazing force of the expansion of the ice, or the 
water in the adt of freezing ; which is sufficient to overcome perhaps any 
resistance whatever; and the consequence seems to be, either that the water 
will freeze, and, by expanding, burst the containing body, be it ever so thick 
and strong ; or else, if the resistance of the body exceeds the expansive force 
of the ice, or of water in the adt of freezing, then, by preventing the expan- 
sion, it will prevent the freezing, and the water will remain fluid, whatever the 
degree of cold may be." (Trans. Roy. Soc. Edin., vol. ii., p. 27.) 
