9 
1879*] Relations to that of the Glacial Epoch. 
instance, the amount carried inward horizontally from the 
outer edge of the cap by conduction must be infinitesimal, 
and indeed can never affect the interior, as the ice moves 
outward more rapidly than the heat can possibly travel 
inward. 
Heat derived from Beneath . — We shall begin with the con- 
sideration of the heat derived by the bottom of the sheet 
from the earth’s crust. The researches of Sir William 
Thomson enable us to determine with a tolerable degree of 
certainty the amount received from this source. He tells us 
that through every square metre of the earth’s surface 
220 metre-tons, or 1,613,700 foot-pounds, of underground 
heat pass upwards annually. Through every square foot, 
therefore, there must come 149,600 foot-pounds. This 
amount is sufficient to melt a layer of ice, already at the 
melting-point, one-fifth of an inch in thickness. But under- 
ground heat would probably be insufficient to melt even so 
small a layer, since a portion of the heat must, doubtless, 
be expended in passing through and maintaining at the 
melting-point a few inches of the ice at the bottom of the 
sheet. 
At first we might be apt to suppose that underground heat 
ought to travel up through the ice in the same way as 
through the strata of the earth below, and to make its 
presence sensibly felt at no great distance from the surface 
of the sheet. This, however, is impossible; for (1) the 
greater part of the heat is spent not in raising the temper- 
ature, but in melting the ice ; and (2) the ice when melted 
immediately runs off, carrying the heat along with it. But it 
will be replied, that, notwithstanding this, if the temperature 
of the ice be much below the freezing-point, the heat con- 
stantly passing into the solid layers at the bottom of the 
sheet, though trifling, ought in course of ages to pass up 
through the ice, affecting its temperature not for a few 
inches, as I have supposed, but for a thickness of a great 
number of feet. Were the ice, like the ground underneath 
on which it rests, to remain immovable, this would no doubt 
be the case ; but the sheet is in a state of constant motion 
outwards from the centre of dispersion, and no sooner is a 
particle of the ice heated than it moves away, and its place 
is supplied by another particle from behind, which in turn 
requires to be heated. Besides, the ice has always a down- 
ward as well as a horizontal motion ; for all the ice found 
at the bottom comes primarily from the top, and that re- 
moved from below is replaced from above. Hence not only 
