214 DISCUSSIONS IN CLIMATOLOGY. 



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 under- 

 ground 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 temperature, 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 

 constantly 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 downward 

 as well as a horizontal motion ; for all the ice found at 

 the bottom comes primarily from the top, and that 

 removed from below is replaced from above. Hence 

 not only 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 



