Solar Energy in Relation to Ice. 147 



tion were the principal cause of the movement, and if 

 we take the very lowest gradient down which ice has been 

 observed to flow, these would require that the Scandinavian 

 ice should have been some miles in thickness. A similar 

 difficulty meets us in attempting to explain the cause of 

 movement of the ice in North America, which, on the 

 supposition that this movement is due to gravitation, would 

 require that its centres of dispersal should have been ten 

 or twelve miles higher than the outer extremities of the 

 mass. It is extravagant suppositions like these that have 

 led many students of glacial geology to believe that these 

 problems cannot be solved, and that have led not a few 

 others to join the ranks of the diluvialists. 



Is it not possible to find a simpler explanation of these 

 facts regarding the uphill and extensive movements of land- 

 ice ? I venture to think that it is ; and, indeed, it is one of 

 the chief objects of this communication to put that explana- 

 tion forward. In approaching that explanation, we must 

 again digress a little, in order to consider some additional 

 facts connected with the relation of Solar Energy to Ice. 



First of these is the fact that ice expands with a rise of 

 temperature, and contracts with a fall, more than any solid 

 known. The change of dimensions referred to is more than 

 three times that of iron, and ten times that of Carrara 

 marble. It may be stated to be about half an inch per 

 thousand feet for each degree of temperature.^ 



Next is the fact that ice is one of the most plastic of sub- 

 stances under pressure, and one of the most brittle under 

 strain, so that while it can mould itself to all the ins and 

 outs of the surface when pressure is applied, it yet gives way, 

 snaps, and forms crevasses, directly any tension is put upon 

 it. The plastic condition referred to probably arises from 

 the fact that ice, like iron, wax, pitch, and some few other 

 substances, does not pass abruptly from the solid to the fluid 

 condition, but gradually softens when it is at temperatures 

 very near the melting point. This latter state may be 

 reached by an increase of pressure ; for both Lord Kelvin 



1 Geol. Mag., dec. iii., vol. viii. (1891), pp. 19-22. 



