﻿of the Motion of Glaciers. 205 



of " crevasses " in glaciers. Take, for example, the transverse 

 crevasses formed at the point where an increase in the inclina- 

 tion of the glacier takes place. Suppose a change of inclination 

 from, say, 4° to 8° in the bed of the glacier. The molecules on 

 the slope of 8° will descend more rapidly than those above on 

 the slope of 4°. A state of tension will therefore be induced at 

 the point where the change of inclination occurs. The ice on 

 the slope of 8° will tend to pull after it the mass of the glacier 

 moving more slowly on the slope above. The pull being con- 

 tinued, the glacier will snap asunder the moment that the cohe- 

 sion of the ice is overcome. The greater the change of inclina- 

 tion is, the more readily will the rupture of the ice take place. 

 Every species of crevasse can be explained upon the same 

 principle. 



This theory explains also why a glacier moves at a greater rate 

 during summer than during winter; for as the supply of heat to 

 the glacier is greater during the former season than during the 

 latter,, the molecules will pass oftener into the liquid state. 



As regards the denuding power of glaciers, I may observe 

 that, though a glacier descends molecule by molecule, it will 

 grind the rocky bed over which it moves as effectually as it 

 would do did it slide down in a rigid mass in the way generally 

 supposed; for the grinding- effect is produced not by the ice of 

 the glacier, but by the stones, sand, and other materials forced 

 along under it. But if all the resistances opposing the descent 

 of a glacier, internal and external, are overcome by the mere 

 weight of the ice alone, it can be proved that in the case of one 

 descending with a given velocity the amount of work performed 

 in forcing the grinding materials lying under the ice forward 

 must be as great, supposing the motion of the ice to be molecular, 

 in the way I have explained, as it would be supposing the ice de- 

 scended in the manner generally supposed. 



Of course, a glacier could not descend by means of its weight 

 as rapidly in the latter case as in the former; for in fact, as 

 Canon Moseley has shown, it would not in the latter case de- 

 scend at all ; but assuming for the sake of argument the rate of 

 descent in both cases to be the same, the conclusion I have 

 stated would follow. Consequently whatever denuding- effects 

 may have been attributed to the glacier, according to the ordi- 

 nary theory, must be equally attributable to it according to the 

 present theory. 



This theory, however, explains, what has always hitherto ex- 

 cited astonishment, viz. why a glacier can descend a slope almost 

 horizontal, or why the ice can move off the face of a continent 

 perfectly level. 



Canon Moseley suggests that heat passing into the ice might 



