102 Mr. H. H. Howorth on 



glacier motion as a motion en masse caused by gravity, due 

 partially to their own weight, partially to the pressure of 

 the higher ice and neve {sqq Stud^, Lehrbuch^ 1844)- 



Forbes summed up his arguments against this theory 

 thus : " If the glacier slides down its bed, why is not its motion 

 continually accelerated, i.e., why does it not result in an 

 avalanche ? and is it not inconceivable that a vast and irregu- 

 lar mass like a glacier, having a mean slope of only 8° and 

 often less than 5°, can slide, according to the common laws 

 of gravity and friction, over a bed of uneven rock, and 

 through a channel so sinuous and irregular that a glacier is 

 often embayed in a valley, whence it can only escape by an 

 aperture of half its actual width? On all mechanical 

 principles, we answer, that is impossible. We may add that 

 many small glaciers are seen to rest upon slopes of from 

 20° to 30° without taking an accelerated motion ; and this 

 is conformable to the known laws of friction. It is known, 

 for instance, to architects that hewn stones, finely dressed 

 with plane surfaces, will not slide over one another until the 

 slope exceeds y^"" {Forbes' Travels, 35). 



He says further "there is no reason to suppose that either 

 Gruner or De Saussure thought it necessary to take into 

 account the varying form of the channel through which the 

 glacier had to pass, and the consequently invincible barrier 

 presented to the passage of a rigid cake of ice through a 

 strait or narrowed aperture where it occurred " {Phil. 

 Trans., 1846, p. 137). 



Hopkins, who resuscitated De Saussure's theory in 

 another form, in replying to Forbes, denied that the bed of 

 a glacier is rough and rugged ; " how," he says, " could the 

 hardest rocks resist for thousands of years the increasing 

 effects of infinitely the most powerful polisher that nature 

 has put in action ; the fact of the existence of roches polies 

 at the end of a glacier and the continuation of glacier 

 valleys proves uncontrovertably that there is some sliding 



