Mr. Hopkins on the Mof.io?i of Glaciers. 3 



tion is independent of both these circumstances. This diffi- 

 cuhy has been hitherto regarded, and with reason, as a most 

 serious if not an insuperable one to tiie sliding theory. An- 

 other has also been frequently urged, for which, however, there 

 is no real foundation. It has been contended that if a glacier 

 moved by sliding over its bed from the mere action of gravity, 

 it ought to move with an accelerated motion, whereas the mo- 

 tion is observed to be unaccelerated* . If the force retarding 

 the motion were solely that of ordinary _//7C//o« of the surface 

 over which it moves, the objection would be valid, because the 

 retarding force of friction is independent of the velocity acquired^ 

 but in the case of a glacier moving down an irregular valley 

 and over an irregular surface, all the retarding forces do not 

 act on the mass in the same manner as friction in the ordinary 

 cases of sliding bodies. Besides the friction, there may be 

 retarding forces acting along the sides or bottom of the glacial 

 valley, and depending on the velocity of the glacier, in which 

 case the whole accelerating force on the mass will be some 

 function of the velocity, and the motion will not necessarily be 

 an accelerated motion f. The difficulty now spoken of, there- 

 fore, seems to have arisen from an imperfect conception of the 

 problem ; but the one first mentioned requires to be removed, 

 if possible, by direct experiment. This I have attempted to 

 ilo in the manner which 1 shall now describe. 



1. Experiments. — A slab of sandstone was so arranged that 

 the inclination of its surface to the horizon could be slowly and 

 continuously varied by the elevation of one edge. The sur- 

 face was in the state in which it had been sent from the quarry, 

 and in which such stones are sometimes laid down as paving 

 stones, retaining the marks of the pick with which the quarry- 

 man has shaped them, without any subsequent process for ren- 

 dering the surface smooth. The slab thus presented a grooved 

 surface (the grooves running in very nearly parallel directions), 

 having some resemblance to those over which existing glaciers 

 move, but having little of the smoothness of roches polies. The 



* " Of tins I was persuaded, that if the motion should appear to be con- 

 tinuous and nearly uniform, it could not be due to the mere sliding of the 

 glacier on its bed, as De Saussure supposed j for, admitting the possibility 

 of gravity to overcome such intense friction as the bed of a glacier presents, 

 it seemed to me quite inconsistent with all mechanical experience that such 

 a motion, unless so rapid as to be an accelerated one, and that the glacier 

 should slide before our eyes out of its hollow bed (which would be an ava- 

 lanche), could take place except discontinuously, and by fits and starts." — 

 Travels through the Alps of Savoy, p. 132. 



t The descent of water along a river-course, or of ice floating down its 

 current, is not necessarily with an accelerated motion, and for a reason ex- 

 actly similar to that assigned in the text. 



B2 



