The Theory of Glacier motion. 1 03 



in glaciers," and he urges that the surfaces of rocks 

 forming the beds of glaciers must necessarily be free from 

 asperities. 



He further showed, experimentally, that, while it is per- 

 fectly true that a smooth hard body will not descend down 

 an equally smooth hard plane at angles considerably greater 

 ■even than that at which some glaciers are formed, ice 

 nevertheless does so ; and he argues that whereas the 

 particles of ice in contact with the plane are capable, so long 

 as they remain a part of the solid mass, of exerting a consi- 

 derable force to prevent sliding, they are incapable of 

 exerting any sensible force when they become detached 

 from the mass by the liquefaction or disintegration of its 

 lower surface, and he contended that the essential condition 

 of glacier motion is that its lower surface is kept continually 

 at or near zero by the conduction of heat from the earth's 

 crust, which is proved by the flow of water from underneath 

 all points of glaciers {Phil Mag. XXVI. i — 16). 



Hopkins also proved experimentally that the argument 

 of Charpentier and Forbes as to the motion of a glacier, 

 if it moved en masse^ being an accelerated motion, is only 

 true of angles greater than that whose tangent determines 

 the co-efficient of friction between the glacier and its bed ; 

 and he further shewed that, for inclinations not exceeding 

 9° or 10°, the motion of a moving mass of ice is approxi- 

 mately proportional to the inclination of the slope on which 

 it rests, and that such velocity is increased by increase of 

 weight, and he succeeded in establishing that sliding due to 

 gravitation is a real and very important element in glacier 

 motion. It seems quite clear that the scratching of the 

 bed and sides of the valley by stones enclosed like planes 

 in the mass of the ice, and the polishing of large surfaces 

 and the rounding of inequalities, shows that in regard to a 

 portion, at all events, of its work, a glacier acts as a rigid 

 body propelled by gravity. 



