Mr. Hopkins on the Motion of Glaciers. 5 



clined plane, if the temperature of the slab and of the air were 

 at or below the freezing temperature, so that no disintegration 

 of the ice should take place. This angle appears to be nearly 

 the same in the case of ice, on the grooved slab I made use of, 

 as for that in which polished marble was the sliding body, and 

 is that whose tangent determines the coefficient of friction be- 

 tween the slab in question and solid ice. When the slab was 

 of polished marble this angle was very small. 



2. In the experiments above detailed we have these re- 

 sults: — 



(1.) For all angles less than that just mentioned the motion 

 was 7iot an accelerated motion. This result was verified in 

 every experiment I made. 



(2.) For inclinations not exceeding 9° or 10°, the velocity, 

 ceeteris paribus^ was approximately proportional to the inclina- 

 tion. This, I doubt not, would hold in all cases in which the 

 inclinations should be sufficiently small compared with the 

 angle of accelerated motion. It is manifestly equivalent to the 

 assertion, that the velocity is proportional to the moving 

 force. 



(3.) The velocity of the mass was increased by an increase 

 of weight. 



(4.) The motion was due to the circumstance of the lower 

 surface of the ice being in a state of constant, though slow 

 disintegration. 



The fact that motion takes place down planes of such small 

 inclination compared with that necessary to make the ice slide 

 independently of its disintegration at the lower surface, may 

 simply be stated as due to this circumstance — 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 

 considerable 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 sur- 

 face. 



When the sliding mass is small (as in the experiments above 

 described) the exact uniformity of the motion will be destroyed 

 by local irregularities in diffijrent parts of the inclined plane 

 down which it takes place, or temporary irregularities in the 

 disintegration ; but where the whole inclined surface on which 

 the motion takes place is always the same (as in the case of a 

 glacier), and the mass is sufficiently large, all local or tempo- 

 rary irregularities will, in a great measure, counteract each 

 other, and will therefore not materially disturb the uniformity 

 of the motion, which will be preserved so long as the intensity 

 of the causes of disinteirration remains unaltered. 



