54 



Mb. HOPKINS, ON THE MOTION OF GLACIERS. 



quarry-man has shaped them, without any subsequent process for rendering 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 poHes. The best measure, however, of the degree of its roughness is this 



when placed at an inclination of about 20", a piece of polished marble would just rest upon it. 



The slab was so placed that: the direction of the grooves coincided with that of greatest 

 inclination. A frame of about 9 inches square and 6 inches in depth, without top or bottom, 

 was then placed on the slab and filled with lumps of ice from a neighbouring ice-house, in 

 such a manner that the ice, and not the frame (which merely served to keep the ice together 

 as one mass) was in contact with the slab. In the experiments in which the following results 

 were obtained, weights were placed on the ice such that the pressure on the slab was at the 

 rate of about 150lbs on the square foot. 



When the inclination was 9° about two-thirds of the weight was removed ; the velocity was 

 diminished by nearly one half. 



When the inclination of the slab did not exceed one degree, there was a small but very 

 appreciable motion. 



On the surface a slab of the same kind of stone smooth but not polished, there was appreciable 

 motion at an angle of 40 minutes. Nor am I prepared to say that either in this, or the preceding 

 case, the angle was the least at which sensible motion would take place. 



When the surface used was that of polished marble, there was sensible motion with the 

 smallest possible inclination. The motion, in fact, afforded almost as sensitive a test of deviation 

 from horizontality as the spirit level itself. 



In all these experiments the ice melted continually but very slowly at its lower surface in 

 immediate contact with the slab. During the night the temperature descended below that of 

 freezing, and the motion entirely ceased. 



The angle at which the accelerated motion just begins to take place is that at which the ice 

 would just rest upon the inclined 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 between the slab in question and solid ice. When the slab was of polished marble 

 this angle was very small. 



