4 Mr. Hopkins on the Motion of Glaciers. 



best measure, however, of the degree of its roughness is this 

 — when placed at an inclination of about 20°, a piece of po- 

 lished marble would just rest upon it. 



The slab was so placed that the direction of the grooves co- 

 incided 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 J 50 lbs 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 1°, there 

 was a small but very appreciable motion. 



On the surface of a slab of the same kind of stone smooth but 

 not polished, there was appreciable motion at an angle of forty 

 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 de- 

 viation 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 in- 



