of the Descent of Glaciers by their Weight only. 119 



difference • and precisely in this difference lies the impossibility 

 of the glacier descending by its weight alone. The block of ice 

 descended bodily ; its parts did not move over one another or 

 alongside one another, but with a common motion of descent; 

 whereas not less than thirteen fourteenths of the motion of the 

 Glacier of the Aar is, according to Professor Forbes, not a common 

 motion, but that of its particles one beside another and one over 

 another, which is called a differential motion, and to which is 

 opposed the resistance to shearing, which is at the rate of not less 

 than 75 lbs. per square inch. 



Mr. Hopkins's experiment leaves therefore more than thir- 

 teen fourteenths of the power necessary to cause a glacier to de- 

 scend unaccounted for. 



In conclusion I will venture to quote a passage from Mr. 

 Croll's paper, although it expresses an opinion favourable to 

 my present argument, because it states in terms more clear 

 and explicit than I can do the little influence that alleged 

 errors in the data on which my conclusions are founded would 

 have, even if their existence were admitted, on the result at 

 which I have arrived*: — "The impression left on my mind 

 after reading Canon Moseley's memoir in the Proceedings of the 

 Royal Society for Jan. 1869 was that, unless some very serious 

 error could be pointed out in the mathematical part of his in~ 

 vestigation, it would be hopeless to attempt to overturn his general 

 conclusion as regards the received theory of the cause of the de- 

 scent of glaciers, by searching for errors in the experimental data 

 on which the conclusion rests. Had the result been that the 

 actual shearing-force of ice is by twice, thrice, four times, or even 

 five times too great to allow of a glacier shearing by its own weight, 

 one might then hope that by some more accurate method of de- 

 termining the unit of shear than that adopted by Canon 

 Moseley, his objections to the received theory of glacier-motion 

 might be met ; but when the unit of shear is found to be not 

 simply by three times, four times, or even five times, but actu- 

 ally by thirty, forty, or fifty times too great, all our hopes of 

 overturning his conclusion by searching for errors in this di- 

 rection vanish, even although there aje some points connected 

 with his unit of shear that are not satisfactory." 



* Phil. Mag. S. 4. vol. xl. p. 154, 



