The Theory of Glacier motion. 141 



The position in which the problem now stood was 

 remarkable. It was admitted by everybody that the general 

 phenomena of the motion of a glacier exactly reproduces 

 that of a viscous body moving through a channel under the 

 influence of its own weight, and that Forbes's view seemed 

 incontrovertible, and every attempt to explain the motion 

 of glaciers by other processes had failed. But for the 

 experiments of Moseley, which went to shew that the shear- 

 ing resistance of ice is not sufficiently low to allow us to 

 treat a glacier as a viscous mass, and that consequently 

 gravity is not strong enough to do the work demanded 

 from it, there would have been no hesitation among scientific 

 men in accepting the plastic theory as alone valid. 



It had not occurred to enquirers to call in question 

 Moseley's experiments. This was now to be done, with the 

 result of completely vindicating Forbes's view. 



Mr. Ball objected, on theoretical grounds, to experiments 

 like Mr. Moseley's upon artificially formed ice being applied 

 to ice of another kind altogether, namely, glacier-ice, a con- 

 clusion for which, as we shall see, he had full warrant. He 

 also urged that while the shearing in a glacier, whatever its 

 amount, takes place very slowly, in Mr. Moseley's experiment 

 it was very fast Moseley's own experiments show, that if we 

 want to shear ice quickly, a weight of 120 lbs. is required, 

 while, if the thing is done more slowly, 75 lbs. will suffice, 

 and this gives point to Mr. Ball's criticism, that to ascertain 

 the resistance opposed to very slow changes in the relative 

 position of the particles, so slight as to be insensible at short 

 distances, Mr. Moseley measures the resistance opposed to 

 rapid disruption between contiguous portions of the same 

 substance {PhiL Mag., XL. 158). 



Mr. Matthews and Mr. Reilly, by careful experiments, 

 •emphasized this argument of Mr. Ball, and showed how 

 important an element time is in the shearing of ice ; the 

 actual differential motion of a glacier ranging for molecules 



