158 Mr. J. Croll on the Cause of the Motion of Glaciers. 



ing the unit of shear on this ground : — The shearing of the 

 ice in a glacier is effected with extreme slowness ; but the shear- 

 ing in Canon Moseley's experiment was effected with rapidity; 

 and although it required 75 lbs. to shear one square inch of sur- 

 face in his experiment, it does not follow that 75 lbs. would be 

 required to shear the ice if done in the slow manner in which it 

 is effected in the glacier. " In short," says Mr. Ball, " to ascer- 

 tain the resistance opposed to very slow changes in the relative 

 positions 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." 

 There is force in this objection ; and here we arrive at a really 

 weak point in Canon Moseley's reasoning. His experiments 

 show that if we want to shear ice quickly a weight of nearly 

 120 lbs. is required ; but if the thing is to be done more 

 slowly, 75 lbs. will suffice*. In short, the number of pounds 

 required to shear the ice depends to a large extent on the length 

 of time that the weight is allowed to act ; the longer it is allowed 

 to act, the less will be the weight required to perform the work. 

 " I am curious to know," says Mr. Mathews when referring to 

 this point, " what weight would have sheared the ice if a day 

 had been allowed for its operation." I do not know what would 

 have been the weight required to shear the ice in Mr. Moseley's 

 experiments had a day been allowed; but I feel pretty confi- 

 dent that, should the ice remain unmelted, and sufficient time 

 be allowed, shearing would be produced without the application of 

 any weight whatever. There are no weights placed upon a glacier 

 to make it move, and yet the ice of the glacier shears. If the 

 shearing is effected by weight, the only weight applied is the 

 weight of the ice; and if the weight of the ice makes the ice 

 shear in the glacier, why may it not do the same thing in the 

 experiment? Whatever may be the cause which displaces the 

 particles of the ice in a glacier, they, as a matter of fact, are dis- 

 placed without any weight being applied beyond that of the ice 

 itself ; and if so, why may not the particles of the ice in the ex- 

 periment be also displaced without the application of weights ? 

 Allow the ice of the glacier to take its own time and its own way, 

 aud the particles will move over each other without the aid of 

 external weights, whatever may be the cause of this ; well, then, 

 allow the ice in the experiment to take its own time and its own 

 way, and it will probably do the same thing. There is something 

 here unsatisfactory. If, by the unit of shear, be meant the 

 pressure in pounds that must be applied to the ice to break the 

 connexion of one square inch of two surfaces frozen together and 



* Philosophical Magazine for January 1870, p. 8; Proceedings of the 

 Royal Society for January 1869. 



