Mr. J. Croll on the Cause of the Motion of Glaciers. 159 



cause the one to slip over the other, then the amount of pressure 

 required to do this will depend upon the time you allow for the 

 thing being done. If the thing is to be done rapidly, as in some 

 of Mr. Moseley's experiments, it will take, as he has shown, a 

 pressure of about 120 lbs. ; but if the thing has to be done more 

 slowly, as in some other of his experiments, 75 lbs. will suffice. 

 And if sufficient time be allowed, as in the case of glaciers, the 

 thing may be done without any weight whatever being applied to 

 the ice, and, of course, Mr. Moseley's argument, that a glacier 

 cannot descend by its weight alone, falls to the ground. But if, 

 by the unit of shear, be meant not the weight or pressure necessary 

 to shear the ice, but the amount oiwork required to shear a square 

 inch of surface in a given time or at a given rate, then he might be 

 able to show that in the case of a glacier (say the Mer de Glace) 

 the work of all the resistances which are opposed to its descent at 

 the rate at which it is descending is greater than the work of its 

 weight, and that consequently there must be some cause, in ad- 

 dition to the weight, urging the glacier forward. But then he 

 would have no right to affirm that the glacier would not de- 

 scend by its weight only; all that he could affirm would simply 

 be that it could not descend by its weight alone at the rate at 

 which it is descending. 



Mr. Moseley's unit of shear, however, is not the amount of 

 work performed in shearing a square inch of ice in a given time, 

 but the amount of weight or pressure requiring to be applied 

 to the ice to shear a square inch. But this amount of pres- 

 sure depends upon the length of time that the pressure is applied. 

 Here lies the difficulty in determining what amount of pressure 

 is to be taken as the real unit. And here also lies the radical 

 defect in Canon Moseley's result. Time as well as pressure 

 enters as an element into the process. The key to the explana- 

 tion of this curious circumstance will, I think, be found in the 

 fact to which reference has already been made, viz. that the rate 

 at which a glacier descends depends in some way or other upon 

 the amount of heat that the ice is receiving. This fact shows 

 that heat has something to do in the shearing of the ice of 

 the glacier. But in the communication of heat to the ice time 

 necessarily enters as an element. There are two different ways 

 in which heat may be conceived to aid in shearing the ice : (1) we 

 may conceive that heat acts as a force along with gravitation in 

 producing displacement of the particles of the ice ; or (2) we may 

 conceive that heat does not act as a force in pushing the particles 

 over each other, but that it assists the shearing processes by di- 

 minishing the cohesion of the particles of the ice, and thus allow- 

 ing gravitation to produce displacement. The former is the 

 function attributed to heat in Canon Moseley's theory of glacier- 



