﻿82 Mr. J. Ball on the Cause of the Motion of Glaciers. 



facts of glacier-motion. The point for discussion is whether the 

 ordinary theory, which affirms that a glacier descends by its weight 

 through the processes of fracture and regelation, has been over- 

 thrown by the arguments and observations opposed to it by Mr. 

 Moseley. Mr. Croll admits the force of some of the objections 

 urged by me to the views of Canon Moseley, but nevertheless 

 retains the opinion already expressed by him in March 1869, 

 that " Canon Moseley has successfully shown the insufficiency of 

 the generally received theory of the descent of glaciers/ 5 



It is known to all who have followed the controversy, that the 

 argument against the received theory depends upon observations 

 made by Mr. Moseley on the shearing-force of ice, determined 

 by the weight or pressure requisite to shear a square inch of 

 that substance. In my last paper I sought indirectly to show 

 that these observations were inapplicable, because the physical 

 fact which Mr. Moseley had in view is not the same, but totally 

 different from that which obtains in a moving glacier. It will 

 perhaps be simpler to say that I demur altogether to the use of 

 the term shearing -force until its meaning is accurately defined, 

 and is shown to be appropriate to the argument to which it is ap- 

 plied. In some substances the amount of resistance opposed to 

 the separation of adjoining particles is nearly independent of 

 temperature, and of the time during which the pressure is applied. 

 In other bodies which oppose a very considerable resistance to 

 fracture, the particles gradually change their relative positions 

 under the prolonged action of even slight pressure ; and in some 

 of them the amount of change depends very much on their tem- 

 perature while exposed to pressure. If we are to use the same 

 term for every process by which the particles of solid bodies 

 change their relative positions, whether by actual fracture or by 

 more or less slow rearrangement, we shall merely introduce con- 

 fusion into our physical conceptions. What, to take a familiar 

 instance, is the unit of shear of ordinary sealing-wax ? Measured 

 by the method employed by Mr. Moseley, it would be repre- 

 sented by a considerable weight, which would not vary very 

 much within the ordinary range of temperature in our rooms. 

 His argument would apply at least as well to show that a glacier 

 of sealing-wax at, say, 20° below its point of fusion could not 

 descend from the mountains into the valleys, though it most 

 certainly would do so far more rapidly than our ice-glaciers do. 



I must be allowed to remind Mr. Croll that the utmost 

 amount of relative displacement of the particles of ice in a 

 moving glacier amounts to a difference of the sixteenth part of a 

 line in twenty-four hours between the motion of two points 

 one inch apart. If any one will make careful observations to 

 ascertain the pressure necessary to produce that amount of dis- 



