Mr. J. Croll on the Cause of the Motion of Glaciers. 161 



the influence of heat ; but according to the molecular theory of 

 glacier-motion which I have adopted, heat will aid the displace- 

 ment of the particles whether the temperature be kept constant or 

 not. In short, it is absolutely impossible in our experiments to 

 be certain that heat is not in some way or other concerned in the 

 displacement of the particles of the ice. But we can shorten the 

 time, and thus make sure that the amount of heat entering the 

 ice during the experiments is too small to affect materially the 

 result. We cannot in this case say that all the displacement has 

 been effected by the weight applied to the ice, but we can say 

 that so little has been done by heat that, practically, we may 

 regard it as all done by the weight. 



This consideration, I trust, shows that the unit of shear 

 adopted by Canon Moseley in his calculations is not too large. 

 For if in half an hour, after all the work that may have been 

 done by heat, a pressure of 75 lbs. is still required to displace 

 the particles of one square inch, it is perfectly evident that if no 

 work had been done by heat during that time, the force required 

 to produce the displacement could not have been less than 75 lbs. 

 It might have been more than that; but it could not have been less. 

 Be this, however, as it may, in determining the unit of shear we 

 cannot be permitted to prolong the experiment for any consider- 

 able length of time, because the weight under which the ice 

 might then shear could not be taken as the measure of the force 

 which is required to shear ice. By prolonging the experiment 

 we might possibly get a unit smaller than that required by 

 Canon Moseley for a glacier to descend by its own weight. But 

 it would be just as much begging the whole question at issue, to 

 assume that, because the ice sheared under such a weight, a glacier 

 might descend by its weight alone, as it would be to assume that, 

 because a glacier shears without a weight being placed upon it, 

 the glacier descends by its weight alone. 



But why not determine the unit of shear of ice in the same 

 way as we would the unit of shear of any other solid substance, 

 such as iron, stone, or wood ? If the shearing-force of ice be 

 determined in this manner, it will be found to be by far too great 

 to allow of the ice shearing by its weight alone. We shall be 

 obliged to admit either that the ice of the glacier does not shear 

 (in the ordinary sense of the term), or if it does shear, that 

 there must, as Canon Moseley concludes, be some other force in 

 addition to the weight of the ice urging the glacier forward. 



Physical objections to the Eev. Canon Moseley's own theory. 



Although Canon Moseley has thus so ably and so successfully 

 shown the insufficiency of the generally received theory of the 

 cause of the descent of glaciers, he has,however,T venture to think. 



