﻿Mr. J. Ball on the Cause of the Motion of Glaciers. 83 



placement in glacier-ice, not in compact ice from our ponds, he 

 will have ascertained a fact that is at least prima facie applicable 

 to the consideration of the cause of glacier-motion ; but as in 

 the present state of our knowledge the observations of Mr. 

 Moseley do not contribute in the slightest degree to determine 

 the amount of that pressure, I must continue to deny the cogency 

 of the argument derived from them. I shall presently endea- 

 vour to show that the determination of the unknown force in 

 question would by no means conclude the controversy, even 

 though the resistance were much greater than the force of gra- 

 vity acting on a given section of the glacier-mass ; but I wish 

 to make a further preliminary remark on this portion of the 

 subject. 



Mr. Croll explicitly admits that Mr. Moseley's argument is 

 defective, inasmuch as he takes no account of the time during 

 which a given pressure must be applied in order to shear ad- 

 joining surfaces of ice ; and, in accordance with the views which 

 he holds as to the molecular action of heat upon ice, he assumes 

 that the reason why time enters as a condition into the pheno- 

 menon is that it is necessary for the communication of heat, 

 and enters as a condition merely so far as it determines the 

 quantity of heat received. Reasoning on this basis, Mr. Croll 

 argues that, inasmuch as in Canon Moseley' s experiments the re- 

 sults obtained were due to pressure plus a certain small amount 

 of heat, if they had been obtained with less pressure acting for 

 a longer time, they would have been the result of a greater 

 amount of heat combined with the pressure. The unit of shear 

 determined by Canon Moseley is certainly not too large. Having 

 concluded from these premises that the existing theory has been 

 overthrown by Canon Moseley's objections, Mr. Croll introduces 

 his own very ingenious views as to the molecular effects of the 

 propagation of heat through ice as affording the only possible 

 explanation of the phenomena of glacier-motion. 



With the most sincere respect for Mr. Croll' s acuteness and 

 the usual accuracy of his reasoning, I think that on this occa- 

 sion there is an obvious begging of the question, in first assu- 

 ming that his molecular hypothesis is alone capable of account- 

 ing for the fact that the effect of pressure upon some bodies 

 largely depends upon the time during which it acts, and then 

 arguing from this fact to show that it is necessary to admit the 

 truth of the hypothesis. There is a large class of bodies which 

 are capable of changing their molecular condition and becom- 

 ing fluid by the application of heat, but which are also capable 

 of modifying their form, or undergoing changes in the relative 

 positions of their particles at temperatures very much below 

 their melting-point. Is Mr. Croll prepared to maintain that in 



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