Review of the Controversy Regarding the Motion of Glaciers. 1 35 



perature of the interior of a glacier made the acceptance of the sug- 

 gestion of DeCharpentier and of Canon Moseley alike impossible. 



The important fact established by the investigations of the last 

 named writer — the high shearing force of ice — raised a new and for- 

 midable difficulty in the way of theorists upon this subject, which it 

 was necessary in some way to evade or to overcome. Accordingly, the 

 most recent suggestion is founded upon a completely different prin- 

 ciple. It is equally novel and ingenious, and comes from the pen of 

 Mr. Jas. Croll, now, and for some years past, engaged on the Geo- 

 logical Survey of Great Britain. Instead of seeking a force sufficient 

 to overcome the high resistance of ice to shearing, he believes that he 

 has found in heat a means of undermining that resistance, and thus, of 

 enabling gravitation, unaided, to produce the movement. 



The papers embodying these views appeared in the London, Edin- 

 burgh and Dublin Philosophical Magazine, for March, 1869, and Sep- 

 tember, 1870. In the earlier number, Mr. Croll says: 



" Although it is demonstrated that glaciers can not descend by 

 means of their weight alone, in the manner generally supposed, still 

 I venture to think, that notwithstanding the demonstration, gravita- 

 tion, after all, may be the only force moving the ice. 



"The correctness of the above conclusion — that the weight of the 

 ice is not a sufficient cause — depends upon the truth of a certain ele- 

 ment taken for granted in the reasoning, that the shearing force of 

 the molecules of icp remains constant. If this force remains constant, 

 then Canon Moseley's conclusion is undoubtedly correct, but not other- 

 wise ; for if a molecule should lose its shearing force, though it were 

 but for a moment, if no obstacle stood in front of it, it would descend 

 in virtue of its weight. 



■ " The fact, that the shearing force of a mass of ice is found to be 

 constant, does not prove that the same is the case in regard to the 

 individual molecule. If we take a mass of molecules in the aggregate, 

 the shearing force of the mass taken thus collectively may remain 

 absolutely constant, while at the same time each individual molecule 

 may be suffering repeated momentary losses of shearing force. This 

 is so obvious as to require no further elucidation. The whole matter 

 therefore resolves itself into this one question, whether or not the 

 shearing force of a crystalline molecule of ice remains constant. In 

 the case of ordinary solid bodies we have no reason to conclude that 

 the shearing force of the molecules ever disappear, but in regard to 

 ice it is very different. 



" If we analyze the process by which heat is conducted through a 

 mass of ice, we shall find that we have every reason to believe that 



