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



not been so fortunate in his attempt to establish a theory of his 

 own. And I cannot help thinking that the influence which his 

 remarkable communication to the Royal Society, on the impossi- 

 bility of the descent of glaciers by their weight alone, would have 

 had on the minds of physicists, has been much impaired by the 

 prominence which he has since been giving to a theory which few, 

 I fear, will ever be able to accept. Whatever may be the fate which 

 awaits the generally accepted theory of the cause of glacier- 

 motion, his own theory seems to be beset by difficulties of a phy- 

 sical nature which will require to be removed before he can expect 

 that it will be received by physicists in general. 



Most of these difficulties have already been noticed and dis- 

 cussed by Professor Forbes, Mr. Mathews, Mr. Ball, and others. 

 I shall therefore only briefly allude to a few of those that more 

 particularly bear on some points which have not already been 

 sufficiently discussed. 



Canon Moseley has shown that the mere weight of the ice is 

 wholly insufficient to overcome the cohesion of the crystalline 

 particles, so as to break their connexion and cause them to be 

 displaced one over the other. This point I regard as fully esta- 

 blished. It is implied in the generally received theory, that, in 

 the descent of a glacier, owing to differential motion the cohe- 

 sion of the particles of the ice is broken, and that these solid par- 

 ticles are forced over one another and alongside one another. Mr. 

 Moseley then concludes that it follows, as a necessary consequence, 

 that there must be some other force, in addition to the weight of 

 the ice, pushing the glacier forward. Here lies the fundamental 

 error. He has not proved that in the descent of the glacier the 

 connexion of the solid particles of the ice has to be broken. 

 True, the ice moves with a differential motion, and, as a necessary 

 consequence, the particles are displaced over each other. Two 

 particles separate, and the one moves past the other ; but the 

 point to be determined is this : — were the two particles at the 

 moment when separation took place both in the hard crystalline 

 and solid state ? Canon Moseley does not prove this ; he merely 

 assumes it to be the case ; but it must be proved to be the case, 

 not assumed to be so, before he can conclude that it necessarily 

 follows that in the descent of the glacier some force in addition 

 to the weight of the ice is required to push the glacier forward. 

 Certainly he is warranted in concluding that it necessarily 

 follows that the generally received theory is incorrect, because 

 in this theory it is assumed that the particles shear in the solid 

 state. He would be warranted in saying to those who believe in 

 the generally received theory, "You assume with me that in the 

 descent of a glacier the cohesion of the solid particles of the ice has 

 to be overcome and the one particle forced past the other. Then 



