Mr. J. Croll on the Cause of the Motion of Glaciers. 157 



moving, everybody (whatever might be his theory as to the 

 cause of the motion of glaciers) would at once admit that the 

 middle of the plank (which, of course, was not fixed) would begin 

 slowly to descend the incline in the manner that the ice of a 

 glacier actually does, and that the plank, not being permitted to 

 move at its ends, would become bent or deflected in the middle. 

 Then, if everybody would admit that the plank would be deflected 

 in the middle notwithstanding the friction of the ice on the in- 

 clined plane, and the diminished pressure of the weight of the 

 ice in consequence of its resting on the slope, surely no one could 

 conclude that, were the inclined plane removed and the plank sus- 

 pended in the air by its two extremities, as in Mr. Mathews's 

 experiment, it would not descend in the middle. 



I shall now briefly refer to Mr. Ball's principal objections to 

 Canon M oseley's proof that a glacier cannot shear by its weight 

 alone. One of his chief objections is that Mr. Moseley has as- 

 sumed the ice to be homogeneous in structure, and that pres- 

 sures and tensions acting within it are not modified by the vary- 

 ing constitution of the mass. Although there is, no doubt, some 

 force in this objection (for we have probably good reason to be- 

 lieve that ice will shear, for example, more easily along certain 

 planes than along others), still I can hardly think that Canon 

 Moseley's main conclusion can ever be materially affected by this 

 objection. The main question is this, Can the ice of the glacier 

 shear by its own weight in the way generally supposed ? Now 

 the shearing-force of ice, take it in whatever direction we may, 

 so enormously exceeds that required by Mr. Moseley in order to 

 allow a glacier to descend by its weight only, that it is a matter 

 of indifference whether ice be regarded as homogeneous in struc- 

 ture or not. Mr. Ball objects also to Mr.Moseley's imaginary gla- 

 cier lying on an even slope and in a uniform rectangular channel. 

 Surely Mr. Ball does not suppose that a glacier would descend 

 more easily in an irregular and broken channel having a variable 

 slope and direction than it would do in a straight channel uni- 

 form in width and slope. And if he does not, why advance such 

 an objection ? Canon Moseley assumed, as he had a perfect right 

 to do, that if the glacier could not descend by its weight in his 

 imaginary channel, it could mucii less do so in its actual one. 



That a relative displacement of the particles of the ice is in- 

 volved in the motion of a glacier, is admitted, of course, by Mr. 

 Ball ; but he states that the amount of this displacement is but 

 small, and that it is effected with extreme slowness. This may 

 be the case ; but if the weight of the ice be not able to overcome 

 the mutual cohesion of the particles, then the weight of the ice 

 cannot produce the required displacement, however small it may 

 be. Mr. Ball then objects to Mr. Moseley's method of determine 



