328 Mr. Hopkins ofi the Mechanism of Glacial Motion 



ing a neutral acetate of lime at a temperature approaching 

 that of the boiling-point by chalk alone, but that they require 

 to add milk of lime in order to overcome the acid reaction. 

 Blond lot has deduced the inference from his experiment, that 

 the stomach owes its acid reaction to the presence of an acid 

 phosphate of lime ; but as the experiments now detailed do 

 not coincide with those of the French physiologist, it is suffi- 

 ciently obvious that they do not support him in his conclu- 

 sions. 



[To be continued.] 



XLIX. On the Mechanism of Glacial Motion, Fourth Letter. 

 By W. Hopkins, Esq., M.A., F.R.S., 4-c. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 ^INCE 1 addressed my last letter to you on the motion of 

 ^^ glaciers, I have devised some experiments, the results of 

 which I wish to communicate, as corroborative of the con- 

 clusions at which I have arrived by mathematical investiga- 

 tion. 



The weight of a solid mass sufficiently small to experiment 

 upon conveniently, is not sufficient to communicate a motion 

 to it which, as in the case of a glacier, shall superinduce any 

 considerable change of form and consequent fracture, the con- 

 ditions under which the mass is placed being analogous to 

 those of a glacier. When the width of a mass under such 

 conditions is increased, the tendency of the forces arising from 

 the weight of the mass to fracture it is also increased ; and 

 lateral obstacles, which would entirely arrest the motion of a 

 mass of limited width, might produce scarcely a sensible effect 

 on the central motion of a mass of large dimensions. When 

 our object, however, is to determine the effects of an assigned 

 motion and consequent change of form, as in the case before 

 us, it is immaterial whether the motion be produced by the 

 action of gravity, or any other cause. In my experiments the 

 mass was about three feet in length and two in breadth, and 

 two inches in depth. It consisted of fine mortar, which was al- 

 lowed to assume different degrees of solidity, according to the 

 particular object of each experiment. 



The experiments were conducted as follows: — A trough of 

 the length and width above-mentioned and five or six inches 

 in depth, was prepared, open at both ends. Along the bot- 

 tom of it was placed longitudinally a layer of straight rods 

 parallel to each other, on which the mortar was poured, its 

 consistency being about the same as that of mortar used in 



