on the Motion of Glaciers. 411 



and convex at the termination, presenting at one view lines of 

 veined structure in tiie direction of those existing on the gla- 

 ciers, and in multitude and delicacy comparable to the slaty 

 cleavage of the ice, with crevasses exactly at right angles to 

 them, the radiating form of the crevasses at the termination, 

 — in short all the leading facts which had been confidently pre- 

 dicted from general mechanical considerations before a single 

 model had been made* ; to explain this in a more precise and 

 mathematical manner than had been attempted, would at least 

 have been a problem suited to Mr. Hopkins's talents for such 

 investigations, though it could add little force to the parallel 

 already established between them and glaciers; but instead 

 of this Mr. Hopkins never alludes to these models, except to 

 find faultf with one deduction, in a note to the final chapter 

 of my Travels. I willingly admit that the strict letter of the 

 inference was incorrect, that " the direction of maximum dis- 

 tension of the particles must be, not parallel to the length of 

 the glacier, but in the direction of the branches of the elongated 

 loopsX" I admit that, though approximating, there is not a 

 necessary coincidence in these directions ; but the substantive 

 fact, that the line of greatest force is not parallel to the axis 

 of the glacier, but inclines towards its centre, remains esta- 

 blished. It would plainly have been better if Mr. Hopkins, 

 instead of dwelling on an oversight, common, as he states §, to 

 himself and me, had attempted to explain something on which 

 these models give indisputable evidence. 



5. But as Mr. Hopkins does sometimes appeal to experi- 

 ment, and has even quoted an experiment of Mr. Hodgkinson's 

 in a note II, it were to be wished that he had distinguished 

 more clearly cases in which an appeal to experiment is the otih/ 

 test of theory, such as those in which the undefined constitu- 

 tion of the mass is one of the most important elements of the 

 problem. If a superficial foot of a body possessing a certain 

 cohesion be wrenched by distorting the angles, I defy any 

 analytical legerdemain to declare whether it will be torn in 

 one line of fracture or a thousand. We know bj/ experience 

 that it depends upon the degree of cohesion and upon the ve- 

 locity with which the distortion is effected, neither of which 

 quantities enter into any of Mr. Hopkins's investigations. 

 To take a familiar instance, a piece of sealing-wax in a warm 

 room may be broken over sharply at one fracture, but if the 



* See my Third Letter on Glaciers, written in 1842, from the remote 

 village of Zermatt, where my mechanical theory is stated as explicitly as I 

 could do it now. — Ed. Phil. Journal, October 1842. 



t Phil. Mag., March 1845, p. 237. 



X Travels in the Alps, p. 378, note. 



§ Pliil. Mag., March, p. 238, note. \\ Ibid, p. 245. 



