1846.] EFFECT OF WATER IN THE SUBSTANCE OF THE GLACIER. 165 



the most solid and ponderous bodies, breaking up their parts, 

 and giving to the motion of the whole a more or less river-like 

 character, is seen in the frequent case of land-slips, as for in- 

 stance that of Goldau. And scarcely less instructive are the 

 numerous examples, cited in the first section of this paper, of 

 huge masses of almost cold and brittle lavas being pressed on 

 with a uniform and graduated motion, by the almost unimpeded 

 hydrostatical communication of pressure from the yet active 

 fluid which circulates unseen in their pores. With this analogy 

 before me, I replied in 1844 in the following terms to the 

 question, " How far a glacier is to be regarded as a plastic 

 mass ?" " Were a glacier composed of a solid crystalline cake 

 of ice, fitted or moulded to the mountain bed which it occupies 

 like a lake tranquilly frozen, it would seem impossible to admit 

 such a flexibility or yielding of parts as should permit any 

 comparison to a fluid or semi-fluid body transmitting pressure 

 horizontally, and whose parts might change their mutual posi- 

 tion so that one part should be pushed out whilst another 

 remained behind. But we know in point of fact, that a glacier 

 is a body very differently constituted. It is clearly proved by 

 the experiments of Agassiz and others, that the glacier is not a 

 mass of ice, but of ice and water ; the latter percolating freely 

 through the crevices of the former to all depths of the glacier ; 

 and as it is matter of ocular demonstration, that these crevices, 

 though very minute, communicate freely with one another to 

 great distances, the water with which they are filled communi- 

 cates force also to great distances, and exercises a tremendous 

 hydrostatic pressure to move onwards in the direction in which 

 gravity urges it, the vast porous crackling mass of seemingly 

 rigid ice in which it is, as it were, bound up.* 



Now the water in the crevices does not constitue the glacier, 

 but only the principal vehicle of the force which acts on it, and 

 the slow irresistible energy with which the icy mass moves 

 onwards from hour to hour with a continuous march, bespeaks 

 of itself the presence of a fluid pressure. But if the ice were 



* Sixth Letter, [page 51 above.] 



