Review of Hie Controversy Begarding the Motion of the Glacier. 77 



so that when turned out it had become a mass of perfectly clear ice — 

 an effect, as he urges, due to its having regealed when the molecules 

 were brought very closely together. In the case of a snowball made 

 by the hands of a boy, this explanation is no doubt true, for the 

 pressure is small ; but when the force of the hydraulic press is brought 

 into use, can Prof. Tyndall assert that the snow did not wholly, or m 

 great part, melt in consequence of the lowering of its congelation point, 

 and freeze again on the removal of the pressure ? If so, the regela- 

 tion to which he appeals is quite a different process from that of 

 Faraday— occuring below, instead of at the freezing point; under 

 great, instead of little external pressure ; and throughout the mass, in- 

 stead of only at the surface. Logically, therefore, it seems as if Prof. 

 Tyndall, in making the suggestion, with a view, apparently, to sup- 

 port the Viscous Theory, has cut away from under the theory the 

 very ground upon which it has previously maintained its foothold. 

 It is, manifestly, impossible to hold at the same time both Forbes' 

 theory and Tyndall's explanation of it. 



It should be borne in mind, that all writers who seek to explain the 

 motion of glacier ice, by a resort to forces sufficient to overcome 

 this length unit of shear, are bound to include in their explanation a 

 solution of the difficulty, that the swiftest movement is found in the 

 middle and at the surface. In seeking such a force in the weight of 

 the ice itself, or in that component of the weight, which acts horizon- 

 tally, they are met by the fact, that at the surface, where the move- 

 ment is swiftest, the weight is least ; in other words, the greatest 

 result ensues when the smallest force is employed. The horizontal 

 component of the weight of the first foot in depth, on a slope so low 

 as that of the xMere de Glace, is almost nothing, while the shearing force 

 of the ice is as great there as at any point of the glacier, supposing it 

 to be in a frozen state. It is impossible, therefore, that the mere 

 weight of the ice can supply any motive power sufficient to shear the 

 upper layers, whatever it may be able to effect at greater depth in the 



mass. 



From this difficulty, however, those are free who attempt to ac- 

 count for the motion, not by introducing a force of high intensity to 

 overcome the full shearing force of the ice, but by resorting to some 

 means of undermining that resistance to such a degree, that the 

 horizontal component of gravity in the glacier may be sufficient of 

 itself to produce the whole movement observed. There has been, 

 therefore, of late years, a disposition to work in this direction for a 

 solution of the problem, and to seek a clue to it in some of the less 

 obvious properties of ice. The further consideration of this will be 

 the subject of the second part of this paper. 



