96 VISCOUS THEORY OF GLACIER MOTION. [1845. 



of the correlative fact of an unequal motion of the sides and 

 centre of the ice, which may in some sense be considered as the 

 geometrical statement of the preceding physical fact. The fact 

 of plasticity was suspected by Basil Hall, and more distinctly 

 announced by Rendu, as shown in the first part of this paper ; 

 but it could not be proved until the geometrical fact of the 

 swifter motion of the centre of the glacier relatively to the 

 sides was established in 1842.* The contrary opinion at that 

 time generally entertained would have been conclusive against 

 the hypothesis of plasticity called forth by the gravity of the 

 mass. 



So far then as appears from his writings, De Saussure con- 

 sidered the ice of glaciers to constitute a mass possessing rigidity 

 in the highest degree, such rigidity in short as common expe- 

 rience assigns to ice tranquilly frozen in small masses, which is 

 sensibly inflexible. It is in this sense in which I have spoken 

 of De Saussure's sliding theory, as one which " supposes the 

 mass of the glacier to be a rigid one sliding over its trough or 

 bed in the manner of solid bodies,"t and I adhere to the 

 definition as excluding the introduction of the smallest flexibility 

 or plasticity, to which the term rigidity is correctly opposed. 

 I consider too, that De Saussure's theory supposes the mass of 

 the glacier to slide over its trough or bed in the manner of solid 

 bodies, that is, not as a heap of rubbish or absolute fragments, 

 such as a glacier sometimes precipitates over a rock, but which 

 evidently did not enter into De Saussure's explanation, nor, in 

 fact, required any theory. 



As to the crevasses which form so prominent a feature of 

 many glaciers (although many are in parts almost devoid of 

 them), I do not recollect that De Saussure alludes to them as 

 facilitating in any way the movement of the glacier, but simply 

 as results of its motion and of the rigid character of ice. And 

 I believe that this view (whether it was held by De Saussure 



* Edinburgh Philosophical Journal, October 1842, and Travels in the Alps of 

 Savoy, p. 134. 



f Travels, p. 362. 



