Mr. HOPKINS, ON THE MOTION OF GLACIERS. 51 



weight of the moving mass, or of the extent of its surface in contact with that over which it moves ; 

 for, according to the observed laws of sliding bodies, the motion is independent of both these 

 circumstances. This difficulty has been hitherto regarded, and with reason, as a most serious 

 if not an insuperable one to the .sliding theory. Another has also been frequently urged, for which, 

 however, there is no real foundation. It has been contended that if a glacier moved by sliding 

 over its bed from the mere action of gravity, it ought to move with an accelerated motion, whereas 

 the motion is observed to be unaccelerated. If the force retarding the motion were solely that 

 of ordinary friction of the surface over which it moves, the objection would be valid, because the 

 retarding force of friction is independent of the velocity acquired ; but in the case of a glacier 

 moving down an irregular valley and over an irregular surface, all the retarding forces do not act 

 on the mass in the same manner as friction in the oi-dinary cases of sliding bodies. Besides the 

 friction, there will be retarding forces acting at an indefinite number of projecting points along 

 the sides or bottom of the glacial valley. Such forces will depend on the velocity' of the glacier, 

 and therefore the whole accelerating force on the mass will be some function of the velocity, and 

 the motion will not necessaril}' be an accelerated motion*. The difficult}' now spoken of, there- 

 fore, seems to have arisen from an imperfect conception of the problem ; but the one first mentioned 

 is sufficient to shew that the solution afforded by De Saussure's theorj' is far from being satis- 

 factory. 



The rejection of the sliding theory has led to the adoption, hy diffisrent persons, of two other 

 theories, which have been denominated respectively the dilatation and expansion theories. They 

 both rest on the same principle — the expansion of water in the act of freezing. The former has had 

 recently for its principal advocate M. Agassiz. It is found that a portion of the water arisin<r 

 from the dissolution of the superficial ice of the glacier by the direct rays of the sun and the 

 warmth of the summer atmosphere, infiltrates into the minute pores and cavities of the ice, where, 

 it is contended, it is frozen by the cold of the glacier, and, in freezing, expands and produces a 

 dilatation and consequent onward motion of the whole mass. According to the expansion theory, 

 the motion is due to the freezing and consequent expansion of water collected, not in minute 

 pores and crevices, but in cavities or fissures of considerable dimensions. A repetition of these 

 processes is supposed to keep up the continuous motion of the glacier. 



These theories appear to me to involve insuperable difficulties, both physical and mechanical. 

 Supposing the capillary cavities in the one case, and the large ones in the other, to become 

 full of water, and that water to be frozen, the cavities will be completely filled with solid ice. 

 How is another set of cavities to be formed for a repetition of the process ? Such an effect cannot 

 be ascribed to an internal dissolution of the ice as a consequence of external temperature, for 

 though the intei-nal temperature of the glacier might be depressed far below the freezing point 

 in winter, it cannot possibly be raised above that point, or even up to it, except at the extreme 

 surface, during the summer. That water does percolate through the pores of glacial ice with 

 extraordinary freedom, M. Agassiz has proved by making the percolation evident to the eye, but he 

 has not proved that it freezes there. The temperature of the upper portion of a glacier, where 

 the percolation has been observed, is, in fact, very little below that of freezing, and does not ap- 

 pear to be sufficiently low to convert water into ice while moving with the freedom with which 

 it descends through the glacier. Wherever congelation does take place the capillary pores must 

 necessarily, I conceive, be filled up, and where it does not, the percolating water must proceed till 

 it meets with the larger fissures, through which it will descend freely to the bottom of the 

 glacier. The existence of the larger internal cavities of the expansion theory is purely hypothetical; 



• The dewent of wntcr .ilong a river-counic, or of ice floating down its current, is not necessarily with an acrekraleil motion, ami 

 for a reason exactly similar to that assigned in the text. 



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