222 Professor Forbes on the Leading Bhenomena of Glaciers. 



TATION and that of gravitation. On the former the ice 

 k supposed to be pressed onwards by an internal swelling of 

 its parts, occasioned by rapid alternations of freezing and 

 thawing of its parts, or rather by the continual formation of 

 minute crevices, into which water, derived from the warmth 

 of the sun and the action of the air on the surface, is intro- 

 duced, and where it is frozen by the cold of the glacier, whose 

 bulk it thus increases. On the theory that gravity or weight 

 is the sole cause of glacier motion, the ice, lying on an inclined 

 plane of rock, is supposed to slide over it, by its natural ten- 

 dency to descend, aided by the action of the earth's warmth, 

 which, on the hypothesis of De Saussure, prevents it from 

 being frozen to the bottom. 



It may be proper now to enquire shortly what light has 

 been thrown upon these two theories by the observations de- 

 tailed in a former part of this volume. 



Of the facts which I have established with respect to the 

 motion and structure of the ice of glaciers, two seem at least 

 to be not opposed to the theory of dilatation. I mean the 

 more rapid movement of the glacier at its centre, and the infiltra- 

 tion of its mass by water permeating the capillary fissures. The 

 former fact having been unknown to the supporters of the dila- 

 tation theory, has not been adduced by them in its favour; which 

 it is, indeed, only thus much, that a body having a certain 

 consistence and variability of form, when subjected to any pres- 

 sure, whether internal or external, will yield soonest in those 

 parts which are least retarded by friction. This fact, how- 

 ever, has no direct bearing on the cause of the pressure. 



The latter fact would be entirely favourable to the theory of 

 De Charpentier and Agassiz, could it be carried out in its con- 

 sequences, in the manner which they suppose. But it is not 

 enough that there be capillary fissures and crevices, and that 

 these be filled with water, — that does not help the matter at 

 all, — it must also be shewn that that water undergoes conver- 

 sion into ice, so as to dilate it at the time, and to the extent, 

 required for the motion. I conceive that the observations which 

 I have made, shew such a cause of motion to be inconsistent with 

 the phenomena; and this inconsistency is two-fold, first, from the 

 direct evidence that, though the ice is permeated by water, yet 



