Professor Forbes on the Leading/ Phenomena of Glaciers. 227 



experiments which confirm this view ; but I have already 

 stated, that my own are entirely at variance with it, the abso- 

 lute level of the ice lowering with great rapidity during the sea- 

 son of most rapid motion ; a conclusion which is entirely con- 

 firmed by the observations of MM. Martins and Bravais, lately 

 published.* 



On these, amongst other grounds deduced from direct ob- 

 servation, I consider the dilatation theory maintained by Scheu- 

 chzer, De Charpentier, and Agassiz, as untenable. 



In the next place, let us consider the sliding theory of Gru- 

 ner and De Saussure. 



As I understand the gravitation theory, it supposes the 

 mass of the glacier to be a ri^id one, sliding over its trough 

 or bed in the manner of solid bodies, assisted, it may be, by 

 the melting of the ice in contact with the soil, which possesses 

 a proper heat of its own, and which lubricates in some degree 

 the slope, as grease or soap does when interposed between a 

 sliding body and an inclined plane. It is only in so far as 

 the theory is considered as applicable to a rigid body, that I 

 have objections to state to it. 



1. In the case of the greater number of extensive glaciers, 

 there are notable contractions and enlargements of the chan- 

 nel or bed down which they are urged. Let any one glance 

 at the Mer de Glace, and see two extensive glaciers meeting 

 at the Tacul, forming a vast basin or pool, from which the 

 only outlet has a less breadth than the narrowest of the tribu- 

 taries ; the idea of sliding, in the common legitimate sense of 

 the word, is wholly out of the question. 



2. We have already seen that the ice does not move as a 

 solid body, — that it does not slide down with uniformity in 

 different parts of its section, — that the sides, which might be 

 imagined to be most completely detached from their rocky 

 walls during summer, move slowest, and are, as it were, drag- 

 ged down by the central parts. All this is consistent with 

 motion due to weight or gravitation ; but not with the sliding 

 of a rigid mass over its bed. 



* AnnaUt des Sciences Geologiqttes, par Riviere, 1842. 



