156 Prof. Forbes's Tenth Letter on Glaciers, 



view of the subject which is given in the Bibliotheque Univer- 

 selle. In the Sixth Letter, written in February 1844, 1 have 

 said, speaking of the rigidity of solid ice, " It is that fragility 

 " precisely which, yielding to the hvdrostatic pressure of the 

 " unfrozen water contained in the countless capillaries of the 

 " glacier, produces the crushing action which shoves the ice 

 " over its neighbour particles and leaves a bruise, within which 

 " the infiltrated water finally freezes and forms a blue vein."* 

 And again, in the following passage, which that in the Bi- 

 bliotheque resembles so nearly that it might almost pass for a 

 translation, — "It is clearly proved, by' the experiments ° of 

 *' Agassiz and others, that the glacier is not a mass of ice, but 

 " of ice and water ; the latter percolating freely through the 

 " crevices of the former to all depths of the glacier. And, as 

 " it is matter of ocular demonstration that these crevices, 

 *' though very minute, communicate freely with one another 

 '* to great distances, the water with which they are filled com- 

 " municates force also to great distances, and exercises a tre- 

 " mendous hydrostatic pressure to move onwards in the direc- 

 " tion in which gravity urges it, the vast porous, crackling 

 " mass of seemingly rigid ice in which it is as it were bound 

 " up.''f It is quite plain, therefore, that M. Agassiz' senti- 

 ments, if correctly expressed, and those formerly published 

 by myself are in all respects identical, and I rejoice that 

 they should be so. The eminent Swiss naturalist will now, 

 I am sure, recall, with very different feelings from what he 

 once did, the hesitation and caution which prevented me 

 from at once subscribing, in 1841, to the Dilatation Theory 

 of Glaciers, which he then believed that he had experimentally 

 proved, but which he has now found it necessary to abandon. 

 On this subject I would only add, that the hydrostatic 

 pressure exerted within the veins and crevices of the glacier 

 itself cannot produce a sliding motion along the bed, except 

 by a plastic change in the figure of the mass. From this 

 simple consideration the ductility of the glacier on the great 

 scale becomes a corollary from the admission of internal 

 hydrostatic pressure as a cause of motion. Of the hydrosta- 



* Edin. Phil. Jouriial> Oct. 1844, p. 235. t Ibid, p. 239-40.. 



