246 Professor Forbes on the Leading Phenomenal)/ Glacier^. 



I feel bound also to quote the significant expressions of 

 Captain Hall, pointing to the conception of a semifluid glacier. 

 " When successive layers of snow," he says, speaking of the 

 Glacier de Miage, " often several hundreds of feet in thickness, 

 come to be melted by the sun and by the innumerable torrents 

 which are poured upon them from every side, to say nothing 

 of the heavy rains of summer, they form a mass, not liquid 

 indeed, but such as has a tendency to move down the highly 

 inclined faces on which they lie, every part of which is not 

 only well lubricated by running streams resulting from the melt- 

 ing snows on every side, but has been well polished by the 

 friction of ages of antecedent glaciers. Every summer a cer- 

 tain but very slow advance is made by these huge, sluggish, 

 slushy, half-snowy, half-icy accumulations."* It is plain, I 

 think, that the author had an idea that liquid pressure might 

 drag a mass over its rocky bed, which would not move upon 

 it as a solid. 



But such speculations could not pass into a theory, until 

 supported by the definite facts of which M. Rendu deplores the 

 want. I too, like my predecessors, though independently of 

 them, had compared the movement of glaciers to that of a duc- 

 tile plastic mass, in 1841, when I spoke of the Glacier of the 

 Rhone as " spreading itself out much as a pailful of thickish 

 mortar would do in like circumstances,'' t and again, when I 

 likened the motion of glaciers to that of a great river, or of a 

 lava stream. J Rut I knew very well that such analogies had 

 no claim to found a theory. I knew that the onus of the 

 proof lay with the theorist, — (1.) To shew that (contrary to 

 the then received opinion) the centre of a glacier moves fastest ; 



des experiences bien faites sur la glace et la neige viendra-t-on a bout de le saisir : 

 toais ces premiers elements nous manquent encore." — Theorie des Glaciers, p. 90, 



* " Patchwork," vol. i., p 104, et seq. The whole passage, which is too long 

 to quote, gives an admirable picture of the glacier w orld. 



t Ed. New Phil. Journal, January 1842. 



X Edinburgh Review, April 1842, p. 54. Both these articles were written in 

 1841. 



