Prof. Forbes's Tenth Letter on Glaciers, 165 



" most rapid when the water is abundant, and is checked 

 *' when it diminishes from any cause ; as, for instance, by 

 " a fall of snow during three or four days of frost, which 

 " prevents the water from arriving at the surface of the gla- 

 " cier. During this time it empties itself of water like a 

 " squeezed sponge.''* 



It appears clearly from this extract (supposing it to re- 

 present accurately M. Agassiz's opinions), that he has at 

 length, and finally, abandoned the dilatation theory, and has 

 embraced that in which the glacier is considered as essen- 

 tially a compound of ice and water, which moves under the 

 impulsion of its own hydrostatic pressure. I have pleasure 

 in thinking, that this change in M. Agassiz' views may be 

 due, in part at least, to the proof contained in the last (ninth) 

 Letter which you did me the favour of publishing, in which I 

 shewed that, from the experiments of M. Agassiz and his 

 friends, the plastic or viscous theory was as clearly deducible 

 as from my own previous ones. So far as can be decided 

 from the passage above quoted, and especially the part in 

 Italics, the Swiss naturalist and myself are now entirely at 

 one. The hydrostatic pressure of the liquid water in the 

 capillary fissures has, in many passages of my writings, been 

 insisted on by me as the main agent in controlling and over- 

 coming the rigidity of the icy mass in which it is bound up, 

 producing by its pressure the veined structure and the con- 

 sequent tendency to fluid-like motion in the glacier mass. 

 In illustration of this, I will take the liberty of quoting two 

 passages from my Letters on Glaciers, taking exactly the 



* *' M. Agassiz considere le glacier comme forme d'un assemblage de 

 fragments angulaires de glace, entre lesquels circule de I'eau dans la- 

 quelle on voit nager des animalcules vivants. Si Ton jette sur le glacier 

 des liquides colores on les voit appartiitre a de grandes distances au 

 fond des crevasses, mais ils ne peuvent penetrer dans Tinterieur des 

 fragments de glace. La quantity d'eau qui gorge le glacier pardit kre la 

 cause de son mouvcment, en raison de la pression hydrostatique qu*elle excrce sur 

 la masse. En effet, ce movivement devient plus rapide lorsque I'eau 

 abonde, et il se ralentit lorsqu'elle vient a diminuer par une cause quel- 

 conque ; par exemple une chute de neige pendant trois au quatre jours 

 do gele'e, ce qui oppose a ce que I'eau arrive a la surface du glacier : 

 pendant ce temps il se vide d'eau comme une eponge pressee." 



