414 Prof. Forbes's Reply to Mr. Hopkins 



plasticity is not sufficient in degree to constitute a cause of 

 motion of the glacier ; and here everything is a question of 

 "degree." It is to this that all the rigour of mathematical 

 demonstration comes at last ! If the plasticity be " high " in 

 degree, we are "justified in asserting that the effectiveness of 

 gravity to put the mass in motion was due to that property;" 

 but if the plasticity is small or "secular," we "have no right 

 thence to conclude that the motion is effectiveiy due to the 

 plasticity of the mass*." Mr. Hopkins gives us no clue for 

 finding, either by theory or experiment, where " high " plas- 

 ticity ends, and "secular" plasticity begins. He is therefore 

 plainly not in a state to affirm that the plasticity of glaciers is 

 not sufficient to be a cause of motion. I leave it to metaphy- 

 sicians to join issue with Mr. Hopkins on his idea of cause ; 

 here I will only state a fact. 



It is this : — Lavas, every one admits to be more or less vis- 

 cous fluids, and that their fluidity is indispensable to their 

 motion. I have shown, in a paper lately communicated to 

 the Royal Society, that whilst some lavas have been so fluid as 

 to describe a space of a thousand feet in a Jew seconds^ others 

 have moved continuously and for a long period, even ^ox yearsy 

 at a rate not exceeding that of some glaciers, or three Jeet a 

 day^ and under. Since cases might be cited with every inter- 

 mediate degree of speed, I shall leave it to Mr. Hopkins to 

 define the effectiveness of plasticity in these instances. 



When I speak of plasticity or viscosity, which is the same 

 property in a higher degree, (I have used the term as it was 

 originally suggested to me by Sir John Herschel for distin- 

 guishing my theory of glacier motion) as being the cause of 

 the descent of glaciers, or effective in producing their descent, 

 I mean this (and whatever my theory may be worth, I claim 

 at least consistency in the interpretation of it, and definiteness 

 in its statements), that plasticity is a quality of glaciers with- 

 out which they would remain stationary or descend in avalanches. 

 In this sense I understand it as a cause and not an accessory 

 of motion. 



Perhaps the following illustration will appear to the impar- 

 tial reader almost a demonstration of this principle. I do not 

 mean to offer it to Mr. Hopkins as such, because he knows 

 of no demonstrations but those clothed in the language of ana- 

 lysis. 



There is a glacier basin in the range of Mont Blanc called 

 the Glacier du Talefre. Its outline is correctly represented 

 in the annexed figure, as well as the relative dimension of the 



* Phil. Mag., February, p. 168. 



