4 10 Prof. Forbes's Reply to Mr. Hopkins 



of fourteen to three, or nearly five to one; and that at the 

 side the motion had almost, if not entirely, vanished in that 

 case? If it escaped his notice in reading the letter, does he 

 not recollect that Dr. Whewell reminded him, at the last 

 meeting of the British Association, that three to fourteen is 

 not a small ratio? If Mr. Hopkins requires further proof than 

 my assertions, let him see if the facts observed by my oppo- 

 nents in theory on the glacier of the Aar will come to his aid. 

 He will find that the side of the Aar glacier moved regularly 

 through one foot, whilst the centre moved through fourteen, 

 or that the effect of plasticity was thirteen-fourteenths of 

 the entire motion*. 



4. Mr. Hopkins does not object to an appeal to experiment 

 to illustrate the complicated subject of the movement of gla- 

 ciers. On the contrary, the foundation of his present specula- 

 tions was an experiment to illustrate De Saussure's opinion, 

 that the melting of the ice in contact with its bed by the heat 

 of the earth greatly facilitates its movement. This fusion of 

 the ice in contact with the soil is what Mr. Hopkins calls its 

 " disintegration." It does certainly appear to me that this ex- 

 periment was not more requisite to show the manner of pro- 

 gression of a mass of ice under these circumstances than ex- 

 periments on semifluid or plastic bodies, and if possible on the 

 mass of the glacier itself, were necessary to determine with cer- 

 tainty the manner in which their internal forces accommodate 

 them to the circumstances of mechanical constraint under 

 vi^hich they move. If calculation and estimation of forces was 

 sufficient in the latter case, it appears to me to be not less so 

 in the former ; but if, on the other hand, the experiment of 

 making ice-masses slide on a rough inclined plane was consi- 

 dered by Mr. Hopkins as the very touchstone of the true 

 theory, and sufficient to put to silence any rival explanation, 

 it would appear that the experiments which I have made 

 upon plastic models and the directions of tearing and maxi- 

 mum distension of semifluid pastes, which are well-known to 

 Mr. Hopkins and other readers of my writings on glaciers, 

 deserved a deliberate consideration and reconcilement with 

 the abstract investigations which Mr. Hopkins elaborated at 

 leisure in his study, whilst others sought to test and correct 

 theirs by a daily and hourly appeal to the multitude of phae- 

 nomena which the glaciers present to those who live amongst 

 them. 



To explain the undoubted facts presented by these plastic 

 models, — the external forms, concave at the origin, swelling 



* Comptes Rendus, Dec. 9, 1844; and my Ninth Letter on Glaciers, 

 Edin. Phil. Journal, April 1845. 



