66 Royal Society. 



sheet of uniform breadth to Montanvert without receiving the tri- 

 butary glacier of the Talefre, or uniting with the Glacier du Geant, 

 its diurnal descent would be given by the same formula, and would 

 be found to be "95487 foot. Reasoning similarly with reference to 

 the Glacier du Geant, supposing it to have continued its course singly 

 from the Col du Geant to Montanvert without confluence with the 

 Glacier de Le'chaud, its length being 40,420 feet, and its mean incli- 

 nation 6° 53', its mean diurnal motion / at Montanvert would, by 

 formula (2) have been 2'3564 feet*. The actual mean daily mo- 

 tion of the united glaciers, between the 1st and the 28th July, was 

 Montanvert (Forbes's ' Alps of the Savoy,' p. 140), — 



Near the side of the glacier 1*44 1 foot. 



Between the side and the centre . . r750 foot. 

 Near the centre 2'141 feet. 



The motion of the Glacier de Lechaut was therefore accelerated by 

 their confluence, and that of the Glacier du Geant retarded. The 

 former is dragged down by the latter. 



I have had the less hesitation in offering this solution of the me- 

 chanical problem of the motion of glaciers, as those hitherto pro- 

 posed are confessedly imperfect. That of De Saussure, which attri- 

 butes the descent of the glacier simply to its weight, is contradicted 

 by the fact that isolated fragments of the glacier stand firmly on 

 the slope on which the whole nevertheless descends ; it being ob- 

 vious that if the parts would remain at rest separately on the bed of 

 the glacier, they would also remain at rest when united. 



That of Professor J. Forbes, which supposes a viscous or semi- 

 fluid structure of the glacier, is not consistent with the fact that no 

 viscosity is to be traced in its parts when separated. They appear 

 as solid fragments, and they cannot acquire in their union proper- 

 ties in this respect which individually they have not. 



Lastly, the theory of Charpentier, which attributes the descent of 

 the glacier to the daily congelation of the water which percolates it, 

 and the expansion of its mass consequent thereon, whilst it assigns 

 a cause which, so far as it operates, cannot, as I have shown, but 

 cause the glacier to descend, appears to assign one inadequate to the 

 result ; for the congelation of the water which percolates the glacier 

 does not, according to the observations of Professor Forbes f, take 

 place at all in summer more than a few inches from the surface. 

 Nevertheless it is in the summer that the daily motion of the glacier 

 is the greatest. 



The following remarkable experiment of Mr. Hopkins of Cam- 

 bridge J, which is considered by him to be confirmatory of the sliding 

 theory of De Saussure as opposed to De Charpentier's dilatation 

 theory, receives a ready explanation on the principles which I have 

 laid down in this note. It is indeed a necessary result of them. 



* On the 1st of July, the centre of the actual motion of the Mer de Glace at 

 Montanvert was 2-25 feet, 

 t Travels in the Alps. 

 X I have quoted the above account of it from Professor Forbes's book, p. 419. 



