16 SECOND LETTER ON GLACIERS. [1842. 



This variation of motion appears to be common to every part 

 of the glacier, as well where compact and completely even, as 

 where most fissured ; nor perhaps is the variation of velocity 

 greater in one case than in the other. (4.) From numerous 

 observations, made in all parts of the glacier, it invariably 

 results as before, that the centre moves faster than the sides of 

 the ice-stream. In the lower and faster-moving part of the 

 glacier, this disproportion is greatest, varying from one-third to 

 one-half of the smaller velocity. Near the origin of the glacier 

 it appears to be one-fourth or one-fifth of the smaller velocity. 

 (5.) The variations of glacier motion affect the central parts most 

 sensibly. (6.) The greatest daily motion which I have observed, 

 nearly opposite the Montanvert, amounts to 27.1 inches. (7.) 

 I have ascertained the velocity of motion much nearer the 

 origin of the glacier than when I last wrote. This, which 

 would appear to be nearly, if not quite, an experimentum crucis 

 between the sliding and dilatation theories, does not yield a 

 result so favourable to the latter as I had at first supposed ; for 

 though it is undoubtedly true, as stated in my last, that the 

 head of the glacier moves slower than the foot, the middle part 

 moves rather slower than either, owing probably to the greater 

 width and thickening of the ice there. This source of error, 

 from the varying section of the glacier, I had fully anticipated ; 

 but still, when we push the experiment to a limit, and take the 

 velocity very near the origin, the velocity ought to diminish, 

 on the theory of Charpentier, with a rapidity not to be mis- 

 taken. Yet very near the head of the Glacier de Lechaud, 

 the diurnal velocity is considerably more than a foot per day. 

 I am far, however, from thinking that I am yet in a position to 

 judge finally of the merits of any theory ; my belief is, that both 

 of those cited will as yet require great modification. 



By insisting upon the treatment of the problem as one of 

 pure mechanics, I am far from denying that the kind of inves- 

 tigations to which the glacier theorist have hitherto almost 

 exclusively referred, are also of great value, such as those on 

 the temperature and structure of the ice. The latter, in par- 



