130 VISCOUS THEORY OF GLACIER MOTION. [1846. 



almost one-sixth of a mile. This is on the Glacier des Bois be- 

 neath the Chapeau, where the inclination of the glacier is very 

 steep, adding a new illustration of the general principle,* that in 

 similar circumstances the velocity increases with the slope. 

 To this cause may be added the high temperature of the air of 

 the valley to which, in this part of its course, it is exposed ; 

 but this last cause is alone insufficient ; for 



II. We find that the lowest part of the same glacier im- 

 mediately behind the Cote du Piget, a little way above the 

 source of the Arveiron, and therefore still deeper in the valley, 

 has a mean velocity nearly four times less, arising solely from 

 the diminished slope and volume of the glacier in that part.f 

 Hence there must be a condensation of the ice here, a pressure 

 a tergo, the quicker moving ice pressing against the slower, con- 

 solidating it, remoulding its plastic material, and sealing the 

 crevasses ; and a slight examination of the state of the glacier 

 at the points in question will show this to be the case. 



III. All that has now been said with respect to the two 

 stations on the Glacier des Bois, may be repeated with only 

 numerical differences with respect to the two stations on the 

 Glacier des Bossons ; the one set of observations confirming 

 the other. 



IV. In both glaciers the summer motion exceeds the winter 

 motion in a greater proportion, as the station is lower, that is, 

 exposed to more violent alternations of heat and cold ; this we 

 shall find to be general. 



Before continuing our deductions, we would call attention 

 to the close relation which may be established between the 

 mean temperature of any portion of the year, and the velocity 

 of the glacier corresponding to it. This is done in Plate 

 IV., exactly in the same way as I did when comparing my 



* Travels, 2d edit,, p. 371. 



f This explains a circumstance which has always hitherto heen a difficulty to 

 me ; the united testimony of the best-informed inhabitants, not only at Chamouni but 

 elsewhere (as at Zermatt and at the Simplon), to the effect that during winter the 

 lowest end of a glacier, which terminates in a valley, does not greatly protrude, nor 

 force the snow before it. This arises in fact from the comparative smallness of the 

 motion which the tongue of such a glacier appears to possess, especially in winter. 



