The Rev. H. Moseley on the Descent of Glaciers. 65 



Geant, Professor Forbes estimates (but not from his own observa- 

 tions, or with the same certainty) to be 24,700 feet in length, and 

 to have a mean inclination of 8° 46' 40". 



According to the observations of De Saussure, the mean daily- 

 range of Reaumur's thermometer in the month of July, at the Col 

 du Geant, is ■i'^-257*, and at Chamouni 10°'092. The resistance 

 opposed by the rugged channel of a glacier to its descent cannot but 

 be different at different points and in respect to different glaciers. 

 The following passage from Professor Forbes's work contains the 

 most authentic information I am able to find on this subject. 

 Speaking of the Glacier of la Brenva, he says, " The ice removed. 

 a layer of fine mud covered the rock, not composed however alone 

 of the clayey limestone mud, but of sharp sand derived from the 

 granitic moraines of the glacier, and brought down with it from 

 the opposite side of the valley. Upon examining the fuce of the 

 ice removed from contact with the rock, we found it set all over 

 with sharp angular fragments, from the size of grains of sand to 

 that of a cherry, or larger, of the same species of rock, and 

 which were so firmly fixed m the ice as to demonstrate the impossi- 

 bility of such a surface being forcibly urged forwards without saw- 

 ing and tearing any comparatively soft body which might be below it. 

 Accordingly, it was not difficult to discover in the limestone the very 

 grooves and scratches which were in the act of being made at the 

 time by the pressure of the ice and its contained fragments of stone." 

 (Alps of the Savoy, pp. 203-4.) It is not difficult from this descrip- 

 tion to account for the fact that small glaciers are sometimes seen to 

 lie on a slope of 30'^ (p. 35). The most probable supposition would 

 indeed fix the limiting angle of resistance between the rock and the 

 under surface of the ice, set all over, as it is described to be, with 

 particles of sand and small fragments of stone, at about 30°, that 

 being nearly the slope at which calcareous stones will rest on one 

 another. If we take then 30° to be the limiting angle of resistance 

 between the under surface of the Mer de Glace and the rock on 

 which it rests, and if we assume the same mean daily variation of 

 temperature (4'257 Reaumur, or 5"321 Centigrade) to obtain through- 

 out the length of the Glacier du Geant, which De Saussure observed 

 in July at the Col du Geant ; if, further, we take the linear expan- 

 sion of ice at 100° Centigrade to be that (-00524) which was deter- 

 mined by the experiments of Schumacher ; and, lastly, if we assume 

 the Glacier du Ge'ant to descend as it would if its descent were un- 

 opposed by its confluence with the Glacier de Lechaud, we shall ob- 

 tain, by substitution in equation (2) for the mean daily descent of 

 the Glacier du Geant at the Tacul, the formula — 



, c.A-nn coo, "00524 tan 8° 46' 

 /=24/00x5-321x^^x-^^^. 



/= 1-8395 feet. 

 The actual descent of the glacier in the centre was 1 '5 foot. If the 

 Glacier de Lechaud descended at a mean slope of 5^, singly in a 

 * Quoted by Profesaor Forbes, p. 231. 



Phil. Mug. S. 4. Vol. 10. No. 63. July 1855. F 



