96 Professor Forbes's Twelfth Letter on Glaciers. 



great fall of snow of the two winters 1843-4 and 1844-5, and 

 the cold wet summers which followed them. The immediate 

 effect of the snow is to protect the ice and diminish the annual 

 ablation. Hence the glacier shoots farther into the valley 

 before the waste suffices to equalize the supply. In the cold 

 spring and summer of 1845, this effect appears to have been 

 most conspicuous, as appears from the decisive observations 

 of Balmat, which I have elsewhere published.t The swollen 

 condition of the Mer de Glace of Chamouni in 1846, was 

 shewn by the fact, that, in spite of the intense and continued 

 heat, it was much higher opposite to the Angle in the middle 

 of August, than it was in June 1842, most of the marks 

 which I then made, for the purpose of estimating the pro- 

 gress of the glacier, being covered by the moraine at the lat- 

 ter period. The extremity of all the glaciers of which I have 

 obtained information, was advancing towards the valley du- 

 ring the summer of 1846 ; and this was even accelerated by 

 the great heat of the season. For though it must increase 

 the ablation of the surface, and the melting of the terminal 

 face, and thus diminish the mass of ice, its immediate effect 

 is to fuse the glacier into a state of pliancy, such as to in- 

 crease its motion in a very perceptible manner (as I have es- 

 tablished by direct experiment), and thus discharging its icy 

 burden into the valley faster than even the increased atmo- 

 spheric heat is capable of dissolving it, it spreads with a ve- 

 locity which, if it could be supposed continual, could not fail 

 to be alarming. Thus it appears, from certain observations 

 made at the desire of M. Carrel, by M. Guicharda, vicar at 

 Courmayeur, that the snout or extremity of the glacier of 

 La Brenva, has protruded iato the valley no less than 22 

 metres, or about 60 feet during the two months of summer, 

 being at the rate of afoot a day. The result of this advance 

 is, that the glacier is rapidly attaining the old moraine of 



Bois at its termination, escapes chiefly at a much higher level, and formed, in 

 1846, a striking cascade on the west side of the glacier, nearly opposite to the 

 Ohapeau. A similar occurrence is said to have happened about 30 years ago. 

 t Phil. Trans., 1846, p. 183. 



