324 THE LIFE OF JAMES D. FORBES. [CHAP. 



glacier stood some 300 feet above *the level at which he 

 then found it: but since 1842 it had again increased in 

 size a condition which appears to have been a general 

 one among glaciers in 1846. 



'The extremities of all the glaciers of which I have 

 obtained observations/ he writes, 1 'were advancing 

 towards their valleys during the summer of 1846. . . . 

 The cause is, no doubt, to be sought partly in the great 

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

 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 

 further into the valley before the waste suffices to 

 equalize the supply. ... In 1846, however, the Mer de 

 Glace, in spite of the intense and continued heat, was 

 much higher opposite to the Angle in the middle of 

 August, than it was in June 1842. . . . Thus, its motion 

 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, thus diminishing 

 the mass of ice, its immediate effect is to fuse the glacier 

 into a state of pliancy, such as to increase its motion 

 in a very perceptible manner (as I have established by 

 direct experiment), and thus discharging its icy burden 

 into the valley faster than even the increased atmospheric 

 heat is capable of dissolving it, it spreads with a velocity 

 which, if it could be supposed to be continual, could not 

 fail to be alarming. Thus it appears from observations 

 made by M. Guicharda, Vicar of Courmayeur, that the 

 extremity of the glacier of La Brenva has protruded into 

 the valley no less than twenty-two metres, or about 

 seventy feet during the two months of summer, being 

 at the rate of a foot a day. . . . The same gentleman 

 has himself made, with considerable labour, observations 

 intended to test the reality of the movement of the 

 glacier during winter, which confirm in every particular 

 those which I have already published regarding the. 

 glaciers of Chamounix/ 



1 Twelfth Letter on Glaciers. 



