Prof. J. D. Forbes's Sixteenth Letter on Glaciers. 173 



thaw there, but finds the usual channels still open for trans- 

 mitting the accumulated snow-water." 



It thus appears quite certain that ice, under the circum- 

 stances in which we find it in the great bulk of glaciers, 

 is in a state more or less softened even in winter; and that, 

 during nearly the whole summer, whilst surrounded by air 

 above 32°, and itself at that temperature, it has acquired a 

 still greater degree of plasticity, due to the latent heat which 

 it has then absorbed. 



I have mentioned that the observations of this and some 

 previous summers have enabled me to extend the survey of 

 the valley of Chamouni beyond the limits to which my Map 

 was originally confined. I have also obtained a great num- 

 ber of approximate altitudes of all the highest summits of 

 the chain of Mont Blanc, from the extended base which the 

 distance from the Mont Breven to the Croix de Flegere 

 (above 15,400 feet) has afforded me. But the results are as 

 yet only partially calculated. I have also made some addi- 

 tions to our knowledge of the geography of the eastern part 

 of the chain of Mont Blanc, by examining the Glacier of La 

 Tour in its whole extent, which proved the configuration of 

 the mountains to be different from what has been represented 

 on all the maps and models which I have seen. The Glaciers 

 of Argentiere and La Tour are separated throughout by a 

 rocky ridge, but the Glaciers of La Tour and Trient all but 

 unite at their highest parts, and the main chain is prolonged 

 with scarcely a break in the north-east direction, sending off 

 only a spur towards the Col de Balme, which, perhaps from 

 being the political boundary of Savoy and Switzerland, has 

 been represented generally on an exaggerated scale. What 

 surprised me most, was the great elevation of the axis of the 

 chain at the head of the Glaciers of La Tour and Trient. I 

 found it barometrically to be 4044 feet above the chalet of the 

 Col de Balme, which, from five comparisons made with the 

 observatory at Geneva, is 7291 English feet, or 2220 metres, 

 above the sea, a result agreeing closely with the recent 

 measurement by M. Favre, which is 2222 metres. Adding 

 this result to the former, we obtain 11,335 English feet for the 

 height of the granitic axis at the lowest point between the 

 Glaciers ofLa Tour and Salena on the side of the Swiss Val 



