Professor Forbes's Eleventh Letter on Glaciers. 415 



2. The subsidence of the glacier in its bed, owing to the melt- 

 ing of its inferior surface, whether by the heat of the earth, 

 or that due to currents of water. 3. The effect of the draw- 

 ing out of the glacier where it is in a state of distension, 

 which tends to reduce the thickness of the mass of ice ; 

 (when a glacier is violently compressed the effect will be 

 contrary, or an elevation will result) ; to which may be added 

 the influence of the slope of the bed of the glacier, by which, 

 as it moves forward, its absolute elevation is diminished, or 

 the contrary if it ascends. I had also pointed out a method* 

 by which the first of these effects, or the absolute ablation 

 of the ice (as it has been termed by M. Agassiz), might be 

 distinguished from the other two, namely, by driving a hori- 

 zontal hole into the wall of a crevasse, and observing the 

 diminution of the thickness of the stratum of ice above it. 

 The partial and total effects I have observed in the following 

 manner, during the present summer, on the Mer de Glace of 

 Chamouni. A crevasse, nearly vertical, and of no great depth, 

 was selected, running in a direction transverse to the glacier. 

 The most vertical wall nearest A (Plate V., fig. 1) is always 

 that the least exposed to the sun, and the waste of its sur- 

 face is very small, unless in the case of rain. In this wall a 

 horizontal hole C was bored, to the depth of at least a foot, 

 and was renewed from time to time. The depth at which 

 this hole existed below the surface of the glacier was deter- 

 mined by stretching a string AB across the crevasse, and 

 measuring by a line the vertical height from C to AB. The 

 yariation of this quantity gives the actual fusion of the sur- 

 face, free from the errors mentioned in my former letter. 

 It is, of course, very variable, depending on the weather as 

 well as on the place of experiment. Opposite the Montan- 

 vert, about 200 feet from the side of the glacier, during the 

 hot weather of July and August 1846, the ablation amounted 

 on an average to 3-62 inches per day ; at a higher station 

 between the Angle and Trelaporte (opposite station Q of 

 the year 1844, see Eighth Letter), it was only 273 inches, 



* Travels, Ist edition (1843), p. 154. 



