170 ELEVENTH LETTER ON GLACIERS. [1846. 



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 VI., fig. 1) 

 is always the [one] least exposed to the sun, and the waste of its 

 surface 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 determined by 

 stretching a string, AB, across the crevasse, and measuring by a 

 line the vertical height from C to AB. The variation of this 

 quantity gives the actual fusion of the surface, 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 Montanvert, 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 2*73 inches, the ice being also remarkably clean and 

 white, and the distance from the western bank of the glacier 

 553 feet. 



The subsidence of the glacier in its bed, or the difference 

 between the geometrical depression of the surface and the 

 ablation, was very easily and most accurately obtained in the 

 following manner : The theodolite being placed and levelled on 

 the ice in the neighbourhood of the place of observation (not 

 necessarily always on the same spot), the height of the horizontal 

 wire of the telescope above the horizontal hole pierced in the 

 side of the crevasse was noted by directing the level upon a 

 measuring tape divided into feet and inches [held vertically by 

 an assistant], the ring at the extremity of which was passed over 

 the boring instrument, which was then firmly adjusted in the 

 horizontal hole. The reading at the telescope gave the height of 

 the eye at the moment above the hole in question. The level 

 was then directed against a fixed object on the moraine, where 

 a cross had been cut in a stone as a point of departure for the 



