416 Professor Forbes' s Eleventh Letter on Glaciers. 



the ice being also remarkably clean and white, and the dis- 

 tance 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 th6 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, 

 the ring at the extremity of which was passed over the bor- 

 ing instrument, which was then firmly adjusted in the hori- 

 zontal 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 vertical height. The height of the eye above or be- 

 low the fixed point was measured, and the sum or the differ- 

 enceT^as the case might be) of this measure and the last gives 

 the difference of the level of the horizontal hole in the ice, 

 and the mark on the moraine. The following may serve as 

 an example : — 



Station Z7, near Montanvert. 



1846. Aug. 1, 4Jh P.M. Aug. 3, 6 p.m. Difference. 



Ft. In. Ft. In. 



Horizontal hole C below A ..B, 9 33 8 7-0 8-3 inches=ra6Za««<m. 



C below Theodolite, 



Cross ( + ) U below Theodolite, 



C below (+) U, . . . I 10 3*4 10 4-8 1*4 \i\c\i^.Bz=. subsidence. 



Of course the horizontal hole may be renewed as often as 

 convenient, all that is necessary being to ascertain the dif- 

 ference of level of the old and new hole. 



In the preceding example (which has been selected by 

 chance), the subsidence bears an unusually small porportion 

 to the ablation. At the station in question, the average daily 

 ablation in July and August was 3*62 inches, the average 

 daily subsidence 1*63 inches. The sum of the two, or the 

 geometrical depression of the surface 5*25 inches, whereof 



