332 Professor Forbes's Fourteenth Letter on Glaciers. 



two stations on the glacier of Bossons, and desired Balmat to select 

 two on the Glacier des Bois (the outlet of the Mer de Glace to- 

 wards the valley of Chamouni) ; all these points being tolerably ac- 

 cessible at every season of the year. 



The general method of observation was the following : — vertical 

 holes were driven into the ice with a 4-foot blasting iron, at the 

 points whose motion was to be determined ; and these holes were re- 

 newed from time to time as the surface of the ice wasted. A staff 

 of wood 5^- feet long, was stuck in each, which projected sufficiently 

 above the snow (which never appears to have exceeded 2^ feet deep 

 on the glacier), to make it visible at all seasons. During winter the 

 staves were frozen into the ice, and the waste being small, the holes 

 did not require renewal. Two marks are then made of a permanent 

 kind on the rocks of the moraine, or two staves driven in, or a dis- 

 tant object on the farther side of the glacier was observed, so as to 

 mark out sufficiently a line transverse to the glacier, the prolonga- 

 tion of which passes over the hole in the ice when first made ; and 

 the advance of the hole in the ice beyond this fixed visual line marks 

 the progress of the glacier. The want of a theodolite is supplied by 

 directing the eye past a plumb-line suspended over the fixed mark 

 on the moraine nearest to the glacier, the eye of the observer being 

 over the farthest mark. As the spaces moved over were in most 

 cases considerable, an error of a few inches, or even a foot, is not 

 important to the result. The progress was in every case determined 

 by means of a line marked with English feet and inches, left by me 

 at Chamouni on purpose. 



The results were communicated to me regularly by letter at in- 

 tervals of a few weeks during the whole year, and all questions asked 

 and explanations required by me were answered by return of post. 



Those who may look with suspicion upon observations made in a 

 remote place by a peasant of the better class, though they may not 

 partake of my security in the results from knowing the character of 

 the individual, will, I believe, have their doubts removed by the in- 

 ternal evidence of this important series of observations, which even a 

 philosopher could not have invented, and which, it will be seen, are 

 confirmed by data of quite another kind over which the observer 

 could have no control, I mean the Meteorological Registers of Geneva 

 and St Bernard. 



The following Table contains, in a condensed form, the results 

 deduced from Balmat's observations at the four following stations :*-^ 



Bois I. On the Glacier des Bois a little way below the Chapeau, 

 at about one-third of the breadth of the glacier from its eastern 

 bank. 



* This table is abridged from those in the Philosophical Transactions for 

 1846, pp. 183 and 184, where the days and hours of each observation are 

 more accurately given, and the total motions. The days of observation are, in 

 jgeneral, the same for all the stations, but not precisely so in a few instapces, 



