182 PROFESSOR FORBES ON THE VISCOUS THEORY OF GLACIER MOTION. 
he actually clambered with a companion up the rugged ascent from the source of the 
Arveiron, plunging continually up to the middle in snow, for no other purpose than 
to make the observation which I had requested of him ; and it would be unjust not 
to mention at the same time the admirable, because rare, generosity, with which he 
positively refused for himself any share of the remuneration which 1 pressed upon 
him the following summer, as some recompense for the fatigues and dangers which 
he had braved to obtain for me this information. With such a person, my confidence 
in the observations which he has since made at points much more accessible, and 
with the experience of some additional years, is complete. I do not mean that 
mistakes may not occur, or even that the measures may not be less exact than I 
might have taken myself ; but from my knowledge of the man, I am nearly as con- 
fident in their being faithfully reported, exactly as they were made, as if I had done 
so myself. 
With a view to lighten the labour as much as possible, I selected 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 towards the valley of Chamouni) ; all these points being 
tolerably accessible 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 deter- 
mined ; and these holes were renewed from time to time as the surface of the ice 
wasted. A staff of wood feet long, was stuck in each, which projected sufficiently 
above the snow (which never appears to have exceeded 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 
distant object on the farther side of the glacier was observed, so as to mark out suffi- 
ciently a line transverse to the glacier, the prolongation 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 sup- 
plied 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 deter- 
mined by means of a line marked with English feet and inches, left by me at Cha- 
mouni on purpose. 
The results were communicated to me regularly by letter at intervals 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 then- 
doubts removed by the internal evidence of this important series of observations, 
