242 
PROFESSOR FORBES ON THE EXTINCTION OF THE SOLAR RAYS 
therefore indicated by Sir John Herschel as the most proper for “ascertaining 
the very important point of the comparative force of solar radiation at great and small 
elevations in the atmosphere*.” 
48. The difficulties in the way of such an attempt are greater than would at first 
sight appear, and probably will ever render satisfactory observations of this kind very 
rare. Two practised and zealous observers must agree to devote a considerable 
time to the experiment ; for the assurance of fine weather to a degree that is very 
seldom met with in mountainous regions, is the first essential. The selection of a 
station elevated and insulated, and affording a permanent shelter to await the op- 
portunity of making the observation, and of making experiments continuously when 
it arrives, is of the highest importance. At great heights on insulated mountains 
such stations are exceedingly rare indeed. The instruments employed must be 
rigorously compared, and the indications afterwards very carefully reduced. All 
these essentials were in a good measure united in the summer of 1832, and it will 
appear that I have not exaggerated the difficulties when I state that, among obser- 
vations made at intervals for some weeks, those of one day only seem sufficiently per- 
fect to yield consistent and trustworthy results -f-. Considering the value of that 
day’s observations, I have spared no pains in analyzing them as completely as pos- 
sible, both for the sake of the conclusions they afford, and also to point out for the 
encouragement of future observers, how well really good observations repay the la- 
bour of a detailed reduction. In meteorology, the making of observations is usually 
by far the least considerable part of the philosopher’s task. 
49. I was fortunate enough, not only to be provided with instruments and full in- 
structions by Sir John Herschel, but likewise to make the acquaintance of a most 
zealous and able coadjutor, Professor Kamtz of Halle, who was about to proceed from 
Geneva (where we accidentally met) to an elevated insulated summit in the Ober- 
land of Berne, called the Faulhorn, for the purpose of prosecuting meteorological 
observations for some weeks. I explained my objects of investigation, and he gene- 
rously offered his best assistance, and soon acquired a knowledge of the instrument 
and its use. The month of September was the one he had selected. The Faulhorn 
is a hill or mountain, which lies exactly between the valley of Grindelwald and the 
Lake of Brientz. It is perfectly insulated, and commanding fine views in all direc- 
tions, it has been found worth while to erect a small inn upon the summit, inhabited 
during a short part of the summer, where travellers can be accommodated. Its height 
above the sea is 8747 English feet;};. The barometer stands at 21^ English inches : 
consequently nearly one-third of the atmosphere was left below. The comparative 
observations were chiefly made at Brientz, on the lake of that name, which has an 
elevation of only 1903 English feet§, consequently 6844 feet below the Faulhorn: 
and the difference of barometers (which had been compared and found accurately to 
* Private letter of 5th August 1832. f See Note B. at the end of this paper. 
X Hoffman in Brugiere’s Orographie; = 2666 metres. § Tralles. 
