[ 44 ® ] 
hammer upon the plain, in fine weather, came off 
of itfelf upon the top of the mountain. When this 
happened, the thermometer, which I called common, 
though expofed to the fun, was only three degrees 
above o ; while on the plain it was at 18 in the 
fhade. This phenomenon, joined with feveral others 
1 obferved at the fame time, confirmed me in my 
opinion, that one of the reafons why the upper parts 
of the atmofphere have lels heat than the lower, is, 
that they contain lefs humor. 
77. With this notion, it became a very intereft- 
ing inquiry to know the different degrees of humi- 
dity habitual to the different heights of the atmo- 
phere. Of courfe therefore this was the firff obfer- 
vation I thought of, as foon as I had added an hy- 
grometer to the other inftruments contained within 
the box of my portable barometer. I undertook 
therefore to afcend Buet (the name of that high 
mountain) a fecond time. My companions in this 
expedition were Mr. Dentan, a very intelligent young 
philofopher, and my brother, who, having aflifted 
me in all undertakings of any difficulty, had been a 
witnefs of that fad which was the objed of my pre- 
fent refearches. 
78. At our fetting out, on the 29th of laft Auguft, 
the hygrometer was at 86 in my apartment, and the 
barometer at 27 p. 1 line. We were in hopes of 
fine weather, becaufe it is generally fair in this coun- 
try when the barometer at Geneva is above 27 
French inches. Soon after we fet out, we began to 
perceive that the power of the fun was greater than 
might have been expeded for the feafon. From this 
circumftance I concluded that the barometer muff 
fall; 
