[ + 5 * 3 
97« I fhall not attribute intlrely to the fame caufe, 
the great difference obferved between my hygrome- 
ters, when one was expofed to the fun, while the 
other flood in the fhade. The immediate adlion of 
the folar rays, or of the luminous heat, produces a 
variety of effe&s, which, as I have faid before, do 
not appear to follow the fame laws as thofe of dark 
heat. And if I may be allowed to propofe a con- 
jecture upon this particular point, before fuller ex- 
periments have been made, it fliould feem, that the 
immediate adtion of the folar rays muft occafion a 
greater evaporation than what is produced by dark 
heat, even when they hold the thermometer at the 
fame height. But let the caufe be as it will, we 
fee by this experiment, that in a fedlion of air about 
a foot wide, through which the folar rays did not 
immediately pafs, the adtion of the humor upon the 
hygrometer was 23 degrees greater than in the 
place round about ; though that of the heat upon 
the thermometers was only a degree and a quarter 
lefs ; which leads us to conceive how many apparently 
fmall caufes may contribute to produce fenfible dif- 
ferences in the diflribution of the difcrete humor. 
98. Another ufe to be made of thefe obfervations 
is, to compare them with thofe that I have made in the 
mountains of Sixt ; in order to form a better judg- 
ment of the proportion between the different de- 
grees of humidity, in the fu peri or and inferior parts 
of the atmofphere. My hygrometer, held in the 
fhade upon the lummit of Buet, rofe to 132$, 
and v/as not yet flationary. This is pretty nearly 
the greatefl: degree of drynefs obferved in the hy- 
- grometer expofed to the fun in the garden 3 while 
