[ 454 ] 
on the higher ones. I fhall add, that though my hy- 
grometer was expofed to the open air on the moun- 
tain, as it was in the plain, yet it was not fo much 
infulated there, being tied to the box of my portable 
barometer. The difference obferved, however, is fo 
confiderable, that, notwithffanding the concurrence of 
all thefe particular caufes, I cannot but afcribe it in 
forne meafure to that general one which I have 
lufpefted, namely, that there is comparatively a lefs 
degree of humidity in the upper than in the lower 
parts of the atmofphere. 
ioi. The oblervation of the 13th of September 
feems likewife to throw fome light upon the phas- 
nomena of dew. We know that when the Iky is 
cloudy, there is little or no dew, and it has likewife 
been obferved from this very circumffance, that the 
air is not fo much cooled after fun-fet. The caufe 
of thefe differences appears to me to be, that when 
there are no clouds in the air at fun-fet, or when 
they are difperfed, the heat of the inferior air, and 
that which rifes from the earth, diffipates itfelf into 
the fuperior regions, and then the vapours which are 
difperfed throughout the air condenfe and fall down 
again in dew ; but when the clouds are continued, 
and thus feparate the inferior from the fuperior air, 
they prevent this diflipation of the heat, and the va- 
pours remain fufpended. And if the Iky grows 
cloudy fome hours after the fetting of the fun, and 
after the heat has fenfibly diminiffied in the inferior 
air, it encreafes again in it; becaufe the heat, which 
continues to rife out of the earth, is accumulated in 
the inferior air. This appears in the oblervation l 
am fpeaking of. The clouds having been fepa- 
5 rated 
