[ i6i ] 
In this manner will aqueous vapours afcend llowly 
into the atmofphere, even when we fuppofe the air 
almoft at reft, for I believe it is never perfedtly fo: 
but the folution of water in air, and the afcent of 
vapours, is greatly promoted by the motion of the 
w'inds, which bring frefh and drier air into the place 
of that which may be already faturated and loaded 
with moifture, carrying it together with its moifturc 
into the higher parts of the atmofphere, and difperfing 
it into all quarters. If we fliould now fuppofe the at-^ 
mofphere to remain always of the fame temperature 
as to heat and cold, and to have always the 
fame denfity; when it was once faturated with 
water, all evaporation would ceafe, and the va- 
pours already raifed would always remain fulpend- 
ed ; for a fluid, while it continues of the fame tem- 
perature and denfity, will never let go the particles 
of a body that it has diflblved. We muft, there- 
fore, confider what are the caufes which occafion 
the air fometimes to part with the water it has dil- 
folved, and which thereby keep up a continual cir- 
culation of vapours. And thefe I fliall Ihew to be 
the frequent vicifiitudes of heat and cold, conden- 
fation, and rarefadlion, to which the atmofphere is 
fubjedt. 
As to the effedis of heat and cold, I have already 
ftiewn that the former promotes, and the latter checks, 
or in fome meafure hinders evaporation, as well as 
more ftrongly than common water does, which is faturated with 
air. And, on this account, I fliould think it probable that ice, 
notwithftanding its hardnefs, will evaporate as faft as common 
water. 
VoL. LV. 
Y 
■other 
