a. ee 
On Evaporation. 5 ot 
from arising under 212°: but it is caused by the 
vis inertice- of the particles of air; and is similar 
to that which a stream of water mects with in 
descending amongst pebbles. 
The theory of evaporation being thus mani- 
fested from experiments in high temperatures, 
I found that -if it was to be verified. by experi- 
ménts in low temperatures, regard must be had 
to the force of vapour actually,existing .in the 
atmosphere at the ime. For instance, if water 
of 59° were the subject, the force of vapour of 
that temperature is ¢s of the force at 212°, and 
one might expect the quantity of evaporation 
4 also; but if it should happen, as it sometimes 
does in summer, that an aqueous atmosphere to 
that amount does already exist, the evaperation, 
instead of being <> of that from boiling. water, 
would be nothing at all: On the other hand, if the 
aqueous.atmosphere were less than that, suppose 
1 of it, corresponding to 39° of heat, then the. 
effective evaporating force would be zs of that 
from boiling water; in short, the - evaporating 
force must be universally equal to that-of the 
temperature of the water, diminished by that 
already existing in the atmosphere. In order 
to find the force of the aqueous atmosphere I 
usually take a tall cylindrical glass jar, dry on . 
