Mr. Tredgold on the Theory of Evaporation. _ 45 
As to the integral which enters into these expressions 
dsds' (s—s' cose) ds’ 
ves 73 Se pret af @ + st +82 —2s5' cose? 
we may obtain by the known method of integration of dif- 
ferentials which comprise a radix of the second order, and 
more easily by a particular process which I shall explain else- 
where. 
V. On the Fheory of Evaporation. By Tuos. Trepveoxrp, Esq. 
: To Mr. R. Taylor. 
Sir, 
BYAPORATION has been considerably attended to, but 
rather as a matter of experimental research than with the 
object of finding those first principles which are essential to 
the process. In the following inquiry it is not intended to 
limit it to a particular case, but simply for illustration the 
vapour is supposed to be from the surface of water. 
When the air in contact with water is saturated with vapour, 
evaporation ceases, or there is an equilibrium between the 
powers which produce and retard the formation of vapour. 
Now conceive a portion of the vapour to be abstracted from 
the air, then the equilibrium will be destroyed; and all other 
circumstances being the same, the tendency to restore the 
equilibrium must be proportional to the quantity of vapour 
removed from the previously saturated air; for no other cir- 
cumstance than the weight of vapour in a given portion of air 
is altered. 
But, the equilibrium being destroyed, evaporation commences, 
and the vapour cannot be formed without a constant supply of 
heat; therefore, to obtain this supply of heat when there 
is no other source than the surrounding bodies of the same 
temperature, the temperature of the surface where the vapour 
forms must be depressed, in order that heat may flow to it 
from the adjoining bodies, or parts of the same body; and as 
the heat required is proportional to the quantity of vapour 
formed in a given time, the depression of temperature will be 
proportional to that quantity. 
It will also be obvious that the vapour formed will be of 
the elasticity corresponding to the temperature of the sur- 
face producing it, and therefore will correspond to the de- 
pressed temperature of the evaporating surface. 
Let T be the general temperature, ¢ the tempevature of the 
evaporating surface at its ultimate depression, and w the weight 
of vapour in grains that would saturate a cubic foot of air ‘e 
the 
