Mr. Daniell on Evaporation. 61 
the aérial currents to which, in my essay upon the constitution of 
the atmosphere, I have ascribed the fluctuations of the barometer. 
Now, I have therein shewn how the gradual spread of a small in- 
crease of temperature, through a considerable stratum, is sufficient 
for the purpose ; and a very little consideration will, I think, con- 
vince any one that the evolution of caloric is by no means so small 
as has been supposed. 
‘The following rough calculation will place the facts in a striking 
point of view :—The latent heat of steam has been proved to be 
somewhere about 970°, and it is known that, whatever be its den- 
sity, or the temperature at which it is produced, the amount will 
differ but little from this estimate. The condensation, therefore, 
of a pound of steam of any degree of elasticity would be adequate 
to raise a pound of water 970°. The capacity of atmospheric air, 
of mean density, for heat, compared to that of water, is as ‘2669 
to 1; therefore the same quantity of heat which would raise a 
pound of water 1°, would raise a pound of air 3°7. The conden- 
sation of a pound of steam would, therefore, elevate the same 
weight of air to 3589°. A pound of air is equal to about 11 cubic 
feet, so that the evolution of heat from the condensation of a 
pound of steam, would be sufficient to raise the temperature of 
3657 cubic feet of air 10°. 
When we now look to the depth of water which falls upon the 
surface of the earth, and recollect that this is not the sole measure 
of the effect we are endeavouring to estimate, but that the unceas- 
ing precipitation and exhalation of the clouds is perpetually ex- 
tending this influence to the most inaccessible heights, we shall, 
perhaps, have a juster notion of the prodigious power of atmo- 
spheric vapour ; and it will, I think, be granted that I have not 
over-rated the impulse which it is calculated to impart. 
