Mr. Daniell on Evaporation. 57 
increasing as the pressure diminishes, would give 2°13 grains per 
minute ; so that the power of evaporation at this stage exceeds the 
supply of moisture, and no cloud could possibly be formed. 
Above the second point of condensation let us now suppose the 
force of the vapour to be determined, in still loftier regions, by a 
temperature of 120. The force of evaporation would then be 0°44 
grains, increased in the proportion of 16 inches to 30, or 0°82 
grains. Here, then, the power of evaporation would be insufficient 
to diffuse in the upper regions the whole of the moisture supplied 
from the surface of the earth, and a cloud, it might be supposed, 
must consequently result. But another modification of the pro- 
cess now ensues; the precipitated moisture has a tendency to fall 
back into the warm air below it, and consequently would again 
assume the elastic form with a rapidity proportioned to the rare- 
faction of the stratum in which it is diffused. There is, I think, 
no difficulty in supposing that no visible cloud, or one of extreme 
tenuity, would be formed during this double process of evapora- 
tion. A yery important re-action, however, must take place upon 
the strata of vapour beneath; the elastic force being increased 
aboye, enables the water below to maintain an atmosphere of a 
higher degree, and the quantity of evaporation must decrease as 
the point of saturation rises. A different arrangement of the points 
of precipitation would ensue in the progress of these effects. 
An important distinction must here be drawn between the ulti- 
mate effects of the superior and inferior evaporation denoted above. 
In the first, the whole weight of water is condensed and simul- 
taneously exhaled ; and although it constitutes steam of an in- 
ferior degree of force, there is little or no difference in the quantity 
of its latent heat, and no effect is tierefore produced upon the 
temperature of that portion of the atmosphere in which the change 
takes place. But in the second, the condensation happens at one 
spot, and the vaporization at another inferior to it; the latent heat 
is therefore evolved at the former and communicated to the air, 
while at the latter the process is reversed, and the air is cooled. 
The process of this operation would, therefore, tend to equalize 
the temperature of the atmosphere, 
