58 Mr. Daniell on Evaporation. 
We will next imagine that the surface of the earth is swept by a 
high wind, and that the atmosphere instead of resting calmly upon 
its base, moves laterally with great velocity. Under these cireum- 
stances experience has shewn that the amount of evaporation will 
be nearly doubled; but the force of evaporation is not altered in 
the upper regions. The inferior exhaling surface being immove- 
able, the motion of the air perpetually changes, and renews the 
points of contact, and prevents accumulation at any one place; 
but in the heights of the atmosphere the exhaling surface of the 
cloud is borne upon the wind, and their relative situations never 
change. 
The progress of precipitation must, therefore, necessarily, under 
these circumstances, outstrip that of evaporation, and the dis- 
turbance of the atmospheric temperature will be greatly accele- 
rated. 
There is another cause which would also quicken evaporation 
below, without equally increasing its power of diffusion at any 
given height above; and that is a decrease in the density of the air 
at the surface of the earth. Under the circumstances of our first 
supposition imagine the barometer to fall to 28 inches, the evapo- 
ration would be increased from 1°74 grains per minute, to 1°86 
grains; but this decline of two inches at the surface would indi- 
cate a contemporaneous fall of little more than one inch at the 
height of 15,000 feet, and the rate of diffusion would vary accord~ 
ingly. When it is considered that great falls of the barometer are 
generally accompanied by high winds, and that this disparity is 
multiplied by the force of the current, it is easy to appreciate the 
influence of this local increase of the power of evaporation. 
The facility of evaporation in the rarer regions of the atmosphere 
will also go far to account for the state of saturation in which the 
air of mountainous countries is generally found, and many minor 
meteorological phenomena might probably meet with their expla- 
nation from variations of the same cause; such as the fogs which 
frequently accompany a very high degree of atmospheric pressure, 
and that peculiar transparency of the air which often precedes | 
yain, and is accompanied by a falling barometer, But te return 
