48 Aimometric and Hygrometric Experiments. 
mer about 40°, the daily exhalation from a sheltered spot 
will amount in winter to a thickness of ‘018, and in sum- 
mer to °048 decimal parts of an inch. If we reckon the 
mean daily evaporation from the ground while screened at 
"030, the waste during the whole year will amount to near 
eleven inches, being scarcely the half perhaps of what, un- 
der the circulation of the. atmosphere, actually obtains. 
The dissipation of moisture indeed is vastly accelerated by 
the action of sweeping winds,—the effect being sometimes 
augmented five or ten times, In general, this. augmenta- 
tion 18 proportional, as in the case of cooling, to the swift- 
ness of the wind, the action of still air itself being reckoned 
eqnal to that produced by a celerity of eight miles each 
hour. Hence the velocity of wind is easily éomputed, from 
a comparison of the indications of an hygrometer with an 
tmometer, or of a sheltered, with those of an exposed, at- 
mometer. Thus, suppose the hygrometer to mark 40 de- 
grees, or the column in a Sheltered atmometer to subside 
at the rate of two divisions each hour, while in one ex- 
posed to the current the descent is 12 divisions; then, as 
2 is to 10, the superadded effect of the wind, so is 8 to 
40 miles, its velocity during the hour. 
** It is curious to remark, what a small proportion of 
any stream of air can acquire heat or moisture, by flowing 
over a warm or a humid sarface. Supposing the air to 
have 20 degrees of dryness, the ordinary evaporation would 
every hour equal a film of the thousandth part of an inch 
er But this portion of moisture would be sufficient, 
ce have seen, to saturate 800 times its weight of air at 
cach a low state of dryness ; or, reckoning the air 850 times 
Jighter than water, this weight would correspond to that of 
a cylinder of air 574 feet high, and having its base equal 
to the surface of the humid ball,—or to a cylinder 230 fect 
bigh, and of the same diameter as that ball. Now, since 
the ordinary evaporation at 20 degrees of the hygrometer 
is equal to the increased effect occasioned by a current of 
air moving with the velocity of eight miles in the hour, 
and forming therefore against the ball a cylinder of 42,240 
feet in height ; it hence follows, that not more than the 
164th part of this advancing column can be humified by 
its streaming over the surface of the ball. Such commnu- 
nication of moisture is no doubt confined within the nar- 
row limits of physical contact. Each minute poruon of 
air which comes to graze along the humid surface has its 
velocity retarded, aad acquiring new elasticity from the 
moisture 
