Atmometric and Hygrometric Experiments. 49 
moisture which it dissolves, it is quickly thrown back into 
the current. On the rapidity of these successive contacts 
will depend the absolute quantity of evaporation. 
*¢ But, in perfectly calm air, the power of evaporation, if 
it be very considerable, will yet, as in the case of a heated 
surface, create an artificial stream, which mingles its in- 
fluence with the ordinary dissipation of moisture. When 
the hygrometer marks 75 degrees, this current will have a 
corresponding velocity of one mile every hour, and must 
therefore augment the regular effect of evaporation by an 
eighth part. In general, “to find the correct hourly evapo- 
sation in a medium of still air, as expressed in atmometric 
divisions, or the thousandths of an inch of superficial thick- 
ness, after having divided by 20. the number of degrees in= 
dicated by the hygrometer, let the quotient be increased in 
the ratio of that number to 600. Thus, if the hygrometer 
were to mark 30 degrees, then 1°5 is the approximate meas 
sure of evaporation ; and sipce 50 is contained 20 times in 
600, the correction to be added to 1°5 is likewise its twen= 
tieth part, or (075; so that the hourly evaporation, esti« 
mated in atmometric divisions, amounts to 1°575, and the 
daily to 18°9. . This correction is however in most instances 
so small, that it may, without material inaccuracy, be en- 
tirely overlooked. Kut, in confined hydrogen gas, at the 
same state of dryness, the atmometer is ‘as much affected 
as if 1t were exposed in open air to a wind having the velo= 
city of 12. miles an bour; and consequently the dispersion 
of moisture in such a powerful medium is, like that of 
heat under such circumstances, two and a half times more 
profuse than in almospheric air. 
“The atmometer is an instrument evidently of extensive 
application and of great. vtlity in practice. To ascertain 
with accuracy and readiness the quantity of evaporation 
from any surtace in a given time, 1s an important acquisi-= 
tion, not only in meteorology, but in agriculture, and the 
various arts and manufactures. The rate of exhalation 
from the surface ot the ground ‘is scarcely of less conse- 
quence than the fall. of rain, and a knowledge of it might 
often direct the farmer advantageously in his operations. 
On the rapid dispersion of moisture, depends the efficacy 
of drying houses, which are too frequently constructed 
most unskilfully, or on very mistaken principles. But the 
purposes to which the atmometer so ‘aptly apples, were 
hitherto supplied in a rude and imperfect manner. The 
joss that water sustains, in a given time from evaporation, 
as commonly been estimated by weight or measure. If a 
Vol, 42. No. 183, July 1813. D piece 
