Experiments on Evaporation with the Atmometer. 47 
that would cover the outer surface of the ball to the thick- 
ness of the thousandth part of an inch. These divisions 
are ascertained by a simple calculation, and numbered 
downwards to the extent of 100 or 200; to the top of the 
tube is fitted a brass cap, having a collar of leather, and 
which, after the cavity has been filled with distilled or 
boiled water, is screwed tight. The outside of the ball 
being now wiped dry, the instrument is suspended out of 
doors, and exposed to the free action of the air. 
‘« Evaporation is always proportioned to the extent of the 
humid surface. If a sheet of wet paper be applied to a plate 
of glass, it will, in a close room, lose its weight exactly at 
the same rate, whether it be held vertically or horizontally, 
and whether it-occupies the upper or the under side of the 
plate. The quantity of evaporation from a wet ball is the 
same as from an equal plane surface, or from a circle having 
twice the diameter of the sphere. In the atmometer, the 
humidity transudes through %he porous substance, just as 
fast as it evaporates from the external surface; and this 
waste is measured, by the corresponding descent of the 
water in the stem. At the same time, the tightness of the 
collar, taking off the pressure of the column of liquid, pre- 
vents it from oozing so profusely as to drop from the ball; 
an inconvenience which, in the case of very feeble evapora- 
tion, might otherwise take place. As the process goes on, 
a corresponding portion of air is likewise imbibed by the 
moisture on the outside, aud, being introduced into the 
ball, rises in a small stream, to occupy the space deserted 
by the subsiding of the water in the tube. The rate of 
evaporation is nowise affected by the quality of the porous 
ball, and continues exactly the same when the exhaling sur- 
face appears almost dry, as when it glistens with abundant 
moisture. The exterior watery film attracts moisture from 
the internal mass with a force inversely as its thickness, 
and will therefore accommodate the supply precisely to any 
given degree of expenditure. When this consumption is 
excessive, the water may be allowed to percolate, by un- 
screwing the cap, avoiding however the risk of letting it 
drop from the ball. 
‘© In still air, the indications of the hygrometer, and 
those of the atmometer, bear the same proportion; and 
the quantity of evaporation for every hour is expressed, in 
thousandths of an inch in depth, by the twentieth part of 
the hyyrometric degrees. For example, in this climate the 
medium dryness in winter being reckoned 15°, and in sum- 
mer 
