05 
Mr Anderson’s New Atmomehfo 
over by the wind. Nor do we conceive, even though this last 
accident were guarded against, that the result would be either 
satisfactory or important. If the object of the atmometer be 
to ascertain the dissolving power of the air, it may be accom~ 
plished by placing the basin in a sheltered situation, as well as if 
it were freely exposed ; and if the quantity evaporated from the 
ground be required, the simplest and the most accurate method 
is that adopted by Mr Dalton. 
An atmometer, upon a very simple principle, was some years 
ago constructed by Mr Leslie. It consists of a ball of po-. 
rous earthen-ware, two or three inches in diameter, into which is 
inserted a glass tube, so graduated, that the quantity of water 
contained between two divisions of the tube, would cover the 
outer surface of the ball to the depth of one thousandth part of 
an inch. The ball and tube being filled with water, the top of 
the latter is covered with a brass cap, which, by means of a 
screw and collar of leather, is made quite air tight, and the in- 
strument is suspended out of doors, freely exposed to the wind. 
In this state the humidity exudes through the surface of the 
ball, just as fast as it evaporates, and the descent of the column 
in the tube indicates the quantity evaporated. As the pressure 
of the atmosphere is in a great measure removed by the tight- 
ness of the collar, the water is prevented from passing through 
the ball so quickly as to drop, while the space which it leaves 
empty at the top, is occupied by the very minute stream of air 
which is imbibed by the moisture on the outside, and may be 
seen rising through the water in the tube. 
This instrument, as we have found from numerous trials, is 
totally useless for a continued series of observations. The ob- 
vious impossibility of using it in frosty weather, is itself an insu- 
perable objection, butithere is another which renders it equally 
unsatisfactory at all seasons of the year. Though a portion of 
air, as we have already remarked, is imbibed by the humid sur- 
face of the ball, and rises into the part of the tube from which 
the water has subsided, it is in so small a quantity as to exert 
very little pressure on the surface of the water: The conse- 
quence is, that during a shower, the rain that falls on the sur- 
face of the ball is forced into the interior, so that the w*ater 
again rises in the tube ; and though the elasticity of the air 
VOL. II. NO. 3. januahy 1820. 
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