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Atmometer

ball, surface and water

ATMOMETER. The name given to an instrument, by its inventor, Pro fessor Leslie, to measure the quantity of exhalation from a humid surface in a given time. A thin ball of porous earthenware, 2 er 3 inches in diameter, with a small neck, has cemented to it a long and rather wide tube of glass, bearing divisions, each of them corresponding to an internal section, equal to a film of liquid that would cover the outer surface of the ball to the thick ness of part of an inch. These divisions are ascertained by a simple calcu lation, and numbered downwards to the extent of 100 or 200. To the top of this 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 out side of the ball being now wiped dry, the instrument is suspended out of doors to the free action of the air. The quantity of evaporation from a wet ball is the same as from a circle having twice the diameter of the sphere. In the atmometer, the humidity transudes through the porous substance just as fast as it evaporates from the external surface ; and this waste is measured by the cor responding descent of the water in the stem. As the process goes on, a corre

sponding portion of air is likewise imbibed by the moisture on the outside, and being introduced into the ball, rises in a small stream to replace the water. The rate of evaporation is nowise effected by the quality of the porous ball, but continues the same, whether the exhaling surface appears almost dry, or glisters with superfluous moisture. When the consumption of water is excessive, it may be allowed to percolate gradually, without dropping, by unscrewing the cap. In a review of Leslie's. Meteorology, published in the Journal of Science, for Oct. 1922, the writer recommends a vessel of porous earthenware, of a given surface, filled with water, to be poised at the end of a balance, and the loss of weight which it suffers by evaporation in a given time, to be noted. A thermometer being inserted into the mouth of the vessel, will indicate the temperature of the evaporating mass, and would form, at the same time, a good hygrometer, on Dr. Black's principle, that the degree of cold generated by evaporation is pro rtional to the dryness of the air. See Leslie on Heat andifoisture ; also Ure's ATTAR