ATMOMETER, an instrument for measur ing the quantity of moisture exhaled in the open air in a given time from any moist surface, or from the surface of a liquid. The first in strument of this kind was invented by Sir John Leslie. It consisted of a very thin ball of porous earthenware, from one to three inches in diame ter, having a small neck firmly cemented to a long and rather wide glass tube, to which is fitted a brass cap with a leather collar. It is filled with distilled or pure water and its cap is screwed on tightly. It is then suspended out of doors where it is exposed to the action of the wind but is sheltered from rain. As the water evaporates from the surface of the ball the waste is indicated by a corresponding de scent of the water level in the glass tube, accu rately measured by a finely-graduated scale. Another form of this instrument consists of a round dish into which is placed an inclined scale, which gives a magnified record of the rate of evaporation from the surface of the liquid in the dish. Sometimes the loss of mois
ture through evaporation is recorded by means of weighing. The Marvin atmometer consists of a surface of water fed from a supply well where the rate of evaporation can be measured. The Piche instrument is a glass tube, closed on top and open below. The open bottom is then rested on a disc of paper and fastened to it. The water is absorbed by the paper, whence it evaporates, the rate of evaporation being indi cated by the descending level in the glass tube as in the Leslie instrument. This instrument, however, is more accurate because of the fact that the paper surface gives off more evapora tion than does the exposed surface of the water in an open dish. There is also the balance in strument invented by Wild, in which a pan of water forms the weight on a balance scale, the action of the scale indicating the loss by evapo ration. For a detailed description of atmom eters consult Russell's 'Meteorology' (New York 1899).