(from atmēs, vapour, and μέτρον, a measure), an instrument lately contrived by Professor Leslie, for measuring the quantity of moisture exhaled from any humid surface in a given time. It consists of a very thin ball of porous earthen-ware, from one to three inches in diameter, having a small neck firmly cemented to a long and rather wide tube of glass, to which is adapted a brass cap, with a narrow collar of leather to fit close. Being filled with distilled or pure water, the waste and descent of this column serves to indicate the quantity of evaporation from the external surface of the ball. The tube is marked downwards through its whole length by the point of a diamond, with divisions across it, amounting from 200 to 500, each of which corresponds to a ring of fluid, that, spread over the whole exhaling surface, would form a film only one thousand part of an inch in thickness. This graduation is performed by previously sealing one of the ends of the tube with wax, and introducing successive portions of quicksilver, to mark every 20, 50, or 100 of those divisions; being calculated of equal bulk to discs of water, that have the surface of the ball (exclusive of the neck) for their base, and so many thousand parts of an inch for their altitude.

The instrument, being thus constructed, has its cavity filled with pure water, and its cap screwed tight, and is then suspended freely out of doors, sheltered indeed from rain, but exposed to the action of the wind. The water transudes through the porous substance of the ball, just as fast as it evaporates from the external surface; and this waste is measured by the corresponding descent of the liquid in the stem. At the same time, the column is suspended in consequence of the tightness of the cap, and prevented from oozing so freely as to drop from the ball. As the process of evaporation goes on, minute globules of air, separated by the removal of atmospheric pressure from the body of the water, or partly introduced by external absorption, continue to rise in fine streamlets to the top, where they partially occupy the space left by the subsidence of the fluid column. We need scarcely observe, that, after the water has sunk to the bottom of the stem, it will be requisite again to fill the cavity.

It is a fact of main importance for the accuracy of the Atmometer, that the rate of evaporation is nowise affected by the quality of the porous ball, and continues precisely the same, whether the exhaling surface appears almost dry or glistens with excess of moisture. This rate must evidently depend on the effect of the dryness of the air combined with its quickness of circulation. In a close room, the instrument might therefore serve the purpose of an hygrometer; and, placed out of doors, first screened, perhaps, for an hour, and then exposed during an equal space of time, it would furnish data for calculating the velocity of the wind.

This elegant instrument is of extensive application, and great practical utility. To ascertain readily and accurately the rate of evaporation from any surface, is an important acquisition, not only in Meteorology, but in Agriculture, and in the various mechanical arts. The quantity of exhalation from the surface of the ground is not of less consequence than the measure of 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 efficiency of drying-houses, which are generally constructed on most erroneous principles.

We shall select a few observations made with the Atmometer, from a register kept last season at Abbotshall, near Kirkcaldy in Fifeshire, by the very intelligent gardener of Robert Ferguson, Esq. of Raith. During the months of July, August, September, October, and November, the mean quantity of evaporation, in twenty-four hours, was respectively .111, .090, .060, .045, and .022; and distinguishing the whole interval into equal spaces of twelve hours, from six o'clock in the morning to six