Supplement to the Fourth, Fifth, and Sixth Editions of the Encyclopædia Britannica/Atmometer

by John Leslie

ATMOMETER (from ατμος, vapour, and μετρον, 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 efficacy 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 o’clock in the evening, for the measure of the day and of the night; the corresponding results are .092, .029,—.078, .012,—.050, .010,—.032, .013, .020. On the 21st of July 1815, the quantity of evaporation, during twenty-four hours, was 208; and, on the 27th of the same month, it was 200, the thermometer having sunk as low during the night as 46° and 49° of Fahrenheit’s scale.—If we reckon the mean daily evaporation through the year at .040, this would give 14⁶⁄₁₀ inches, for the whole amount, or about half the annual quantity of rain that falls in this climate.

The atmometer, in its most compendious form, is admirably fitted for delicate experiments on the evaporation which takes place in close vessels, when absorbent substances are introduced. Let the ball of the instrument, for example, be immersed in air variously rarefied or condensed, under a receiver covering a surface of sulphuric acid, which has different degrees of strength; and, things being thus disposed, on extracting the common air, and introducing hydrogen gas, the rate of evaporation will then be nearly tripled. But we purpose to take some future opportunity of stating the results of such curious and interesting researches.

We shall close this short notice, with mentioning a striking fact, which shows the necessity of extreme caution in all physical inquiries. Let the ball of an atmometer be cemented to a narrow glass tube of three or four feet in length, and the whole capacity filled with fresh distilled water. Now invert the instrument upright in a basin of quicksilver, and secure it in that situation; the quicksilver, following the column of water, will rise at first quickly, and then by degrees more slowly till it reaches, perhaps, an elevation of 28 inches, where it will remain stationary and afterwards sink down, when the evaporation is nearly spent. Ice-water is raised in this way about 26 inches only, and common water scarcely 24 inches; the air separated from such liquids forming near the top of the ball, a thin medium, which, by its elasticity, counteracts in part the pressure of the external atmosphere supporting the mercurial column. But a similar experiment, where the shoot of a vine was cemented to a tube holding quicksilver, has been thought, by Dr Hales and M. Du Hamel, quite conclusive, in proving the power of the living principle of vegetation. It is obvious, that the force of evaporation alone was sufficient to explain the facts advanced by those ingenious philosophers. See Leslie’s Short Account of Experiments and Instruments depending on the Relations of Air to Heat and Moisture. (D.)