HYGROMETER. This is an instrument serving to determine the quantity of aqueous vapour in the atmosphere or other gas under examination. M. De Luc employed for this purpose a thin slip of whalebone, the con tractions of which indicated the variations of moisture; and De Saussure had recourse to a human hair, by means of which he con structed a far more delicate instrument. Leslie's hygrometer consists of a glass tube, bent so as to form two equal branches parallel to one another, and each terminating with a hollow ball, in which is introduced sulphuric acid coloured. One of the balls is covered with cambric, which is kept constantly moist by water from a neighbouring vessel ; and the evaporation of the water, by cooling that ball, allows the air in the other, by its superior elasticity, to depress the acid in the tube be low. This depression, being measured by a convenient scale, affords an indication of the relative dryness of the atmosphere.
All these instruments have however been nearly superseded by the hygrometer invented by the late Professor Daniell. It consists of two thin glass balls one inch and a quarter in diameter, connected by a glass tube about 7 inches in length. The tube is bent in two places at right angles, so as to form three arms of unequal length, the longest of which contains a small thermometer, whose bulb de scends into the lower of the two glass balls.
This ball, after being filled about two-thirds with tether, is placed over a spirit-lamp until the vapour of the tether has expelled the con tained air through a capillary tube which is left open for the purpose, and afterwards her metically sealed. The other ball is then covered with a piece of muslin, and the instru ment thus adjusted is placed upon a stand, to which is attached a small thermometer, in dicating the temperature of the external air. When about to be used, a small portion of [ether is poured upon the muslin, which, by evaporating, lowers the temperature of the glass ball, and thereby occasions a rapid con densation of the [ethereal vapour contained within the instrument. The condensation of the vapour within the tube produces a conti nuous evaporation from the surface of the nether in the lower ball, whereby the tempe rature of the included [ether is continually reduced until a deposit of moisture from the surrounding atmosphere is observed to take place upon the exterior of the glass. At this instant the inner thermometer, which always