HYGROMETER (Gr. gmoisture-meas ure), an instrument for determining the degree of humidity or of moisture present in the at mosphere; when of primitive form, calculated only to show the difference between a dry day and a damp day, called hydroscope as when constructed of catgut, hair or other fibrous material, having the property of lengthening when wet and contracting when dry. Sausson's hydroscope employs a human hair, the stretch ing or shortening of which with variations of moisture in the air moves a pointer on a grad uated arc. A similar machine is Richmond's hydrograph, in which the hair is wrapped around a screw-thread, turning a cylinder as it lengthens or contracts, so that a connecting pencil may make a record on the cylinder. The first hygrometer properly so-called was made by Professor Daniell. It consists of a glass tube, bent at right angles into arms of unequal length. Each arm terminates in a bulb, one bulb (usualy black) being two-thirds filled with sul phuric ether, and the other bulb being, at the commencement of an experiment, empty. In process of construction the tube is exhausted of air, and is thus filled with vapor of ether through its entire length. A thermometer with a bulb immersed in the ether of the lower arm is inserted in the tube to register variation of temperature, and a second thermometer is at tached to the stand of the instrument to show the temperature of the outer air. For use one bulb containing the sulphuric ether has a zone of polished gold, and the other bulb a muslin cover. If sulphuric ether be dropped on the latter, as it evaporates the bulb is cooled, and the vapor of ether is condensed within it from the other bulb; the temperature of which rap idly falls owing to evaporation from it. The operation is carried on, .ether being dropped on the second bulb as is required, till the tempera ture of the first is so far reduced that dew from the surrounding air just begins to condense upon it. By means of the thermometer con tained in the first bulb the temperature is read off at the instant at which vapor begins to con dense, and the dew-point is thus obtained. The hygrometric condition, that is, the ratio between the quantity of moisture that the air actually contains and the quantity which it is capable of containing at the existing temperature is then easily deduced.
Regnault's hygrometer, shown in the above figure, is an important modification of Daniell's instrument. D and 1111 are two precisely similar
cups or thimbles of polished silver; each is sur mounted with a similar glass tube into which, by means of a cork, two thermometers are fitted, and the bulbs of the thermometers are covered with ether. Through the cork in one of the tubes a small glass tube t passes, and is carried down below the surface of the ether; while a side tube establishes communication with the vertical tube u v which is connected with an aspirator A (or vessel into which air is sucked at the top to supply the place of water which escapes at the bottom). There are no cor responding side tubes connecting the left-hand tube of the hygrometer to'. By means of the aspirator a current of air is drawn through t; it therefore bubbles through the ether, causing evaporation and cooling the ether till the dew point is reached. This is observed with great nicety by means of a silver cap; for the in stant the dew commences to deposit, the bril liant polish of the silver is dulled. The temper ature of the air is at the same time read off by means of the other thermometer in 0'. Reg nault's hygrometer, both from its construction and from the use of the aspirator, avoids the too great proximity of the observer, which from the nature of the experiments, is objec tionable.
Mason's dry and wet bulb hygrometer (some times called psychrometer or hydrodeik) con sists of two thermometers arranged side by side as in the figure. The dry bulb gives the tem perature of the air at the time of observation; and the other bulb, which is covered with mus lin, and kept moist by filaments of cotton car ried from it into a small cistern of rain or distilled water, reduces the height of the mer cury in its tube in proportion to the capacity of the air for drying or taking up additional vapor. This instrument does not give the dew point directly. The difference between the read ings of the two thermometers is multiplied by a special factor for every temperature of the dry bulb. The United States Weather Bureau psychrometer has two thermometers mounted parallel on a jointed handle that permits their being whirled. One thermometer bulb is cov ered with wet muslin, the other exposed to the air; the difference in the record of the two thermometers is greater as the air is drier.