Hygrometer \Hy*grom"e*ter\, n. [Gr. ? wet, moist + -meter: cf. F. hygrom[`e]tre.]

(Physics) An instrument for measuring the degree of moisture of the atmosphere.

Daniell's hygrometer, a form of hygrometer consisting of a bent glass tube terminating in two bulbs, the one covered with muslin, the other of black glass, and containing ether and a thermometer. Ether being poured on the muslin, the black ball, cooled by the evaporation of the ether within, is soon covered with dew; at this moment, the inclosed thermometer gives the dew-point, and this, compared with the reading of one in the air, determines the humidity.

Source: Webster's Revised Unabridged Dictionary (1913)