*hygrometer*

n. [ Gr. &unr_; wet, moist + -meter: cf. F. hygromètre. ] (Physics) An instrument for measuring the degree of moisture of the atmosphere. [ 1913 Webster ]

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.