1437. The only perfect hygrometer, however, is that of Professor Daniell, which is a modification of that proposed by Leslie; it depends in principle on the fact that visible dew will be deposited on an artificially cooled surface at a temperature exactly proportioned to the quantity of moisture in the air. Leslie's consists of a siphon tube, with A ball blown at each end (fig. 246.) and filled with air. A coloured liquid fills one leg of the siphon; the ball on the opposite limb, smoothly coated with tissue paper, is the evaporating surface; this is kept perpetually moist by means of a thread passing from a jar with water as high as the instrument to the covered ball. The cold produced by evaporation causes the air in the ball to contract, and the coloured liquid is forced into that stem by the elasticity of the air included in the naked ball. This rise is exactly proportional to the dryness of the air. (T.)
