Such instruments, however ingenious, are not of scientific value; because they do not admit of rigid comparison, are liable to alter in their contractile and expansive properties, and cannot be made to indicate precisely alike.
99. Dew-Point.—The amount of water which the air can sustain in an invisible form increases with the temperature; but for every definite temperature there is a limit to the amount of vapour which can be thus diffused. When the air is cooled, the vapour present may be more than it can sustain; part will then be condensed as dew, rain, hail or snow, according to the meteorologic circumstances. The temperature which the air has when it is so fully saturated with vapour that any excess will be deposited as dew, is called the dew-point.
100. Drosometer.—“To measure the quantity of dew deposited each night, an instrument is used called a Drosometer. The most simple process consists in exposing to the open air bodies whose exact weight is known, and then weighing them afresh after they are covered with dew. According to Wells, locks of wool, weighing about eight grains, are to be preferred, which are to be divided [formed] into spherical masses of the diameter of about two inches.”—Kœmtz.
101. Humidity.—The proportion existing between the amount of vapour actually present in the air at any time, and the quantity necessary to completely saturate it, is called the degree of humidity. It is usually expressed in a centesimal scale, 0 being perfect dryness, and 100 complete saturation.
The pressure, or tension, of vapour at the dew-point temperature, divided by the tension of vapour at the air temperature and the quotient multiplied by 100, gives the degree of humidity. (Regnault’s Tables should be used.)
Hence the utility of instruments for determining the dew-point.
Fig. 76.
102. Leslie’s Hygrometer.—This instrument consists of a glass syphon tube, terminated with a bulb or ball at each end, turned outwards from each other, as in fig. 76. The tube is partly filled with concentrated sulphuric acid, tinged by carmine. One of the balls is covered smoothly with fine muslin, and is kept continually moistened with pure water, drawn from a vase placed near it by the capillary attraction of a few strands of clean cotton-wick. The descent of the coloured liquid in the other stem will mark the diminution of temperature caused by the evaporation of the water from the humid surface. The drier the ambient air is, the more rapidly will the evaporation go on; and the cold produced will be greater. When the air is nearly saturated with moisture, the evaporation goes on slowly; the cold produced is moderate, because the ball regains a large portion of its lost heat from surrounding bodies; and the degree of refrigeration of the ball is an index of the dryness of the air.
“Should the water become frozen on the ball, this hygrometer will still act; for evaporation goes on from the surface of ice in proportion to the dryness of the air. Leslie estimates, that when the ball is moist, air, at the temperature of the ball, will take up moisture equal to the sixteen-thousandth part of its weight, for each degree of his hygrometer; and as ice in melting requires one-seventh of the caloric consumed in converting water into vapour, when the ball is frozen, the hygrometer will sink more than when wet by 1° in 7°; and hence, in the frozen state, we must increase the value of the degrees one-seventh: so that each of them will correspond to an absorption of moisture equal to one-fourteen-thousandth part of the weight of the air.