In our climate the air is never completely dry, nor completely saturated with moisture, and the amount of aqueous vapour held in suspension is very variable. This fact has important bearings on many branches of industry, as also on the hygienic qualities of the atmosphere. The consideration that a certain amount of moisture in the air is necessary to the continuance of health will suggest the importance of maintaining that due proportion in the atmosphere of sick rooms, where the artificial heat so injudiciously used, often disturbs the healthful hygrometric condition of the air. Mr. GLAISHER is of opinion that the medical profession should enforce, as far as lies in their power, the use of this simple and effective instrument, which gives indications so important to the comfort of the patient. The _amount_ of moisture in the air is _estimated_ by the use of instruments called Hygrometers, which may be thus classified:— 1. Hygrometers of Absorption.—Made with hair, oatbeard, catgut, seaweed, grass, chloride of calcium. 2. Hygrometers of Condensation.—Regnault’s, Daniell’s, Leslie’s, Dyne’s. 3. Hygrometers of Evaporation.—Mason’s Psychrometer, or Wet and Dry Bulb Thermometers. By an ingenious application of the affinity of the oatbeard for moisture, Damp Detectors are constructed for tourists, commercial travellers, &c., to test moisture and avoid the consequences of sleeping in damp beds. They are strongly gilt, and resemble in size and shape a lady’s watch. [Illustration: 38. Damp Detector. Scale about 2/3.] In Saussaure’s Hygrometer the frame is of brass, and the scale of the same metal silvered. It has an attached thermometer, and the indications are the result of the contraction and expansion of a prepared human hair, consequent upon its absorbing or yielding moisture. The scale is divided on the arc of a circle, and an index needle, working on an enlarged arc, multiples the indications. Regnault’s Hygrometer (Fig. 39) consists of a thin and highly polished silver tube or bottle, into the neck of which is inserted a delicate thermometer. The bottle has a lateral tubular opening, to which is attached a flexible tube with an ivory mouthpiece. Ether is poured into the silver tube in sufficient quantity to cover the bulb of the thermometer. The ether is then agitated by breathing through the flexible tube until, by the rapid evaporation thus produced, a condensation of moisture takes place, readily observable on the bright polished silver surface, and the temperature indicated by the thermometer at that moment is the dew-point. [Illustration: 39. Regnault’s Hygrometer. Scale about 1/10.]