The relative humidity usually varies greatly through the day, being generally lowest when the temperature is highest, and _vice versa_. It is an element of much practical interest, because it is one of the main factors in determining the drying power of the air, the other important one being wind. The air feels dry when evaporation proceeds rapidly from our skin, either on account of low relative humidity, brisk air movement, or both. People are hardly conscious of high relative humidity except when, in hot weather, it retards the evaporation of perspiration, and the latter collects in liquid form on the skin. [Illustration: THERMOGRAPH] Relative humidity does not owe its importance in human affairs solely to its physiological effects, for it plays a prominent part in numerous industries--textile, metallurgical, chemical, leather, food, and all those employing drying processes. In the spinning of cotton and wool, for example, the humidity of the workroom greatly affects the weight of the material, the size of the yarn, and the length and flexibility of the fibers. Humidity must likewise be taken into account in such diverse industries as manufacturing candy, bread, high explosives and photographic films, drying macaroni and tobacco, and operating blast furnaces. There are engineers who specialize in the business of installing “humidifying” and “dehumidifying” systems in workshops, and also, for hygienic purposes, in schoolhouses and other public buildings. The absolute humidity, the relative humidity and the _dew point_ (the temperature to which the air must be brought to start condensation of its moisture) are all determined by means of instruments called _hygrometers_. The hair hygrometer depends for its action upon the fact that a hair, freed from oil, not only absorbs moisture from the atmosphere, but elongates when damp and contracts when dry. The instrument, which includes a single human hair or a bundle of such hairs, is so designed that these changes move an index over a graduated scale. This and other types of hygrometer can be arranged to record their own readings continuously, constituting a _hygrograph_.