=105. Temperature of Evaporation.=--When the air is not saturated with vapour, evaporation is going on with more or less activity, according as the temperature is high or low, rising or falling. Now vapour cannot be formed without an expenditure of heat; as we invariably find that the process of evaporation lowers the temperature of the liquid from which the vapour is produced, and, by communication, that of contiguous substances also. Thus the emigrant, crossing the line under the scorching influence of the vertical sun, wraps a wet towel round his can of water, swings it in the breeze, to evaporate the moisture of the towel, and obtains a glass of cool water. So also, European residents in India, during the hot season, spread out mats in their apartments, and keep them wet, in order that the evaporation may cool the air. This principle has been applied, for the purpose of ascertaining the hygrometric condition of the air, in the instrument known as Mason's hygrometer, or psychrometer, which is now in general use, from its simplicity, accuracy, and ease of observing. 106. MASON'S HYGROMETER. =The Dry and Wet Bulb Hygrometer, or Psychrometer=, known also as Mason's hygrometer (fig. 79), consists of two parallel thermometers, as nearly identical as possible, mounted on a wooden bracket, one marked _dry_, the other _wet_. The bulb of the wet thermometer is covered with thin muslin, and round the neck is twisted a conducting thread of lamp-wick, which passes into a vessel of water, placed at such a distance as to allow a length of conducting thread, of about three inches; the cup or glass is placed on one side, and a little beneath, so that the water within may not affect the reading of the _dry bulb thermometer_. In observing, the eye should be placed on a level with the top of the mercury in the tube, and the observer should refrain from breathing whilst taking an observation. [Illustration: Fig. 79.] The _dry_ bulb thermometer indicates the temperature of the air itself; while the wet bulb, cooled by evaporation, shows a lower temperature according to the rapidity of evaporation. _To find the Dew-point._--From the readings of the two thermometers, the dew-point can be deduced by formulae (that known as Apjohn's is considered the most theoretically true), or from the valuable Hygrometric Tables by J. Glaisher, Esq., F.R.S. For practical purposes in estimating the comparative humidity, the annexed table, which is a reduction from Mr. Glaisher's elaborate work, will be sufficient; it will at least serve to assist in familiarising the inexperienced in the value of the psychrometer's indications:--