1374. Vapour is water rarefied by heat, in consequence of which, becoming lighter than the atmosphere, it is raised considerably above the surface of the earth, and afterwards by a partial condensation forms clouds. It differs from exhalation, which is properly a dispersion of dry particles from a body. When water is heated to 212ﾦ it boils, and is rapidly converted into steam; and the same change takes place in much lower temperatures; but in that case the evaporation is slower, and the elasticity of the steam is smaller. As a very considerable proportion of the earth's surface is covered with water, and as this water is constantly evaporating and mixing with the atmosphere in the state of vapour, a precise determination of the rate of evaporation must be of very great importance in meteorology. Evaporation is confined entirely to the surface of the water; hence it is, in all cases, proportional to the surface of the water exposed to the atmosphere. Much more vapour of course rises in maritime countries, or those interspersed with lakes, than in inland countries. Much more vapour rises during hot weather than during cold: hence the quantity evaporated depends in some measure upon temperature. The quantity of vapour which rises from water, even when the temperature is the same, varies according to circumstances. It is least of all in calm weather, greater when a breeze blows, and greatest of all with a strong wind. From experiments, it appears, that the quantity of vapour raised annually at Manchester is equal to about twenty-five inches of rain. If to this we add five inches for the dew, with Dalton, it will make the annual evaporation thirty inches. Now, if we consider the situation of England, and the greater quantity of vapour raised from water, it will not surely be considered as too great an allowance, if we estimate the mean annual evaporation over the whole surface of the globe at thirty-five inches.
