EVAPORATION, in natural into phy, is the conversion of water nto va pour, which, in consequence of becoming lighter than the atmosphere, is raised considerably above the surface of the earth, and afterwards by a partial conden sation forms clouds. It differs from exha lation, which is properly a dispersion of dry particles from a body. When water is heated to 212°, it boils, and is rapidly converted into steam ; and the same change takes place in much lower tem peratures ; but in that case the evapora tion is slower, and the elasticity of the steam is smaller. As a very considerable proportion of the earth's surface is co vered 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 eva poration must be of very great import ance in meteorology. Accordingly, ma ny experiments have been made to de termine the point by different philoso phers. No person has succeeded so com pletely- as Mr. Dalton : but many curious particulars had been previously ascertain ed by the labours of Richman, Lambert, Watson, Saussure, De Luc, Kirwan, and others. From these we learn that, 1. the 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 ma ritime countries, or those interspersed with lakes, than in inland countries. 2. Much more vapour rises during hot wea ther than during cold; hence the quan tity evaporated depends in some measure upon temperature. The precise law has been happily discovered by Mr. Dalton, who says, in general, the quantity evapo rated from a given surface of water per minute, at any temperature, is to the quantity evaporated from the same sur face at 212°, as the force of vapour at the first temperature is to the force of vapour at 212°. Hence, in order to discover the quantity which will be lost by evapora tion from water of a given tempera ture, we have only to ascertain the force of vapour at that temperature. Hence, we see that the presence of atmospheric air obstructs the evaporation of water ; but this evaporation is overcome in pro portion to the force of the vapour. Mr. Dalton ascribes this obstruction to the via inerti,e of air. 3. 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. Mr. Dalton has given a table, that shews the quantity of vapour raised from a cir cular surface of six inches in diameter in atmospheric temperatures. The first co lumn expresses the temperature ; the se cond the corresponding force of vapour ; the other three columns give the number of grains of water that would be evapo rated from a surface of six inches in dia. meter in the respective temperatures, on the supposition of there being previously no aqueous vapour in the atmosphere. These columns present the extremes, and the mean of evaporation likely to be no ticed, or nearly such ; for the first is-cal. culated upon the supposition of 35 grains loss per minute, from the vessel of 3i inches in diameter; the second 45, and the third 55 grains per minute. 4. Such is the quantity of vapour which would rise in different circumstances, on the suppo sition that no vapour existed in the atmo sphere. But this is a supposition which can never be admitted, as the atmosphere is in no case totally free from vapour. Now, when we wish to ascertain the rate at which evaporation is going on, we have only to find the force of the vapour al ready in the atmosphere, and subtract it from the force of vapour at the given temperature ; the remainder gives us the actual force of evaporation ; from which, by the table, we readily find the rate of evaporation. Thus, suppose we wish to know the rate of evaporation at the tem perature 59°. From the table, we see that the force of vapour at 59° is 0.5, or "s--s its force at 212°. Suppose we find, by trials, that the force of the vapour already existing in the atmosphere is 0.25, or the half ofs-tr. To ascertain the rate Of eva poration, we must subtract the 0.25 from 0.5 ; the remainder 0.25 gives us the force of evaporation required ; which is pre cisely one half of what it would be, if no vapour had previously existed in the at. mosphere. 5. As the force of the vapour actually in the atmosphere is seldom equal to the force of vapour of the tempera. ture of the atmosphere, evaporation, with a few exceptions, may be considered as constantly going on. Various attempts have been made to ascertain the quantity evaporated in the course of a year ; but the difficulty of the problem is so great, that we can expect only an approxima tion towards a solution.