EVAPORATION. 



I 



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 1 and mixing 1 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 vis 

 inertia 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 3 

 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 

 - g -\_ 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 of ^ 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 ; biit 

 the difficulty of the problem is so great, 

 that we can expect only an approxima- 

 tion towards a solution. 



The most exact set of experiments on 

 the evaporation from the earth was made 

 by Mr. Dalton and Mr. Hoyle, during 

 1796, and the two succeeding years. The 

 method which they adopted was this .- 

 having got a cylindrical vessel of tinned 

 iron, ten inches in djameter, and three 

 feet deep, there were inserted into it two 

 pipes turned downwards, for the water to 

 run off into bottles : the one pipe was 

 near the bottom of the vessel, the other 

 was an inch from the top. The vessel 

 was filled up fora few inches with gravel 

 and sand, and all the rest with good fresh 

 soil. It was then put into a hole in the 



