238 WRITINGS OF JOSEPH HENRY. [1855- 



The surface of the earth in this experiment was at first 

 completely soaked with water. 



It is evident, on account of the slowness with which vapor 

 diffuses itself through still air, that a much greater evapora- 

 tion will be produced during a brisk wind, particularly if it 

 be from a dry quarter, than during calm weather. If the 

 vapor which is formed is allowed to accumulate over the 

 evaporating surface, it will by its re-action retard the free 

 ascent of the other portions of vapor; but if it be constantly 

 removed as fast as it is formed the process will evidently 

 go on more rapidly. 



Vapor as we have seen contains a large amount of latent 

 heat, and water cannot be converted into an aeriform state 

 without the supply of the necessary quantity of this princi- 

 ple. Hence the higher the temperature, or the more freely 

 the evaporating surface is supplied with heat, the greater 

 will be the amount of vapor in a given time. 



We have seen that water immediately flashes into vapor 

 in a vacuum, and we might infer from this that the rarer 

 the air, or the more nearly it approximates to a void, the 

 less obstruction would it offer to the free production of vapor, 

 and the correctness of this inference has been satisfactorily 

 shown by direct experiment. 



We owe to Dalton a series of precise experiments on 

 the evaporation of water in air of different degrees of dryness 

 and at different temperatures. He employed in his investi- 

 gations a circular dish or pan 6 inches in diameter, about 

 an inch deep, and suspended from the beam of a balance, by 

 * which the loss of water could be accurately ascertained from 

 the variations of the weight in a given time. With this in- 

 strument he made a series of experiments while the air con- 

 tained different quantities of moisture, the amount of which 

 was ascertained by means of the dew-point method we have 

 before described in a perfectly still place and with the appa- 

 ratus exposed to a rapid draught of air. At the boiling point 

 the evaporation in still air was 120 grains in a minute ; in a 

 gentle wind, 154 grains; and with a strong wind, 189 grains. 

 A similar difference existed at the evaporating temperature 



