CHAP. 1. 10. OF CLOUDS. 37 



version of fluids into elastic vapour is attended 

 with a loss of heat ; the vapour from water, by 



same evaporating force, a strong wind will double the effect 

 produced in a still atmosphere. Thus, if the aqueous atmo- 

 sphere be correspondent to 40' of temperature and the air be 

 CO , the evaporation is the same as if the aqueous atmosphere 

 were at 60 of temperature and the air 72; and in a calm air 

 the evaporation from a vessel of six inches in diameter in such 

 circumstances would be about -9 of a grain per minute, and about 

 1 -8 grains per minute in a very strong wind ; the different imme- 

 diate quantities being regarded solely by the force of the wind. 



" Having quoted so much of this essay as may suffice to 

 exhibit the principles on which we may proceed, it may be 

 useful, before we do this, to recapitulate the following cir- 

 cumstances respecting the atmosphere of aqueous gas, or (for 

 brevity) the aqueous atmosphere. 



" 1st. It is supplied by the process of evaporation, which 

 by this theory appears to be reduced to the immediate union 

 of water with caloric into a binary compound, aqueous gas. 



" 2dly. The supply of vapour (by which term, for the 

 purposes of meteorology, we may denote aqueous gas,) is 

 regulated by the following circumstances: 1. Temperature 

 of the evaporating water, being greater as this is higher, and 

 vice versa. 2. Quantity of surface exposed. Since it is from 

 the surface only of the mass that the vapour in common 

 cases can escape, the supply is in direct proportion thereto. 

 3. Quantity of vapour already subsisting in the atmosphere : 

 the evaporation being less (with equal temperature and sur- 

 face) in proportion as this is greater, and vice versa. 



" 3dly. The vapour thus thrown into the atmosphere is 

 diffusable therein by its own elasticity, which suffices for its 



