350 On the Moiifivailons of Clouds, and 



and made to be suspended from a balance. When any ex- 

 periment, designed as a test of the theorv, was made, a 

 quantity of water was put into one of these (generally the 

 six-inch one, which I preferred), the whole was weighed to 

 a grain ; then it was placed in an open window or other 

 exposed situation for 10 or 15 minutes, and again weighed 

 to a-certain the loss by evaporation; at the same time the 

 temperature of the water was observed, the force of th.e 

 aqueous atmosphere ascertained as above, and the strength 

 of the current of air noticed. From a great variety of ex- 

 periments made both in the winter and summer, and when 

 the evaporating force was strong and weak, I have found 

 the results entirely conforniable with the above theory. The 

 same quantity is evaporated with the same evaporating force 

 thus determined, whatever be the temperature of the air, 

 as near as can be judged; but witli the same evaporating- 

 force, a strong wind will double the effect produced in a 

 still atmosphere. Thus, if the aqueous atmosphere be cor- 

 respondent to 40° of temperature and the air be 60°, 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 •() of a grain per minute, 

 and about 1-8 grains per minute in a very strong wind ; the- 

 different intermediate quantities being regulated solely by 

 the force of the wind." 



Of the Aqueous Atmosphere. 



Hanng quoted so much of this essay as may suffice to ex- 

 hibit the principles on which we shall proc.'d, 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 

 bv this theory appears to be reduced to tn- immediate union' 

 of water with caloric into a binaiy cnuqiound, aqiHOUSgas. 



2dly, The supply of vapour (by wl)ich Icrm, ibr the pur- 

 poses of meteorology, v.'e may denote aqueous g'-s), is re- 

 gulated by the following circumstances: — 1 . Temperature 

 of thccvaporativig water, being greater as this i^ higher, and 

 vice versa. 2- {Quantity of surl'acs'cxposcd.. Smoe 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 subsisur.g m the atmo.-phere t 

 the evaporation licing i'.'ss (wilh equal temperature and sur- 

 face) ill pioporlioa as this is greater, and vuj versa. 



3dly, 



