On Rain, Evaporation, ec. . 351 
excess may stand against the rain that is lost by 
evaporation from the surface of the rain-gage each 
time it rains.* Wherefore, upon the whole, we 
* Since writing the above paragraph on dew, I have 
had occasion to make several experiments on the subject 
of aqueous vapour, as it exists in the atmosphere, the re- 
sult of which will, I am persuaded, materially illustrate 
this important question in physics.—At present I shall 
only observe, that the following conclusions seem de- 
ducible from the experiments above referred to. 
1. That aqueous vapour is an elastic fluid suz generis, 
diffusible in the atmosphere, but forming no chemical 
combination with it. 
2. That temperature alone limits the maximum of va- 
“284 in the atmosphere. 
. That there exists at all times, and in all places, a 
ee, of aqueous vapour in the atmosphere, variable 
according to circumstances, 
4, That whatever quantity of aqueous vapour may 
exist in the atmosphere at any time, a certain temperature 
may be found, below which a portion of that vapour 
would unavoidably fall or be deposited in the form of 
rain or dew, but above which no such diminution could 
take place, chemical agency apart. This point may be 
called the extreme temperature of vapour of that density. 
5. And that whenever any body colder than the ex- 
treme temperature of the existing vapour is situated in the. 
atmosphere, dew is deposited upon it, the quantity of 
which varies as the surface of the body and the degree 
of cold be!ow the extreme temperature. 
N.B. The extreme temperature of vapour in the atmos- + 
phere varies all the way from the actual temperature of 
the atmosphere to 10, 15, 20 or more degrees below it.— 
The point may generally be found in the hottest months 
E 
