98 Mr. Webster's experiments respecting dew. 
the turning up of the moist and cool earth below the surface, render-’ 
ed the air under the plates cool enough to make them condensers. 
The fourth experiment may possibly favor the same theory, on a 
supposition, that a green leaf, which in summer is much cooler than 
dry earth, is a good condenser of vapor, and the better condenser at 
a little distance from the dry, warm earth, than near it, or in contact 
with it, But there are many other facts, within every man’s obser- 
vation, which strongly support the hypothesis, that dew is produced 
by a condensation of surrounding vapor. The copious ‘moisture 
upon the outside of vessels, filled with cold water in hot weather, 1 is 
certainly the effect of condensation, and is no feeble confirmation. of 
the hypothesis respecting dew. The hoar frost is merely frozen dew, 
that is, vapor first condensed into water, and then frozen, or vapor 
congealed without condensation. | This frost appears when the sur- 
face of the earth is sealed with frost, and of course the vapor, from which 
it is formed, cannot, at the time, perspire from the earth. 
T have lately moved into a new house,* the plaistering of which 
was not thoroughly dry, when I came into it. When I arose in the 
morning, I observed a copious dew upon the glass windows of the Keep- 
ing room, on the inside, The phenomena of this dew correspond ex- 
actly with those of the ordinary dew upon the earth. 
In the first place the moisture upon the windows, which were ex- 
posed to the external air and the action of cold, was copious in clear 
weather, and sometimes would collect in such quantities, as to run 
down in drops. But at the same time glass within the room, as the 
looking glass, and the face of the clock, was perfectly dry. This was 
uniformly the fact for a number of nights successively. | Hence the 
conclusion, that the exposure of the glass to the cold external air was 
x Written in December, 1791. 
