Mr. Webster's experiments respecting dew, 97 
covered with dew? And even the under side of such leaves, as moist 
as the upper side, when a direct ascent of ‘the vapor is intercepted 
by the leaves below? I was led by these facts to suspect that dew 
may be partly, if not wholly, the effect of condensation by cold. 
The first experiment. indeed does not seem to favor the theory of 
condensation; for the papers under cover, and the China vessels them- 
selves were moist only on the under side, or side next the earth. Now 
if dew is produced by the condensation of vapor, why should not the 
upper side of a vessel, which is exposed to the external air, and thus 
to a greater degree of cold, be the best condenser antl Gorse cau. 
form the most dew ?* 2s 
ond experime x “may Ge however ex- 
plained upon this hypothesis. At the time this was made, a severe 
drowth prevailed; the earth to a depth of five or six inches was al- 
most totally exhausted of its moisture, and of course must have been 
heated during the day to a very considerable degree. Being thus 
heated and covered at sun set from the surrounding atmosphere, the 
carth under the plate might retain warmth enough during the night | to 
prevent any condensation. 
- The third experiment differed from the second in two particu- 
lars, viz. in that the moist earth from below was brought up 
to the surface, and the plates were not laid over it until ten 0” 
clock at night. ‘In this case, the atmosphere, which cools immedi- 
ately after sun set, had access to the earth more than two hours later, 
than in poner second experiment ; and cae this circumstance, with 
* Some remarks hereafter may solve this difficulty. The vapae toider the 
plates is supposed to perspire from the earth, during, the night, and a» be warmer; 
: nere 5 if so, 
ceil tes tard condensable. "The plates might not be cold enough condense 
the cooler vapor above the gue a re = 
