128 Mr. B. Prevost on Dew. [Fas. 
rence of temperature of the sides of the glass at the same time, 
which I think would have given me instructive results; but 1 was 
not fortunate enough to succeed. 
I mark down with regularity, in a journal containing the height 
of the internal and external thermometer, the dryness or moisture 
of the glass. I find in this journal several confirmations of what I 
have said. I even find some days in which the glass was wet without 
and moist within. 
It may be objected to me that these observations prove only the 
co-existence of this external humidity, and an external temperature 
lower than the internal; and not that this external humidity was 
deposited while the external temperature was lower. I must ac- 
knowledge that I do not recollect to have seen the moisture depo- 
siting itself on the glass, while the external temperature was lower 
than the internal; and I cannot at this moment, for reasons that I 
shall mention to you afterwards, consult the original journal of my 
experiments. But the following observation, which I have an op- 
portunity of repeating every year, does not appear to me to agree 
‘avith the empyrical principle established by Dr. Wells, “ that bodies 
are not covered with dew unless they be colder than the air.” The 
window shutters of the country house in which my experiments on 
dew are made are painted green with oil paint. When open, they 
are applied (at least in part) to the wall of the house. Now in the 
cold season, when the nights are fine, they are often so covered 
with moisture as to be dropping with water in the morning, while 
the air has been getting colder during the whole time, as is shown 
by thermometers fixed against them. This happens principally 
when the preceding day has been fine, especially if it has been hot. 
But as in this case the external air deposits moisture on these bodies 
while cooling, it follows that dew (or humidity) is deposited on 
bodies hotter than the air which surrounds them, and which gives 
out this moisture. 
It will be said, perhaps, that it is the dry and cold air of the 
higher parts of the atmosphere which, descending during the night, 
produces the cold, while the humidity is deposited from the hot and 
moist air which ascends. But upon the whole the air which descends 
uniting with the ascending air must always cool it. That air ac- 
cordingly is colder, &c. 
My chamber looks towards the north, and the roof of the house 
projects some feet over the building, so that in winter the ground 
at the bottom of the wall is in the shade. Though I have a fire in 
the room during the day, and often till pretty late at night, if in 
the evening I shut the shutters, though I leave the window open, 
still a great deal of moisture is deposited on the outside in circum- 
stances similar in other respects to those mentioned above. Here 
the external air appears to be colder than the window shutter, and 
yet it deposits a good deal of humidity on it. Yet, as I have 
already observed, all the ironwork of the shutter, whether painted 
or nol, open or shut (with the exception of some pendent or very 
a ee ee eS 
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