1816.} Mr. B. Prevost on Dew. 129 
salient parts, not painted, and covered with black oxide, as iron 
usually is) remains absolutely dry. Among other parts, the heads 
of nails, whether higher than the surface of the wood, or sunk 
below it. 
I may observe, likewise, that the water deposited on the shutters 
during the night is often exhaled in the morning in the midst of a 
thick fog. 
Perhaps in explaining these phenomena we ought to consider 
separately the effect of the cold air which descends, and of the hot 
vapour which ascends. ‘The first probably cools the surface of the 
shutter only toa very minute depth, in consequence of its incon- 
ductibility, so that its temperature is lower than that of the vapour 
which ascends, although the air of the room and the inside of the 
shutters be hotter than this vapour. This seems to be confirmed 
by the dryness of the ironwork. This, I conceive, would make 
my observations agree with those of Dr. Wells. In that case, both 
of us ought to modify a little our general formula. The air has 
often less to do with these phenomena than would appear at first 
sight. As there is probably no action between the molecules of air 
and those of vapour (except what those of air may produce on each 
other, and those of vapour.on each other; and philosophers, I 
believe, are sufliciently agreed on the subject), while no chemical 
combination takes place, it is possible that the particles of aqueous 
vapour distributed through the cold air may preserve a higher tem- 
perature for a time long enough to rise to a certain height; so that 
the air in the neighbourhood of the window shutter, though colder 
than it, may notwithstanding contain a hotter vapour, &c. 
I here subjoin some observations found in my meteorological 
journal after the above was written, by inspecting it more carefully. 
Exter,| Inter, 
Hour, |Therm, | Therm. 
— — 
1813, 
Noy, 25, Evening..| 6° 30’); 8:39 | 8L {Almost calm. 
8-10) 7 83 |G.W,E. Fine. Almost calm, 
ll OO} 61 81 |G.M.&, Fog. Some stars seen, 
»| Almost calm. 
Dec, 28, Evening..| 6 45], 1°7 46. |G. W, E. Fine. Some light clouds, 
Slight fog. Calm. 
7 eo ah 3°8* |G. W. E, Very fine. Slight fog. 
Calm, 
Dec, 29, Morning,.| 8 Oj} — A] 
Eyening,.| 0 40 | — A2 
6 15] — 42 
Dec, 30, Morning..| 6 45) — 4 
Evening..|} 3 O]; — 43 
56 3] — 46 
Dec, 31, Morning..| 8 25) — 4+ 
Hvening..1 3 O1 — 43 
* The inside thermometer is suspended against the glass itself, 
* 
