Dr. Eafon on the Ajcenl of Vapour. 403 
denly change to fouth weft, in a few hours, black 
clouds begin to gather, vegetables look fickly, 
and droop their leaves ; and, foon after, comes on 
a violent ftorm of thunder, with heavy rain.* 
This change, I imagine, is not fo much owing 
to the fouth weft wind bringing rain, as to the 
atmofphere’s being changed from an eledtric 
ftate, capable of fufpending vapour, to a ftate 
of parting .with its moifture. As foon as the 
ftorm is going off, vegetables revive from their 
languid ftate, and the air recovers its ufual 
■afpedt. From this we may conclude, that no 
inftrument can be made to afcertain the quantity 
of moifture in the air: all that is, or ought to 
be expedted from a hygrometer, is to fhew, 
whether the air be in a ftate to retain, or part 
with its moifture. In apparent dry weather it 
may point to rain ; and when it rains, it may 
point to fair. For this reafon, the ftones of halls, 
and fmooth fubrtances, are often bedewed with 
Wet, in dry warm weather, (that is, the air is in 
a ftate to part with its moifture) and, vice verfd , 
they will dry in the time of rain. 
Left this paper fhould exceed the common 
limits of time in reading, I ftiall pafs over thofe 
obfervations, which might be made on fogs or 
mifts; a few excepted, which I {hall fubjoin in a 
• “ Ingeminant Aullri, et denfiffiraus imber.” Virg. 
D d 2 note. 
