404 Dr. Edfon on the Jfcenl of Vapour'. 
note.* I (hall, therefore, conclude with a fhort 
fummary of the whole. 
1. That, heat is the great caufe, by which 
water is converted into vapour, which is con- 
denfed by cold. 
2. That, ele&ricity renders vapour fpecifi- 
cally lighter, and adds to its abfolute heat, 
repelling its particles; which particles would 
be condenfed by cold : and that, electricity is 
the great agent by which vapour afeends to the 
upper regions. 
* Fogs are produced by two caufes as different as their 
effe&s are oppofite. A fog may be be produced by a preci¬ 
pitation of rain, in very fmall particles, like a cloud floating 
on the furface of the earth. In this cafe the air is moift 
and damp, and never fails to wet a traveller’s cloaths; the 
ftones of the ftreet, painted doors, and hard, cool, fmooth 
bodies are generally covered with moifture, which ofteri 
runs in large drops: this, I dare fay, has been obferved 
by every perfon. Secondly, a fog may be produced by 
the abforption of moifture, when the air is too dry, and 
differs from the other juft deferibed; for it will not im¬ 
part any of its moifture even to dry bodies, ho damp is to 
be met with on ftones, polifhed marble, See. This' fad is 
well known to the inhabitants on the fea coaft of Fifefhire, 
who during their fummer months, have frequent opportu¬ 
nities of obferving a fog in the afternoon, driving up the 
Firth of Forth, with a drying eaft wind, which often blafts 
the trees and young vegetables, and therefore, in a fmall 
degree, refembles the Harmattan in drying up the ground, 
and robbing vegetables of their moifture. 
3. That 
