402 Dr. Eafon on the Afcent of Vapour. 
heat of the fun. We know, that tranfparenJ 
bodies are not heated by the fun, but opaque 
ones are; the clouds being opaque bodies, are 
warmed by the rays of the fun fhining on them, 
and any additional quantity of heat will rarify 
the vapour, and occafion its expanding in the 
air, which will foon become tranfparent. When 
vapour is made to expand, more than it would 
otherwife do, a certain quantity of abfolute heat 
is neceflary to keep it in the form of vapour 5 
therefore, when the receiver of an air-pump is 
exhaufting, it appears muddy, and a number of 
drops are found within it; the moifture contained 
in the air, in the form of vapour, being made to 
occupy a greater fpace than what is natural to 
it, and receiving no addition of heat, a part of it 
it condenfed.* 
If, therefore, the air is fuddenly rarified, a few 
drops of rain will defcend, as may often be ob- 
ferved in the fummer feafon. 
I have repeatedly obferved, efpecially during 
the fummer, when the wind is at north eaft, 
that the weather is, in general, cold and dry, 
with a clear atmofphere. Should the wind fud- 
* On this principle, we can readily account for the mill, 
which appears, on difcharging an air-gun : the condenfed 
air in the chamber of the barrel, on being fet free, will ex¬ 
pand fuddenly, occupying a larger fpace, and no additional 
heat being acquired, the vapours mult neceifarily be con¬ 
denfed in the form of mill. 
denlv 
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