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was the opinion moft commonly received, as Do( 5 lor 
Defaguliers tells us in his differtation on this fub- 
je(fl, publiflied, in the Philofophical Tranfaitions, in 
tlie year 1729, in which he examines and refutes 
thefe two former opinions, and endeavours to eflablifli 
his own. He afcribes the afcent of aqueous vapours 
to their being turned into an elaftic fteam, and al- 
ways rarefied more than the air is, by the degrees 
of heat, to which bodies are ufually fubjedl in the 
different feafons of the year. 
This opinion, I find, has been as ill received by 
fubfequent writers as the former ones. Mr. Clare, 
in his Treatife on the Motion of Fluids, has brought 
many objedlions againfl it ; as Mr. Rowning has 
alfb done in his Syffem of Natural Philofophy not 
long fince publifbed ; who fays, that the caufe of 
the afcent of vapours has been much difputed, but 
not yet fettled, by philofophers, and owns that he 
cannot think of any true principle of philofophy, 
upon which it may be accounted for. 
I fhall not now repeat the objedlions made by 
thofe gentlemen ; but muft beg leave to add only 
the two following; which, among many others 
that might ffill be urged, they have not taken 
notice of. 
Firfl ; If heat was the only caufe of evaporation, 
water in a clofe warm room would evaporate fafter 
than when expofed in a colder place where there is 
a conflant current of air ; which is contrary to ex- 
perience. 
Secondly ; The evaporation of water is fo far 
from depending upon its being rarefied by heat, that 
it is carried on even whilft water is condenfed by the 
U 2 coldnefs 
