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4 



of air, when faturated with vapor in the temperature of 32°, con- 

 tains about 4 grains of water, and gains 0,1109 of a grain by 

 faturation at every degree between 32*^ and 80°, the barometer 

 being at 28,77 ; fo that at 66'^ it contains when faturated 7,7 

 grains of moifture, and about 8,7 if the barometer be at 30 

 inches*. I have often indeed found a greater proportion of 

 moifture in a cubic foot of air than is here mentioned, but then 

 it was on dark days, in which the air was not perfectly tranfpa- 

 rent, and confequently abounded in veficular vapor. 



Fourthly. The fame excellent philofopher has difcovered 

 that in the temperature of 65 '^j the elafticity of air which paffes 

 from a ftate of abfolute drynefs to that of faturation with moif- 

 ture, is encreafed yV. or as I compute ^V ; and hence he infers 

 that the weight of vapor is to that of air of the fame tempera- 

 ture as 10 to 14 j however, as the weight of common air 

 appears by my own experiments .to be much lower than he fup- 

 pofes it, I conclude the weight of vapor to be to that of common 

 air as 10 to 12. Mr. De Luc, reafoning from Mr. Watts's expe- 

 riment?, makes the ratio much greater ; but as thofe experiments 

 •were made on vapor in a boiling heat, they do not appear to 

 me to warrant that conclufion. The elaflicity of vapors differs 

 much from that of air in this refped, that any confiderable accef- 

 fion of preilion will reduce them in fome degree to the ftate of 

 veficular vapor, particularly if they arc nearly in a ftate of fatu- 

 ration in the compreffed air. 



* The weights and meafures are reduced to the Englifh ftandard. 



Fifthly. 



