. [ 345 ] 



According to Mr. Watt (as ftated by De Luc, 3 Meteorolog: 

 p. 145) the fpecific gravity of pure vapour is to that of air as 4 to g, 

 I fuppofe he compares it with air at the ufual denfity of 30 or 

 29, and at fome particular temperature whicli is not mentioned j 

 for at high temperatures the difference mult be much greater, as 

 appears by the foregoing tables. 



Ma. Saussure, Hygrometer: p. 284, has given us the fpecific 

 gravity, not indeed of pure vapour, but of vapour diffolved in 

 air, with more precifion, for he tells us, 10. That a cubic foot of 

 perfedly dry air has its volume augmented by y'4 when faturated 

 with ten grains of moixlure at about 6^'' Fahr. of heat, and ba- 

 rometer 28,77 inches (Englifli). 



2do. That a cubic foot oi pure or perfedly dry air of that den- 

 fity and at that temperature weighs 751 grains (French), and after 

 diflblving 10 grains of moiflure, by which it is dilated Vt) this 

 new volume weighs 751 +10= 761 grains; but a cubic foot of 

 pure air augmented by an accelTion of i of its bulk of pure air 

 would weigh 751 + _'_ 751 =765 grains, that is 14 grains more. 

 Hence he infers that in this cafe the fpecific gravity of the dif- 

 folved, moiflure is to that of dry air as lo to 14, for 7^ of a cubic 

 foot in the one cafe weighs 10 grains, and in the other 14 grains 

 nearly. 



But 



