426 Weight of Air. [Book V. 



afterwards appear) to the neck of which is annexed a 

 itop-cock, the air may be exhaufted either by means 

 of the air-pump, or by filling the flafk with mer- 

 cury, and emptying it gradually into a veflel containing 

 utity of that Quid, and turning the cock before 

 the neck is entirely extricated, which produces a more 

 perfect vacuum than that made by the air-pump. 

 The veflel thus emptied of its air may be weighed by 

 a nice balance , and this done, re-admit the air by 

 turning the cock, when it will rufh in with confider- 

 able violence j and though the fiafk was balanced be- 

 fore, it will now become heavier, and preponderate. 

 The air contained in a quart flafk will by this experi- 

 ment be found to weigh about fourteen grains and a 

 halt 



To find the fpecific gravity of the air, the flafk muft 

 be Mired with pure water, and again weighed. The 

 weight of a cubic foot of pure diitilled water is about 

 ijOoo ounces avoirdupois, and of a cubic inch 253 

 grains and not quite one-fifth *. Dividing the weight 

 of the water contained in the flafk, therefore, by this 

 number of grains, will give the number of its cubic 

 inches ; and as this furnifhes us with the number of 

 cubic inches of air as well as of water, their relative 

 gravity is eaiiiy known. By feveral very accurate 

 experiments, Mr. Haukfbee fixed the fpecific gravity 

 of r.ir to that of water to be in proportion as i to 885. 



By means of its gravity, the atmofphere preffes 

 with great force upon all bodies, according to the ex- 

 tent. of their furface. According to M. Pafchal, the 

 quantity of this pre fibre is not Ids than 2,232 pounds 

 upon every fquare foot of furface, or upwards of fif- 



* 253-18 grains. Decimal arithmetic .fhould always be em- 

 ployed in philoibphical calculations, for the fuke of accuracy. 



teen 



