AEROSTATICS. 



The weight of air is known from the Torricellian ex- 

 periment, or that of the barometer. The air pres- 

 ses on the orifice of the inverted tube with a force 

 just equal to the weight of the column of mercury 

 sustained in it. 



A bottle, weighed when filled with air, is found heavier 

 than after the air is extracted. The pressure of 

 the atmosphere is about 14 Ib. on every square 

 inch of the earth's surface. Hence the total 

 pressure on the convex surface of the earth 

 = 10,686,000,000 hundreds of millions of pounds. 



The elastic force of the air is proved, by simply in- 

 verting a vessel full of air in water : the resistance 

 it offers to farther immersion, and the height to 

 which the water ascends within it, in proportion as 

 it is farther immersed, are proofs of the elasticity of 

 the air contained in it. 



When air is confined in a bent tube, and loaded with 

 different weights of mercury, the spaces it is com- 

 pressed into are found to be inversely as those 

 weights. But those weights are the measures of the 

 elasticity ; therefore the elasticities are inversely as 

 the spaces which the air occupies. 



Now the densities are also inversely as those spaces ; 

 therefore the elasticity of air is directly as its den- 

 sity. This law was first proved by MABIOTTE'S ex- 

 periments. 



In all this, the temperature is supposed to remain un- 

 changed. These properties seem to be common to 



all elastic fluids. 

 i 



Q 2 Air 



