PNEUMATICS. 35 



PNEUMATICS. 



Air a compound body Figure, Height, Weight, Pressure, &c. of the Atmosphere 

 Air the supporter of animal and vegetable life Expansion and condensation 

 of Air Refractive and Reflective power The Air-Pump Air-Gun Con- 

 densing Syringe Barometer, &c. explained. 



THIS science treats of the weight, pressure, elasticity, 

 and other phenomena of that invisible fluid that sur- 

 rounds our globe. It used to be supposed that the at- 

 mosphere consisted only of two distinct substances ; air, 

 and water in a state of vapour. But it is now known 

 that atmospheric air is a compound of two distinct gases,* 

 oxygen, and nitrogen or azote, in proportion of about 

 twenty-two parts of the former, to seventy-eight parts 

 of the latter ; it also contains about one part in one thou- 

 sand of carbonic acid gas, and one part in seventy of 

 aqueous vapour. There is also in the atmosphere hy- 

 drogen* formed from the decomposition of water, and 

 other gases, together with particles of different kinds of 

 bodies constantly floating in it. 



The figure of the atmosphere, if the earth were at rest, 

 would be spherical ; but as the earth is in motion, as also 

 because of the action of the sun heating and expanding 

 the parts about the equatorial regions, it assumes that 

 of a flattened spheroid. 



The extreme height of the atmosphere may be nearly 

 fifty miles, but at this distance from the earth it must be 

 exceedingly rare ; so much so as to have no sensible effect 

 on the rays of light as to refraction or reflection ; for at 

 the height of two miles, it is seldom sufficiently dense to 

 support the clouds, although an aeronaut has ascended 

 as high as/our miles and a third. It has been calculated 

 that at the height of three miles and a half the air is 

 only one-half of the density of what it is on the surface of 

 the earth ; at seven miles it is one-fourth, at fourteen miles 

 one-sixteenth, and so on decreasing in density in a qua- 

 druple ratio, while the distance increases in a duplicate 

 ratio, so that at the height of forty-nine miles it will be 



* See the article Gaseous Bodies. 



