PNEUMATICS. 67 



parts of atmospheric air, it contains other substances, 

 which may be regarded as adventitious, and liable 

 to vary in quantity. Of these carbonic acid and 

 aqueous vapours are the most important and con- 

 stant. The quantity of the first may be usually 

 considered as less than one per cent. 



Height of the Atmosphere. * 



One hundred cubic inches of air, at the level of 

 the sea, when the thermometer is at GO , weigh 30 j 

 grains. The whole atmosphere is equal in weight to 

 a sheet of mercury, 30 inches, or to a sheet of water, 

 34 feet deep, and were its density every where the 

 same, would reach no higher than 27,000 feet or five 

 miles. But its extreme elasticity causes the upper 

 strata to expand indefinitely, and the phenomena of 

 meteors show that it exists in a state of extreme 

 attenuation at a height a hundred times greater than 

 this. 



The density of the air is reduced one half by an 

 ascent of about 1 8000 feet, while that of vapour under-, 

 goes the same degree of attenuation at the height of 

 about 4,500 feet. 



Meteors. 



Whatever is the nature of meteors, it is incontes- 

 table that they, as well as what are called falling stars, 

 come from beyond the atmosphere, and that they 

 inflame on penetrating it. Their rapidity necessarily 

 proves great projectile force. 



Atmospheric Changes. 



There can be no doubt that the series of atmos- 

 pheric changes, however apparently complicated and 

 perplexing, are as regular and determinate in their 

 natures as tides of the ocean, or the revolutions of 

 the celestial bodies, i 



