PNEUMATICS, &c. 



A BRANCH of mechanics which considers the 

 doctrine of air, or the laws according to which 

 that fluid is condensed, rarefied, or gravitates. 



Atmosphere. 



The air that encompasses the solid earth on all 

 sides. 



Its constituent parts. 



Mr. Cavendish ascertained that there is no sensible 

 difference in the constituent parts of the atmosphere, 

 under circumstances the most dissimilar. The air 

 of London, with its half million of blazing fires, equals 

 in purity the freshest breezes of the country ; nor 

 could any difference be discovered between the che- 

 mical composition of the air of a crowded room in a 

 fever hospital, and the common open atmospheric air. 

 It has also been found to be extremely uniform in 

 all parts of the world, and at all heights above its 

 surface, consisting invariably of 



By measure. By weight. 

 Oxygen - - - 21 parts 23 parts. 



Nitrogen or Azote 79 77 



100 parts. 100 parts. 



Extraneous Ingredients. 

 Although the above are the essential component 



