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PNEUMATICS. 



x neumatics is the science which treats of the 

 mechanical properties of elastic or aeriform fluids ; 

 such as their weighty density, compressibility, and 

 elasticity. The other properties of elastic fluids 

 will be treated of under chemistry : a knowledge 

 of that science being necessary in order to under- 

 stand them. 



The air is a fluid in which we live and breathe : 

 it entirely envelopes our globe, and extends to 

 a considerable height around it. Together with 

 the clouds and vapours that float in it, it is called 

 the atmosphere. As it is possessed of gravity, in 

 common with all other fluids, it must press upon 

 bodies in proportion to the depth at which they 

 are immersed in it ; and it also presses in every 

 direction, in common with other fluids. 



It differs from other fluids in the four following 

 particulars : 1. It can be compressed into a much 

 less space than what it naturally possesses j 2. It 

 is of an elastic nature, by which it returns to its 

 former volume when the pressure is removed, the 

 force of its spring being equal to its weight ; 3. It 

 is of a different degree of density in every part 

 upwards from the earth's surface, decreasing in 



