THE CONQUEST OF NATURE 



pressure of the atmosphere at sea level, as measured 

 by these experiments, is about fifteen pounds to the 

 square inch. 



Boyle's further experiments with the air and with 

 other gases developed the fact that the pressure ex- 

 erted by any given quantity of gas is proportional to 

 the external pressure to which it is subjected, which, 

 after all, is only a special application of the law that 

 action and reaction are equal. The further fact was 

 developed that under pressure a gas decreases at a 

 fixed rate in bulk. A general law, expressing these 

 facts in the phrase that density and elasticity vary 

 inversely with the pressure in a precise ratio, was 

 developed by Boyle and the Frenchman, Mario tte, 

 independently, and bears the name of both of its dis- 

 coverers. No immediate application of the law to 

 the practical purposes of the worker was made, how- 

 ever, and it is only in recent years that compressed 

 air has been extensively employed as a motive power. 

 Even now it has not proved a great commercial success, 

 because other more economical methods of power pro- 

 duction are available. In particular cases, however, 

 it has a certain utility, as a relatively large available 

 source of energy may be condensed into a very small 

 receptacle. 



A very striking experiment illustrating the pressure 

 of the air was made by a famous contemporary of 

 Boyle and Mario tte, by the name of Otto von Guericke. 

 He connected an air pump with a large brass sphere, 

 composed of two hemispheres, the edges of which 

 fitted smoothly, but were not connected by any mech- 



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