BOYLE AND MARIOTTE'S LAW. 95 



umn of mercury about 30 (exactly 29.922) inches, and to a 

 column of water about 34 (exactly 33. 9) feet high. A press- 

 ure of two atmospheres, therefore, would mean a pressure 

 of 29.4 Ibs. on each square inch, and a pressure of six atmos- 

 pheres would mean a pressure of 88.2 Ibs. on each square 

 inch. (See Weisbach's Mechs., p. 777.) 



EXAMPLES. 



1. If the elastic force of a gas is 2 atmospheres, find its 

 pressure in Ibs. on each square inch. Ans. 36.75 Ibs. 



2. If the elastic force of steam in a boiler be 5 atmos- 

 pheres, find the pressure on a safety-valve whose area is 

 5.4 sq. ins. Ans. 436.59 Ibs. 



48. Boyle and Mariotte's Law.* Gases readily con- 

 tract into smaller volumes when compressed. When a gas 

 is compressed, its elastic force is increased ; and when it is 

 allowed to expand, its elastic force is diminished. -The 

 statement of the law which expresses the relation between 

 the pressure and the volume, or the pressure and the density, 

 of gases is the following : 



The pressure of a given quantity of air, 

 at a given temperature, varies inversely 

 as its volume, and directly as its density. 





N 



-K 

 F 



B 



Let ABCD be a bent glass tube, the shorter 

 branch of which can have its end D closed, 

 and both branches being vertical. Let a little M 

 mercury be poured in at A, and let it stand at 

 the same level EF in both branches. Now 

 close the end D ; a definite volume of air is Fi fl- 33 

 thus enclosed in DE under a pressure equal to 

 that of the external air, i. e., the elastic force of the enclosed 

 air DE is equal to the atmospheric pressure exerted on F in 



* The experimental proof of this law was discovered about the same time in 

 England by the Hon. Robert Boyle, and in France by Mariotte. 



