94 THE ATMOSPHERIC PRESSURE. 



tube DE, one end of which E opens into the vessel contain- 

 ing the fluid, and the other end D enters above the level of 

 the mercury B in the cistern. By this means 

 the gas from the vessel flows through the tube 

 ED into the cistern, and presses a column of 

 mercury into the tube AB, the height of which 

 measures the elastic force of the gas or vapor in 

 the vessel. 



When the elastic force of the fluid is consid- 

 erable, it is usual to estimate it as so many 

 atmospheres : for instance, steam, in the boiler 

 of an engine, having a pressure of two atmos- 

 pheres, signifies that its elastic force would sus- 

 tain a column of about 60 inches of mercury. 

 If it is said to have a pressure of 6 atmospheres, it means 

 that its elastic force would sustain a column of about 180 

 inches of mercury ; and so on. 



47. The Atmospheric Pressure on a Square Inch. 



This may be found at once by observing that it is the 

 weight of a cylindrical column of mercury whose base is a 

 square inch, and whose height is equal to that of the 

 barometric column. 



Since the specific gravity of mercury is 13.595, that of 

 water being 1, it follows that the pressure of the air on a 

 square inch, taking 30 inches as the height of the barometer 

 at the sea level, 



= (30x13. 595x62.5-j- 1728) Ibs. 

 = 14.7 Ibs., 



and this is called the pressure of one atmosphere. 



Sen. This pressure varies from time to time, but is gen- 

 erally between 14 and 15 Ibs. The standard usually 

 adopted where the English system of measure is used is 

 14.7 Ibs. upon the square inch, which corresponds to a col- 



