THE STEAM ENGINE. 



Fig. 6. 



imes the actual pressure of the steam in 'the boiler. Various 

 methods are used for this purpose. In boilers where steam of 

 great pressure is used, the pressure is indicated by a spring gauge, 

 similar in its principle to those used for steel yards to weigh bodies 

 in commerce. The pressure of the steam acts against a valve 

 which is connected with the arm of the lever of the steel-yard, 

 the other arm being connected with the spring. In this way the 

 varying tension of the spring is made to measure the pressure on 

 the valve. 



When steam of low pressure is used, an expedient called a mer- 

 curial steam gauge is used. A bent tube containing mercury is 

 inserted into some part of the apparatus, which has free commu- 

 nication with the steam. Lot 

 ABC (fig. 6), be such a tube. The 

 pressure of the steam forces the 

 mercury down in the leg A B, and 

 up in the leg B c. If the mercury 

 in both legs be at exactly the 

 same level, the pressure of the 

 steam must be exactly equal to 

 that of the atmosphere ; because 

 the steam pressure on the mer- 

 cury in A B balances the atmo- 

 spheric pressure on the mercury 

 in B c. If, however, the level of 

 the mercury in B c be above the 

 level of the mercury in B A, the 

 pressure of the steam will exceed 

 that of the atmosphere. The ex- 

 cess of its pressure above that of 

 the atmosphere may be found by 

 observing the difference of the 

 level of the mercury in the tubes 

 B c and B A, allowing a pressure 

 of one pound on each square 

 inch for every two inches in the difference of the levels. 



If, on the contrary, the level of the mercury in B c should fall 

 below its level in A B, the atmospheric pressure will exceed that 

 of the steam, and the quantity of the excess may be ascertained 

 exactly in the same way. 



If the tube be glass, the difference of levels of the mercury would 

 be visible ; but it is most commonly made of iron ; and, in order to 

 ascertain the level, a thin wooden rod with a float is inserted in the 

 open end of B c, so that the portion of the stick within the tube 

 indicates the depth of the level of the mercury below its mouth. 



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