FARM MOTORS 381 



height of the water within the boiler. If the gauge cock 

 below the surface of the water be opened, a cloud of white 

 vapor will be emitted; if the cock in connection with the steam 

 space be opened, a colorless gas will escape. In this way 

 the height of the liquid may be determined at any time. It 

 is customary to put the lower gauge cock slightly above the 

 level of the crown sheet or upper tubes. 



The Gauge Glass. In addition to the gauge cocks, the 

 gauge glass is provided, which shows directly the height of 

 the water in the boiler. Care should be taken to see that the 

 gauge glass does not become clogged with sediment and thus 

 fail in accuracy. The low water condition is reached when 

 the water does not cover the heated plates of the boiler. 

 Steam is not a good conductor of heat; so if the plates become 

 uncovered they are quite sure to become so hot as to be 

 softened and perhaps destroyed by the pressure of the steam. 

 Low water is one of the common causes of boiler explosions. 



The Pressure Gauge. Another essential accessory for the 

 steam boiler is the pressure gauge. This instrument indicates 

 the pressure of the steam within the boiler in pounds per 

 square inch. The usual pressure gauge consists of a hollow 

 brass tube curved to a circle, which tends to straighten as the 

 pressure within increases. By connecting this tube with a 

 needle over a graduated dial, by suitable mechanism, the 

 pressure may be indicated directly. A siphon directly below 

 the gauge prevents steam from entering and heating the tube 

 and changing its elasticity. 



The Safety Valve. Every boiler should be provided with 

 a safety valve, which will permit the escape of the steam as 

 fast as generated, after a certain pressure has been reached, 

 in order that the pressure shall not exceed the strength of the 

 boiler. The usual safety valve is held closed by a spring 

 which may be adjusted for the desired pressure. Care should 



