DEVELOPMENT OF HOUSE HEATING 



137 



the curved tube is a short connecting rod, B; the other 

 end of the rod is hinged to one end of the lever C; the 

 opposite end of the lever C carries a circular set of cogs; (3) 

 these cogs on the end of the lever C 

 mesh into the cogs on the cog wheel, 

 D, which carries the pointer or index. 



As the steam pressure rises within 

 the boiler, the pressure tends to 

 cause the curved tube to straighten 

 out. The tube then pulls up on the 

 rod. The rod lifts the right-hand 

 end of the lever. The cogs at the 

 other end of the lever cause the cog 

 wheel to rotate clock- wise. The 

 index points to a scale printed on 

 the face of the gauge. There is 

 always 1 atmosphere of pressure on 

 the exterior of the boiler. Now the pressure gauge indicates 

 the excess of pressure, i.e., the pressure over and above 1 

 atmosphere, on the inside of the boiler. 



158. The Safety Valve. Safety valves are of two types: 

 The BALL AND LEVER TYPE and the POP VALVE TYPE. In the 



L-JASHER 



VALVE: 



FIG. 101. Pop safety 

 valve. 



FIG. 99. The pressure gauge. 



VALVE 



FIG. 100. Ball and lever 

 safety valve. 



ball and lever type, Fig. 100, the valve is held down by the 

 weight of the ball upon the lever. The pressure is increased 

 by slipping the ball farther out on the lever. Explain why. 



