336 



FARM MOTORS 



FIG. 233 STEAM GAUGE 



boiler the air will tend 

 to straighten the tube, 

 and hence the tube acts 

 upon the needle. If it 

 is found by comparison 

 with another gauge 

 that the needle does not 

 indicate the actual 

 steam pressure it can 

 be regulated by sliding 

 the link up or down in 

 the slot at the end of 

 the pinion, thus chang- 

 ing the throw o f t h e 

 needle. 



457. Fusible plug. 

 As a safeguard against low water a fusible plug is put in 

 the boiler. In fire-box boilers it is placed in the crown 

 sheet directly over the fire, and in return-flue boilers 

 it is placed in the back end just above the upper row of 

 flues. The plug is generally made of brass about one inch 

 in diameter and with a tapered hole bored through its 

 center (Fig. 254). The tapered hole is filled with some 

 metal, generally Banca tin, which will fuse at a low tem- 

 perature, so that when the water has become so low that 

 the metal melts and runs out the steam will flow through 

 the opening and put out the fire. 



458. Safety or pop valve. It is essential that in every 

 boiler there be a safety valve so that 

 the steam may be released before too 

 high pressure has been reached. There 

 are two distinct types of these valves, 

 the ball and lever valve and the spring 

 pop valve. The former (Fig. 235) is 



FIG ' 2 ~ 



