CHAPTER VIII. 

 BOILER PARTS. 



FUSIBLE PLUGS. 



A very necessary adjunct to the boiler, especially 

 in the case of traction engines, is the fusible plug, 

 which is placed in the crown sheet to give notice of 

 low water. The plug consists of a brass bushing filled 

 with an alloy of tin, lead, and bismuth which melts 

 at a low temperature. So long as the furnace crown 

 is kept well covered with water, its comparatively low 

 temperature prevents the plug from melting and it 

 remains intact. But the moment that the water gets 

 low enough to uncover the top of the plug, it melts 

 quickly and the combined water and steam from the 

 boiler rushes down on the fire and so reduces the heat 

 before the crown sheet is damaged. This fusible 

 plug of alloy is inserted in a bushing which is screwed 

 home in a hole in the crown sheet. 



GRATES. 



The ordinary type of furnace grate is made of cast 

 iron bars which are placed side by side in the furnace. 

 The thickness of the lugs cast on the sides of the 

 bars determines the width of the air spaces between 

 the bars. These spaces admit air to the bed of fuel; 

 consequently the width of the spaces depends upon 

 the fuel to be burned and the amount of air it needs. 



in 



