31(3 APPLIED SCIENCE 



particularly in the parts near the opening of the feed pipe, 

 causing these parts to contract. This contraction strains the 

 seams and the plates more or less severely, according to the 

 temperature and volume of the water introduced. Draughts 

 of cold air have the same effect, often resulting in leaky tubes 

 and seams. 



350. Boiler Repairs. Boilers may be repaired by placing 

 a hard or soft patch on the defective part. For a hard patch, 

 the defective part is cut out, rivet holes are drilled or punched 

 around the opening, and the patch is applied and calked. A 

 soft patch is made by placing a piece of boiler plate over the 

 place that threatens to give way. The plate is held in place 

 with ^s or % in. countersunk screw bolts. A piece of sheet 

 packing covered with red lead is often put under the patch, or 

 the red lead is used alone. 



351. Principal Parts of a Boiler. The principal parts of 

 .a boiler are the shell, tubes, fusible plug, hand-hole, safety 

 valves, and water gauge. The shell and tubes have already 



been explained. A fusible plug is a 

 brass plug with a tapering center of 

 Banca tin (Fig. 164). The large end 

 is put next to the pressure to prevent 

 the soft metal from blowing out. This 

 plug is screwed into the rear head of a 

 boiler not less than 2 in. above the top 



FIG. 164.-Fusible Plug. row of tubes > and extends 1 in. into 

 the water to prevent its becoming 



scaled. If the water "shrinks" below this plug the soft 

 metal melts, allowing steam to escape, and thus giving 

 timely warning. 



