THE BOILER III 



into the crown sheet, see that it goes in a sufficient dis- 

 tance, so that the remaining lower end does not extend 

 into the fire-box so far as to incur danger of its "melting 

 out" when there is plenty of water in the boiler. The plug 

 shown in Fig. 36 has no thread at the upper end so this 

 cannot happen, but on some plugs the thread extends 

 nearly the whole length. If a plug "melts out" in the 

 field, it may be temporarily filled with lead or babbitt 

 metal if no tin is to be had. Putty or moist clay will stop 

 up the bottom of the hole while pouring. When cool, 

 rivet it a little to be sure that it is tight. A shovel or long- 

 handled iron spoon will serve to melt the metal if a ladle 

 be not at hand. The melting point of lead is 610 degrees 

 F., and of babbitt metal about 650 degrees F. A proper 

 material for soft plugs is commercial tin, the melting point 

 of which is about 450 degrees F., or an alloy of two parts 

 of lead and one of tin, having a melting point of 440 de- 

 grees F. Either will melt before the steel sheet is injured. 

 In some places the law requires the use of Banca tin in 

 fusible plugs. This metal has a melting point of 450 

 degrees F. 



Foaming. When a boiler is "foaming," the water in 

 the glass appears roily and the level changes rapidly, the 

 glass appearing full one moment and nearly empty the 

 next. Dirty water is usually the cause of foaming, alkali 

 or soap in any quantity being especially bad. No one 

 should be allowed to wash in the tank, as even a small 

 amount of soap is liable to cause trouble. On account 



