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 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.

Fig. 164.   Fusible Plug.

Fig. 164. - Fusible Plug.

The manhole, through which it is necessary to enter to inspect the inside of the boiler, is cut in the top or in one of the heads, and is made steam-tight by a rubber gasket. Hand-hole plugs are located in the bottom of the front and rear heads for the purpose of permitting the boiler to be cleaned. The blow-off is connected, at the bottom of the shell at the rear end, with a valve on the pipe outside of the brickwork called the blow-off valve. This valve is designed to empty the boiler and should be used every morning, so that the sediment that has settled at the bottom of the boiler overnight may be blown out. The boiler should be emptied and washed out at least once a month.