at A; and in the engines using coal, the heat of which can not be so perfectly controlled as that of gas, a plug of fusible metal is set into the crown-sheet, which melts when the temperature rises above a certain point, and allows the water to flow out and extinguish the fire.

The design of the boiler of the power-engine is somewhat different from that of the pumping one, but the means of automatically regulating both the steam-pressure and the water-supply are substantially the same. This engine has in addition a very ingenious governor which acts