All About Engines

Raising Steam 83 in charge is at once drawn by the noise of the escaping steam to the steam and watergauges. For locomotives and marine boilers the dead- weight and lever safety valves are unsuitable, and the valves are held down on their seatings by springs. The tendency of the age is to make fool-proof as far as possible every mechanical device the use of which is attended with danger. So in spite of water gauges, water-level alarms, and water-level regulators, and men in charge, there is always one other fitting to a boiler m which, while it may not prevent an accident, reduces the damage that results from it. On the top of the fire- box, which is liable to be- come overheated if the water level falls too low, is a safety plug. This consists merely of a stud with a hole through the centre, and the hole is Fig. 46.—Safety valve with high and low-water floats filled with an alloy which remains solid so long as it is covered by water, but melts if it is uncovered. If it melts a jet of steam enters the firebox or furnace, the pressure in the boiler is relieved, and the fire is put out. No insurance company will accept risks unless this precaution is adopted. Prevention of Waste We have considered the various contrivances which are necessary to raise steam with safety, and