THEORY OF BOILER EXPLOSIONS. 201
than it can bear, the explosion has happened. To avoid danger from this cause, the flues must be properly con- structed, stayed, and strengthened by rings of strong angle iron at every 10 feet. They must be round, not elliptical. The vacuum valve must be kept in working order to prevent boilers collapsing. 227. () Bad Management.-As long as ignorant and careless men can obtain charge of boilers, accidents will certainly happen. Let us hope that, as education spreads- it has perhaps now a fair chance--no such persons will find employment where so much depends upon their intelligence, care, and attention. When men cease fastening down the safety valve, either wilfully or by neglecting to raise it from its seat, and no more surreptitiously alter the weight, we shall have fewer accidents. Such things have been done, improbable as they may appear-the American phrase of sitting on the safety valve is too true. Ignorance leads to most accidents. It was only a few years ago in Plymouth Sound, that on board one of H.M.'s vessels the water was lost through the gauges not acting properly, when the engineer, aware of what was the matter, ignorantly turned on the feed instead of taking out the fires. The remedy for bad management exists in education. But excellent authori- ties say, that the introduction of cold water has nothing to do with the explosion when plates are overhoated, for they say the water is introduced at the bottom, and only rises slowly over the surface, and gradually cools it. Tho experi- ment of putting cold water into red-hot boilers has been repeatedly tried without producing any explosion. Mr. 228. () Mr. Colburn's and the Astronomer-Royal's Theory of Boiler Explosions can hardly be called a theory on explosions, but rather a theory to account for the large amount of mischief that a boiler explosion creates. Colburn is of opinion that boiler explosions take place at but little above ordinary pressure by the rupture of a defec- tive point close to the water line, the defect being generally caused by corrosion; that as soon as the rupture takes place, immediately part of the steam escapes; instantaneously, as the pressure is taken off the boiler water, the large store of heat in the water above the boiling point generates a largo
