Steam boiler explosions, in theory and in pactice; . of safety of two, un-der load momentarily sustained, may not actually mean a factor exceeding-one for permanent loading. Materials of Engineering, Vol. I., §133;Vol. II., §295. DE VEL OPED WE A KNE SS. working-pressure not very long after the regular officialinspection had taken place. Such an example was that of the explosion of theboiler of the Westfield, in New York harbor, in June,1876. The steam ferry-boat Westfield, is one of threeboats which have formed one of the regular lines betweenNew York and Staten Island. The Westfield madeher

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Steam boiler explosions, in theory and in pactice; . of safety of two, un-der load momentarily sustained, may not actually mean a factor exceeding-one for permanent loading. Materials of Engineering, Vol. I., §133;Vol. II., §295. DE VEL OPED WE A KNE SS. working-pressure not very long after the regular officialinspection had taken place. Such an example was that of the explosion of theboiler of the Westfield, in New York harbor, in June, 1876. The steam ferry-boat Westfield, is one of threeboats which have formed one of the regular lines betweenNew York and Staten Island. The Westfield madeher noon trip up from the Island to the city, on Sunday, July 30th, and while lying in the New York slip, herboiler exploded, causing the death of about one hun-dred persons and the wounding of as many more. The boiler is of a very usual form, as represented inFig. 29, and is known as a Marine return-flue boiler.The diameter of its shell-—the cylindrical part was rup-tured—is ten feet: its thickness, No. 2 iron, twenty-eight hundredths of an inch.. Fig. 29.—Boiler of the Westfield. The evidence indicated that the explosion occurredin consequence of the existing of lines of channeling andlong existing cracks, by which the boiler was gradually H2 STEAM BOILER EXPLOSIONS. so weakened, that, six weeks after its inspection andtest, the pressure of steam being allowed by the engineerto rise slightly above the pressure allowed, the boilerwas ruptured, giving way along a horizontal seamand tearing a course out of the boiler. The common lap-joint, customarily adopted in theconstruction of boilers, is liable to such serious distortionunder very heavy pressure, as to produce leakage be-fore actually yielding, and this leakage is sometimes sogreat as to act as a safety-valve. Thus, suppose astraight strip of plate riveted up in parts as in Fig. 30.*A heavy pull will cause distortion as shown, in all casesexcept where a butt-joint is made with a covering stringon each side. If the metal