The "horse of a boiler is often spoken of ; but the term is a loose one, with out any definite significance, because the horse power that can be realized from a boiler de pends to a great extent upon the engine that is used to develop the power and upon how hard the boiler is forced. The Centennial Commission adopted, as the definition of a horse power (when that expression is used in connection with a boiler), the "evaporation of 30 pounds of water per hour, when the tem perature of the feed water is 100° F., and the pressure of the steam is 70 pounfls per square inch, as read from the gauge' Steam boilers may explode from any one of a great variety of causes. Of these three are specially worthy of mention: (1) The boiler may be poorly made or poorly designed, so that even when it is new it is not capable of safely withstanding the load that is put upon it. All boilers, however well made, should have a "factor of safety') of five; that is, they should be able to sustain, without bursting, a pressure five times as great as the regular working pressure. (2) A boiler, originally good, may be wasted away, either locally or generally, by corrosion or other form of de terioration, or it may develop defects in serv ice, which detract from its original strength sufficiently to lead to explosive failure. Com petent periodical inspection will materially les sen the liability to explosion from causes of this sort. (3) The water in the boiler may become low, through neglect or through the failure of the feed-apparatus, so that the metal becomes overheated or burned and loses its strength. This is the cause almost invariably assigned by the general public and even by minor "experts," when a boiler explosion oc curs, and the attendant is frequently censured for his carelessness when the explosion was really due to some totally different cause.
When an explosion is attended by great mani festations of force and energy, it is safe to conclude that a plentiful supply of water was present; for a boiler full of heated water contains vastly more energy than one that is merely filled with steam at the same tempera ture. (Consult Thurston, 'Steam Boiler Ex plosions'). Pound for pound, steam contains more energy than water, when the two are at the same temperature; but cubic foot for cubic foot (and this is the way that the comparison should be made in reasoning about a boiler explosion), the water has an enormous advan tage, owing to its greater density.
Barr, 'Boilers and Fur naces' (Philadelphia 1899); Bertin, 'Marine Boilers' (London 1898) ; Collins, 'Boilers, Piping and Pumps' ; Gebhardt, 'Steam-power Plant Engineering); Gray, 'Practical Design of Marine Single-Ended and Double-Ended Boilers' (London 1912); Haven and Swett, 'Steam Boilers and Pressure Vessels' ; Hut ton, F. R 'Mechanical Engineering of Power Plants' ; Hutton, W. S., 'Steam Boiler Con struction' ; Kent, 'Steam Boiler Economy' (New York 1910) ; Parsons, 'Steam Boilers' (ib. 1904) ; Peabody and Miller, 'Notes on Steam Boilers' ; Robertson, Leslie S., 'Water Tube Boilers' ; Skealy, 'Steam Boilers' (New York 1912) ; Thurston, R. H., 'Steam Boiler Explosions); the Code of the American So ciety of Mechanical Engineers entitled 'Stand ard . Specifications for the Construction of Steam Boilers' ; and 'Steam Boiler Rules,' formulated by the Massachusetts Board of Boiler Rules.