The material chiefly employed in the manu facture of boilers is steel, but wrought-iron, cop per, bronze, brass, east-iron, and malleable iron are sometimes employed. In order to make cer tain that the material used has the proper strength and toughness, it is usually required that the plates be tested by cutting off small pieces and breaking them in a testing machine (sec TESTING MACIIINES) previous to use. After passing the tests the plates, which are shipped. to the boiler-shop perfectly flat and of carious sizes to suit the different sizes of boilers and the different uses to which they are put. are cut or sheared to the proper dimensions, the are planed. flanged, punehed for riveting, and bent by means of heavy rolls to the proper curve. The next step is to assemble the different parts and rivet them together. Finally, when the boiler is complete, it is tested by filling it with water under pressure, which determines whether it has the proper strength to carry the steam pressure. and whether there are any leaks. Very often, a boiler is also tested after it has been installed ready for operation to determine its economy and efficiency in generating steam. It is the usual praetiee to rate and sell boilers by the horse-power, this custom having originat ed early in the days of the steam-engine. The
term horse-power, as applied to a steam boiler, has a purely arbitrary significance only, since the power from the steam is developed hi the engine. and since the economy in the use of the steam depends on the engine only and often arises in different types of engines and under different conditions of maintenance and opera tion. The rule established by the American So ciety of Mechanical Engineers is to consider 30 pounds of water evaporated per hour from a temperature of IOU' F. under a pressure of 70 pounds by the gauge as equivalent to one horse-power. See lIonsE-PowEn.
Boiler explosions may occur from various causes, but the most frequent cause is the very simple one that the boiler is too weak to endure service of the regular working pressure. A new boiler may be weak through defective design or workmanship, and all old boiler is liable to be come weak through rust and general deteriora tion. The safety of a boiler from explosion is insured only by careful and correct design, hon est and thorough workmanship, and intelligent care in service. See STEAM ENUINE.