BOILER. 239 BOILING-POINT. boilers there are two or more shells or drums connected by tubes of small diametei". These drums and tubes are tilled with water and the hot gases from the furnace pass around and between the tubes, and so heat the water con- tained within them. A great variety of water- tube boilers are used, some having only one drum, and others having as many as four drums. The tubes connecting these drums vary from ' i: J SCOTCH MARINE BOILEK, LO.NlilTUDINAL, SECTION. perfectly straight tubes to tubes bent to various degrees of curvature, approaching in some boilers a complete spiral. Water-tube boilers may be made light, powerful, and compact, and are well adapted for forced draught; steam may be raised rapidly from cold water or a sudden demand for increased pressure quickly met. For these rea- sons, particularly, they are largely used on war- vessels, especially of the torpedo-boat and tor- pedo-boat-destroyer classes, though most vessels in the merchant marine continue to employ the Scotch boiler. The water-tube boiler is also ex- tensively employed in large power-generating plants on land. Boiler-settings are the supports of masonry or metal upon which the boiler rests and by which it is oftentimes wholly or partly inclosed. Large stationary boilers usually have masonry settings, while portiible and semi-portable boilers have settings of cast-iron. Locomotive boilers can hardly be said to have settings unless the locomotive frame upon which they are carried may be saiil to be one. Marine boilers are seated on jdates and fastings, which are built into the bottom framing of the ship. In e.tcr- nally fired boilers the setting usually con- tains the furnace. (See Fubxack.s.) Boiler fittings or accessories comprise steam-pipes and blow-ofT pipes, safet}', check, and gate valves for various purposes (see Valves) ; dampers and regulators; feed-pumps and feed-water heaters; steam-traps and steam-separators, and steam- gauges and water-glasses, besides a number of minor contrivances for oiu^ purpose or another in the operation of the boiler. The material chiefly employed in the manu- facture of boilers is steel, but wrought-iron, cop- per, bronze, brass, cast-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 (see Testing Machines) previous to use. After passing the tests the plates, which are shipped to the boiler-shop perfectly flat and of various sizes to suit the dift'erent sizes of boilers and the different uses to which they are put, are cut or sheared to the proper dimensions, the edges are planed, flanged, punched for riveting, and bent by means of heavy rolls to the proper curve. The next step is to assemble the ditlereut parts ami 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 practice 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, lias a purely arbitrary significance only, since the power from the steam is developed in the engine, and since the economy in the use of the steam depends on the engine only and often arises in difl'erent 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 temjierature of 100° F. under a pressure of 70 [(ounds by the gauge as equivalent to one horse-power. See Horse-Power. 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 an 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 E.nuine. BiBiaoGR.-vPHT. The literature on boilers is very extensive, and students have the choice of a number of standard books treating of the his- tory, design, and construction of such apparatus. The following works will be found to cover the subject thoroughly: Thurston, .1 Manual of the Steam Hoilcr ('Sew Vork, ISOG) ; Ban% Boilers and Furnaces (Philadelphia, 1899) ; Ber- tin. Murine Jioilers (London, 1898), BOILING. See Cookery. BOILING-POINT. The temperature atwhieh a liquid substance boils. Evaporation of water takes place, more or less ra7)idly, at all tem- peratures (even at temperatures at which it rapidly freezes), no matter how great the pres- sure of air or other gases on its surface. When some water is placed in a clo.sed vessel, evap- oration will go on until the escaping vapor has attained a certain definite pressure of its own, whose magnitude is dependent on nothing but the temperature of the water; this pressure of water-vapor is called the vapor-tension of water at the given temperature. N'ow, though the m.agnitude of vapor-tension has nothing to do with the presence or absence of air in the vessel, the rate at which evaporation proceeds is greatly influenced by the pressure which air inclosed within the vessel may exert on the sur- '
