334 STEAM BOILER water-tubular boilers having moderate draught. In locomotive and other boilers with forced draught, the ratio of heating to grate surface rises to from 50 to 100 to 1. For the sizes of the parts of steam boilers exposed to strain, see STRENGTH OF MATERIALS. Burned in the furnace of good steam boilers, a cord of dry yellow pine, in the experiments of Prof. Wal- ter R. Johnson, evaporated 12,618*3 Ibs. of water. A cord of dry yellow pine is approxi- mately equal in heating power to 0'6 of a ton of coal, and a ton of good coal is equal in cal- orific power to 1'66 cord of soft wood. As an average, a pound of dry wood is theoretically capable of evaporating 6'66 Ibs. of water from and at 212 F. Similarly, a pound of good anthracite should evaporate 13*5 Ibs. of water. (See FUEL.) Incomplete combustion is caused by an insufficient supply of air, by imperfect intermixture of air and combustible gases from the fuel, and by the falling of fuel through the grate into the ash pit. These losses are usually largely due to unskilful management, and they amount frequently to 15 per cent. They are sometimes due to defects of design. Loss of efficiency is also produced, as already shown, by excessive air supply, which, while insuring complete combustion, lowers the temperature of the furnace. Losses occur by conduction and radiation of heat from the boiler, the fur- nace, or the flues. This can usually be reduced to a very small amount by properly protecting the apparatus by non-conducting covering. Loss may occur by the passage of the gases to the chimney before their temperature has been reduced to that required for draught. This can be prevented by providing a sufficient extent of heating surface. Incrustation and depos- its are produced by the precipitation upon the interior of the boiler of substances held in solution or in suspension by the water. Sea water precipitates sulphate of lime, and, pass- ing a concentration of f, or when it contains 36-37 per cent, of salt, it precipitates the ex- cess. River waters produce scales composed of lime carbonates and sulphates and various other mineral salts. The horse power of a steam boiler is an indefinite and inappropriate term. It was formerly assumed that the evap- oration of a cubic foot of water would yield sufficient steam to drive an engine of one horse power one hour. A moderately good modern engine should not use more than half this amount, and a good boiler should evaporate half a cubic foot an hour for each 12 sq. ft. of heating surface. A good engine of 100 horse power would therefore be supplied with steam by a good boiler of 1,200 sq. ft. area of heat- ing surface. The most economical engines recently built use only about one fourth of a cubic foot or 16 Ibs. of water per horse pow- er per hour. Steam boiler explosions occur as a consequence of ignorance or carelessness in design, in construction, or in management. Experimental explosions in Great Britain, and notably in the United States, have shown that even low pressures are sufficient to produce very violent explosions. The explosion experi- ments of Francis B. Stevens, in November, 1871 (reported by R. H. Thurston in " Journal of the Franklin Institute," 1872), were consid- ered to indicate: 1, that a most violent explo- sion may occur in a boiler well supplied with water; 2, that what is generally considered a moderate steam pressure may produce a very violent explosion of a weak boiler containing a large "body of water, and having all its flues well covered. The same writer estimated that one of the boilers exploded by Mr. Stevens contained 40,000 Ibs. of water; and that when the steam pressure was, as at the time of ex- plosion, 53 Ibs. to the square inch, the heat stored in the boiler amounted to 2,674,080 British thermal units, equivalent in mechani- cal energy to about 2,064,389,760 foot pounds, or, if wholly so expended, sufficient to raise the whole boiler, weighing 70,000 Ibs., to a height of 29,491 ft., or more than five miles. The conclusion reached was : " That it is very certain that the energy of this explosion, and all of its tremendous effects, were principally due to the simple expansion of a mass of steam suddenly liberated at a moderate pressure, by the general disrupture of a steam boiler of very uniform but feeble strength." When steam boilers are locally weak, explosion rarely oc- curs. The steam pressure produces rupture at the weakest point, and, the strength of sur- rounding parts being sufficient to prevent ex- tension of the break, no explosion occurs. Where the weakest portions of the boiler are more extended and more uniformly weak, the extent of the rupture which finally occurs be- comes greater, and the accident is attended with greater violence of disruption, and more serious results follow. Where considerable portions of the boiler are weak, or long lines of weakness exist uninterrupted by points much more defective, disastrous explosions aro very likely to take place with old boilers and at moderate pressures. The most terrible ex- plosions occur with good and uniformly strong boilers, in which, by accident or mismanage- ment, steam has been allowed to accumulate until a fatally high pressure produces rupture and drives the fragments of the boiler in all direction's. It has been shown by compiling the statistics of explosions, that the gradual ac- cumulation of steam until a pressure is reached under which the weakest portion of the boiler gives way, is by far the most usual cause. Prof. Trowbridge has shown that the time of accumulation may be calculated by a formula, mulation, in minutes, from the pressure corre- sponding to the temperature t to that of the temperature t, F. ; W is the weight of water in the boiler, and Q the quantity of heat in British units transferred to the bo-Her per min- ute. He shows that T=9'l minutes in a large marine boiler, containing 79,000 Ibs. of water,
