The steam delivered by a boiler is the basis of the measurement of its capacity or power. A boiler horse-power is the delivery of 33,320 B.T.U. per hour. It is approximately equal to the raising of 30 pounds of water per hour from a temperature of 100° Fahrenheit, to steam at a pressure of 70 pounds per square inch, or to 34 pounds of water per hour changed to steam from and at 212° Fahrenheit, at atmospheric pressure. The horse-power of a boiler is sometimes approximated by the area of its grate or heating surface. Such a method of measuring has a low degree of accuracy on account of the variations in the quality of the fuel, and the rate of combustion. For example, the rate of combustion under a locomotive boiler is high and there is less than ⅒th of a square foot of grate area and about 4.5 square feet of heating surface per boiler horse-power. The Scotch Marine type of boiler used on steam ships, has slightly more grate area and slightly less heating surface than the locomotive type of boiler, because the rate of combustion is lower. Stationary water-tube boilers may have 2 to 3 times as much grate area and heating surface per horse-power as is found in locomotive boilers. If a poor type of fuel is to be used the area of the grate should be increased about inversely as the heat content of the fuel. The approximate heat content of various types of fuels is shown in Table 29. TABLE 29 APPROXIMATE HEAT VALUE OF FUELS ─────────────────────────────────────┬────────────────┬──────────────── Fuel │ │Pounds of Water │ │Evaporated from │ │ and at 212° F. │ │ All heat │B.T.U. per Pound│ utilized ─────────────────────────────────────┼────────────────┼──────────────── Anthracite │ 13,500│ 14.0 Semi-bituminous, Pennsylvania │ 15,000│ 15.5 Semi-bituminous, best, West Virginia │ 15,000│ 15.8 Bituminous, best, Pennsylvania │ 14,450│ 15.0 Bituminous, poor, Illinois │ 10,500│ 10.9 Lignite, best, Utah │ 11,000│ 11.4 Lignite, poor, Oregon │ 8,500│ 8.8 Wood, best oak │ 9,300│ 9.6 Wood, poor ash │ 8,500│ 8.8 ─────────────────────────────────────┴────────────────┴────────────────