273. Q.—­But are 33 cubic feet of steam expended per minute equivalent to a cubic foot of water expended in the hour?

A..—­Not precisely, but nearly so.  A cubic foot of water produces 1669 cubic feet of steam of the atmospheric density of 15 lbs. per square inch, whereas a consumption of 33 cubic feet of steam in the minute is 1980 cubic feet in the hour.  In Watt’s engines about one tenth was reckoned as loss in filling the waste spaces at the top and bottom of the cylinder, making 1872 cubic feet as the quantity consumed per hour without this waste; and in modern engines the waste at the ends of the cylinder is inconsiderable.

274. Q.—­What power was generated by a cubic foot of water in the case of the Albion Mill engines when working without expansion?

A.—­In the Albion Mill engines when working without expansion, it was found that 1 lb. of water in the shape of steam raised 28,489 lbs. 1 foot high.  A cubic foot of water, therefore, or 62-1/2 lbs., if consumed in the hour, would raise 1780562.5 lbs. one foot high in the hour, or would raise 29,676 lbs. one foot high in a minute; and if to this we add one tenth for waste at the ends of the cylinder, a waste which hardly exists in modern engines, we have 32,643 lbs. raised one foot high in the minute, or a horse power very nearly.  In some cases the approximation appears still nearer.  Thus, in a 40 horse engine working without expansion, Watt found that .674 feet of water were evaporated from the boiler per minute, which is just a cubic foot per horse power per hour; but it is not certain in this case that the nominal and actual power were precisely identical.  It will be quite safe, however, to reckon an actual horse power as producible by the evaporation of a cubic foot of water in the hour in the case of engines working without expansion; and for boiling off this quantity in flue or wagon boilers, about 8 lbs. of coal will be required and 9 square feet of flue surface.

MODERN MARINE AND LOCOMOTIVE BOILERS.

275. Q.—­These proportions appear chiefly to refer to old boilers.  I wish you to state what are the proportions of modern flue and tubular marine boilers.

A.—­In modern marine boilers the area of fire grate is less than in Mr. Watt’s original boilers, where it was one square foot to nine square feet of heating surface.  The heat in the furnace is consequently more intense, and a somewhat less amount of surface suffices to evaporate a cubic foot of water.  In Boulton and Watt’s modern flue boilers they allow for the evaporation of a cubic foot of water 8 square feet of heating surface, 70 square inches of fire grate, 13 square inches sectional area of flues, 6 square inches sectional area of chimney, 14 square inches area over furnace bridges, ratio of area of flue to area of fire grate 1 to 5.4.  To evaporate a cubic foot of water per hour in tubular