If in a low-pressure engine the pressure of steam in the cylinder be taken at 17 lbs. per square inch, then the volume of steam will be about fifteen hundred times that of the water which produces it. For every cubic foot of water, therefore, in the effective evaporation of the boiler, 1500 cubic feet of steam will be passed through the cylinder. If it be intended that the motion of the piston shall be at the rate of 25 strokes per minute, or 1500 strokes per hour, then the capacity of that portion of the cylinder between the steam-valve and the piston at the end of the stroke, must consist of half as many cubic feet as there are cubic feet per hour evaporated in the boiler. If the steam, therefore, be cut off at half stroke, the number of cubic feet of space in the cylinder will be equal to the number of cubic feet of water effectively evaporated by the boiler; and if a cubic foot of water effectively evaporated be taken as the measure of a horse-power, then there would be as many cubic feet in the capacity of the cylinder as is equal to the nominal power of the engine. (177.) The duty of engines varies according to their form and magnitude, the circumstances under which they are worked, and the purposes to which they are applied. In double-acting engines working without expansion, the coal consumed per nominal horse-power per hour varies from 7 to 12 lbs. An examination of the steam-logs of several government steamers made by me a few years since, gave, as the average of consumption of fuel at that time of the best class of marine engines, about 8 lbs. per nominal horse-power per hour. Since, however, no account could be obtained of the actual evaporation of water in the boiler, nor, with the necessary degree of precision, of the quantity and pressure of the steam which passed through the cylinders, this estimate must be regarded as an approximation subject to several causes of error. The question of the duty of boilers and engines applied to the [Pg297] general purposes of manufactures and navigation, is one which has not yet been satisfactorily investigated; and it were much to be desired that the proprietors of such engines should combine to establish a strict analysis of their performance in reference to their consumption of fuel, their evaporation of water, and their useful effects. The results of such an investigation, if properly conducted, would perhaps tend more to the improvement of the steam engine than any discoveries in science, or inventions in mechanical detail likely to be made in the present stage of the progress of that machine.