(174.)

There are many circumstances which obstruct the practical application of any standard of engine-power: the magnitude of furnace, and the extent of heating surface necessary to produce any required rate of evaporation in the boiler, are unascertained; each engine-maker has his own rule in these matters, and all the rules are equally unsupported by any experimental test entitled to respect. Thus the circumstances that govern the rate of evaporation in the boiler may be regarded as almost wholly unknown. But supposing the rate of evaporation to be ascertained, the amount of power absorbed by the condensation of steam on its passage to the cylinder, the imperfect condensation of the same steam after it has worked the piston, the friction of the various moving parts of the machinery, and, above all, the difference of effect of these losses of power in engines constructed on different scales of magnitude, are absolutely unknown. We are, therefore, not placed in a condition to assign any thing more than a general account of what has been the practice of engine-makers in constructing engines which are nominally of a certain power.

In common low-pressure engines of the larger kind, to which class alone we at present refer, it has been usual, with the same fuel and under like circumstances, to allow from 10 to 18 square feet of heating surface in the boiler for every nominal horse-power of the engine. Within these wide limits the practice of engine-makers has varied. It is not, however, to be supposed, that the boiler with 18 square feet of surface per horse-power has the same evaporating power as that which has but 10. This difference, therefore, amounts to nothing more than different manufacturers of steam-engines putting into circulation boilers having powers really different while they are nominally the same. The magnitude of the cylinder is regulated by the nominal power of the engine, and it is usual so to regulate the evaporating power of the boiler, that the piston shall move at the average rate of 200 feet per minute. This being assumed, it is customary to allow about 22 square inches of piston [Pg293] surface for every nominal horse-power of the engine. If this power were in conformity to the standard already defined, this amount of surface moved at 200 feet per minute would be impelled by a pressure amounting to 71⁄2 lbs. per square inch. The safety-valve of the boiler of such engines is usually loaded at from 4 to 5 lbs. per square inch, and consequently the steam in the boiler will have a pressure of from 19 to 20 lbs. per square inch. If, therefore, the effective pressure on the piston be really only 71⁄2 lbs. per square inch, the pressure expended in overcoming the friction of the engine, and the loss consequent on the partial condensation of steam on one side and its imperfect condensation on the other, would amount to from 12 to 13 lbs. per square inch, or nearly double the assumed useful effect of the engine.

Messrs. Maudslay and Field are accustomed to allow an evaporation of ten gallons, or 1·6 cubic feet of water per hour, for each nominal horse-power of the engine. They also allow about 22 square inches of piston surface per nominal horse-power, the piston being supposed to move at the rate of 200 feet per second.[24]

The quantity of grate surface necessary in proportion to the power of the engine, has been equally unascertained, and engine-makers vary in their practice from half a square foot to one square foot per nominal horse-power.

The proportion which the magnitude of the heating surface of the boiler, and the fire surface of the grate bears to the evaporating power of the boiler, has not been determined by experiment, nor, so far as we are informed, by any well-ascertained practical results.

The estimates or rather conjectures of engine-makers, of the evaporation necessary to produce one horse-power, vary from one to two cubic feet of water per hour. It has been [Pg294] already shown that the evaporation of 900 cubic inches, or little more than half a cubic foot per hour, evolves a gross mechanical effect representing one horse-power; from which it appears, that if the evaporation of the boilers of steam engines were what engineers suppose them to be, the gross mechanical power produced in them for every nominal horse-power of the engine varies in actual amount from the power of two to that of four horses.

The above estimates must be understood as referring to double-acting steam engines above thirty-horse power. The circumstances attending the performance of single-acting engines applied to the drainage of mines, have been ascertained with much greater precision. This has been mainly owing to a spirited system of general inspection, which has been established in Cornwall, to which we shall hereafter more particularly advert.