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 horsepower 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.

(175.) In expressing the duty of engines, it would have been desirable that the duty of the boiler should have been separated from that of the engine.

The duty of a boiler is estimated by the volume of water evaporated by a given quantity of fuel, independently of the time which such evaporation may take. The duty, therefore, will be expressed by the number of cubic feet of water evaporated, divided by the number of bushels of coal necessary for that evaporation, supposing the bushel of coal to be the unit of fuel. It will be observed that the duty of an engine or boiler is entirely distinct from, and independent of, its power. One boiler may be greater than another in power to any extent, while it may be equal to or less than it in duty. A bushel of coals may evaporate the same number of cubic feet of water under two boilers, but may take twice as great a time to produce such evaporation under one than under the other. In such a case the power of one boiler will be double that of the other, while their duty will be the same.

In like manner, a bushel of coals consumed in working two engines may produce the same useful effect, but it may produce that useful effect in the one in half the time it takes to produce it in the other. In that case the duty of the engines will be the same, but the power of the one will be double that of the other.

In fine, power has reference to time,-duty, to fuel. The more rapidly the engine produces its mechanical effect, the greater its power will be, whatever may be the fuel consumed. in working it. And, on the other hand, the greater the useful effect produced by a given weight of fuel, the greater will be the duty, however long the time may be which the fuel may take to produce the useful effect.

(176.) The proportion of the stroke to the diameter of the cylinder must be determined by the velocity intended to be given to the piston. With the same capacity of cylinder, and the same evaporation in the boiler, the velocity of the piston will augment as the magnitude of its diameter is diminished.

The proportion of the diameter to the stroke of the cylinder is very various. In engines used for steam-vessels the length of the cylinder very little exceeds its diameter. In land engines, however, the proportion of the length to the diameter is greater. It is maintained by some that the proportion of the diameter and length of the cylinder should be such as to render its surface exposed to the cooling of the external air, the smallest possible. Tredgold has maintained that since, during the stroke, the steam is gradually exposed to contact with the surface of the cylinder from the top to the bottom, the mean surface exposed in contact with steam being half that of the entire cylinder, the proportion of the diameter to the stroke should be such that the surface of half the length of the cylinder, added to the magnitude of the top and bottom, shall be a minimum. If this principle be admitted, then the best proportion of the diameter to the stroke would be that of one to two, the length of the stroke being twice the diameter of the cylinder; but since the whole surface of the cylinder is constantly exposed to the cooling effects of the air, and since in the intervals of the stroke there is no sensible change of the temperature of the surface, the loss of heat by cooling will in effect be the same, especially in double-acting engines, as if the cylinder were constantly filled with steam. If this be admitted, then the object should be to give the cylinder such a proportion, that its entire surface, including the top and bottom, shall be a minimum.

The proportion given by this condition would be very nearly that which is observed in the cylinders of marine engines, viz. that the length of the cylinder should be equal to its diameter.

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

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.

(178.) A strict investigation of this kind has been for many years carried on respecting the performance of the steam engines used for the drainage of the mines in Cornwall; and it has been attended with effects the most beneficial to the interests of those concerned in them. The engines to which this important inquiry has been applied being used for the purpose of pumping, are generally single-acting engines, in which steam is used expansively to a great extent. The steam is produced under a very high pressure in the boiler, and being admitted to the cylinder is cut off after a small portion of the entire stroke has been made, the remainder of the stroke being produced by the expansion of the steam.

About the year 1811, a number of the proprietors of the principal Cornish mines agreed to establish this system of inspection, under the management and direction of Captain Joel Lean, and to publish monthly reports. In these reports were stated the following particulars:-1. The load per square inch on the piston; 2. The consumption of coal in bushels; 3. The number of strokes made by the engine; 4. The length of the strokes in the pumps; 5. The load in pounds; 6. The duty of the engine, expressed by the number of pounds raised one foot high by the consumption of a bushel of coals; 7. The number of strokes per minute; 8. The diameter and stroke of the cylinder, and a general description of the engine. When these reports were commenced, the number of engines brought under inspection was twentyone. In the year 1813 it increased to twenty-nine; in 1814 to thirty-two; in 1820 the number reported upon increased

to forty; in 1828 the number was fifty-seven; and in 1836 it was sixty-one. This gradual increase in the number of engines brought under this system of inspection, was produced by the good effects which attended it. These beneficial consequences were manifested, not only in the improved performance of the same engines, but in the gradually improved efficiency of those which were afterwards constructed.

The following table taken from the statement of the duty of Cornish engines by Thomas Lean and brother, lately published by the British Association, will show in a striking manner the improvement of the Cornish engines, from the commencement of this system of inspection to the present time. The duty is expressed by the number of pounds raised one foot high by the consumption of a bushel of coals.