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 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.

As an example of the beneficial effects produced upon the efficiency of an individual engine by the first application of this system of inspection, the case of the Stray Park engine may be mentioned. This engine, constructed by Boulton and Watt, had a sixty inch cylinder, and when first reported in 1811, its duty amounted to 16,000,000 pounds. After having been reported on for three years, its duty was found to have increased to 32,000,000; this estimate being taken from the average result of twelve months' performance. Its duty was doubled in less than three years.

It will appear, by inspection of the duties registered in the preceding table, that the augmentation of the efficiency of the engines has not been the effect of any great or sudden improvement, but has rather resulted from the combination of a great number of small improvements in the details of the operation of these machines. In these improvements more is due to the successful application of practical experience than to any new principles developed by scientific research. Mr. John Taylor, in his "Records of Mining," has traced the successive improvements on which the increased duty of engines depends, and has connected these improvements with their causes in the order of their dates. The following results, abridged from his estimates, may not be uninteresting:

In 1769, soon after the date of the earliest discoveries of Mr. Watt, but before they had come into practical application, Smeaton computed that the average duty of fifteen atmospheric engines, working at Newcastle-on-Tyne, was 5,590,000. The duty of the best of these engines was 7,440,000, and that of the worst 3,220,000.

In 1772, Smeaton commenced his improvements on the atmospheric engine, and raised the duty to 9,450,000.

In 1776, Watt obtained a duty of 21,600,000.

At this time Smeaton acknowledged that Watt's engines gave a duty amounting to double that of his own.

In 1778-79, Watt reported a duty of 23,400,000.
From 1779 to 1788, Watt introduced the application of

In 1798, an engine by Boulton and Watt, erected at Herland, was reported as giving a duty of 27,000,000.

This engine, which was probably the best which at that time had ever been erected, attracted the particular attention of Mr. Watt, who, on visiting Cornwall, went to see it, and had many experiments tried with it. It was under the care of Mr. Murdock, the agent of Messrs. Boulton and Watt in Cornwall. When Mr. Watt inspected it he pronounced it perfect, and that further improvement could not be expected. How singular an instance this of the impossibility, even of the most sagacious, to foresee the results of mechanical improvement! In twenty years afterwards the average duty of the best engine was nearly 40,000,000, and in forty years it was above 84,000,000.