(140.) A given weight or measure of fuel burnt under the boiler of an engine is capable of producing a mechanical effect through the means of that engine, which, when expressed in an equivalent number of pounds' weight lifted a foot high, is called the duty of the engine. If all the heat developed in the combustion of the fuel could be imparted to the water in the boiler, and could be rendered instrumental in producing its evaporation; and if, besides, the steam thus produced could be all rendered mechanically available at the working point; then the duty of the engine would be the entire undiminished effect of the heat of combustion; but it is evident that this can never practically be the case. In the first place, the heat developed by the combustion can never be wholly imparted to the water in the boiler: some part of it will necessarily escape without reaching the boiler at all; another portion will be consumed in heating the metal of the boiler, and in supplying the loss by radiation from its surface; another portion will be abstracted by the various sources of the waste and leakage of steam; another portion will be abstracted by the reaction of the condensed steam; and another portion of the power will be consumed in overcoming the friction and resistance of the engine itself. It is apparent that all these sources of waste will vary according to the circumstances and conditions of the machine, and according to the form and construction of the furnace, flues, boilers, &c. The duty, therefore, of different engines will be different; and when such machines are compared, with a view to ascertain their economy of fuel, it has been found necessary carefully to register and to compare the fuel consumed with the weight or resistance overcome. In engines applied to manufactures generally or navigation, it is not easy to measure the amount of resistance which the engine encounters, but when the engine is applied to the pumping of water, its performance is more easily determined.
In the year 1811, several of the proprietors of the mines in Cornwall, suspecting that some of their engines might not be doing a duty adequate to their consumption of fuel, came to a determination to establish a uniform method of testing the performance of their engines. For this purpose a counter was attached to each engine, to register the number of strokes of the piston. All the engines were put under the superintendence of Messrs. Thomas and John Lean, engineers; and the different proprietors of the mines, as well as their directing engineers, respectively pledged themselves to give every facility and assistance in their power for the attainment of so desirable an end. Messrs. Lean were directed to publish a monthly report of the performance of each engine, specifying the name of the mine, the size of the cylinder, the load upon the engine, the length of the stroke, the number of pump lifts, the depth of the lift, the diameter of the pumps, the time worked, the consumption of coals, the load on the pump, and, finally, the duty of the engine, or the number of pounds lifted one foot high by a bushel of coals. The publication of these monthly reports commenced in August, 1811, and have been regularly continued to the present time.
The favourable effect which these reports have produced upon the vigilance of the several engineers, and the emulation they have excited, both among engine-makers and those to whom the working of the machines are intrusted, are rendered conspicuous in the improvement which has gradually taken place in the performance of the engines, up to the present time. In a report published in December, 1826, the highest duty was that of an engine at Wheal Hope mine in Cornwall. By the consumption of one bushel of coals, this engine raised 46,838,246 pounds a foot high, or, in round numbers, forty-seven millions of pounds.
In a report published in the course of the present year (1835) it was announced that a steam engine, erected at a copper mine near St. Anstell, in Cornwall, had raised by its average work 95 millions of pounds 1 foot high, with a bushel of coals. This enormous mechanical effect having given rise to some doubts as to the correctness of the experiments on which the report was founded, it was agreed that another trial should be made in the presence of a number of competent and disinterested witnesses. This trial accordingly took place a short time since, and was witnessed by a number of the most experienced mining engineers and agents: the result was, that for every bushel of coal consumed under the boiler the engine raised 125-1/2 millions of pounds weight one foot high.
(141.) It may not be uninteresting to illustrate the amount of mechanical virtue, which is thus proved to reside in coals, in a more familiar manner.
Since a bushel of coal weighs 84 lbs. and can lift 56,027 tons a foot high, it follows that a pound of coal would raise 667 tons the same height; and that an ounce of coal would raise 42 tons one foot high, or it would raise 18 lbs. a mile high.
Since a force of 18 lbs. is capable of drawing 2 tons upon a railway, it follows that an ounce of coal possesses mechanical virtue sufficient to draw 2 tons a mile, or 1 ton 2 miles, upon a level railway.[53]
The circumference of the earth measures 25,000 miles. If it were begirt by an iron railway, a load of one ton would be drawn round it in six weeks by the amount of mechanical power which resides in the third part of a ton of coals.
The great pyramid of Egypt stands upon a base measuring 700 feet each way, and is 500 feet high; its weight being 12,760,000,000 lbs. To construct it, cost the labour of 100,000 men for 20 years. Its materials would be raised from the ground to their present position by the combustion of 479 tons of coals.
The weight of metal in the Menai bridge is 4,000,000 lbs., and its height above the level of the water is 120 feet: its mass might be lifted from the level of the water to its present position by the combustion of 4 bushels of coals.[54]