466. It may be remarked that the time of the year is the most unfavorable to the work of the engines; as, from the abundance of water in the mines, many are pushed beyond their most advantageous rate. 467. Duty of the best whim engines, rotatory, double, 6,000,000 drawn one foot high, by each bushel of coals, or thirty kibbles, drawn from the depth of 100 fathoms by ditto. Duty of the best stamping engines, rotatory, double, 15,000,000 lifted one foot high, by each bushel of coals. 468. The monthly report of the duty of steam engines in Cornwall is taken and computed by Messrs. John and Thomas Lean, who are specially appointed and paid for that purpose by the adventurers in the mines, whose object is to obtain a correct comparative statement, by which they may ascertain the merits of the respective engines, and may judge of the skill and care of the engineers they employ. 469. Messrs. Leans have the custody of the keys of the counters on the engines, they themselves measure the capacity and lengths of the pumps, and they receive the returns of the quantity of coals consumed from the persons who measure it, and make oath of the consumption at the custom-houses for the debenture which is allowed. 470. The engineers whose names are given are not manufacturers of engines, nor are they allowed to participate in any business of that kind; they plan the construction and superintend the execution and erection of engines, for which they are paid according to the power of each; and they have the care of them after being erected, and direct repairs, &c., for which they receive regular salaries from the mines. 471. The Royal Academy of Paris has been called upon by the government, to report on the means proper to be adopted for the prevention of accidents and injury from the explosion of steam engine boilers. The means proposed had the double object of preventing the rupture of the boilers, or, in case of their destruction, preventing injury to neighbouring buildings. They directed that the boiler should be proved by the hydraulic press, with a force five times that which they would have to bear during the working of the engines: that a safety valve should be attached to the boiler and locked up, the valve being so loaded as to open at a pressure just above that by which the boilers have been tried that the boiler should be surrounded by a wall of masonry one metre (39-371 inches) in thickness; an interval of a metre being left between the boiler and the wall, and again between the wall and the neighbouring buildings. Another precaution has been added by M. Dupin, and adopted by the academy; namely, the introduction of a metallic plug into the upper surface of the boilers, formed of such an alloy as should melt at a temperature a few degrees above that at which the engine is intended to work. 472. In consequence of this application, it became necessary to form a table of the pressure and temperature of vapor. The academy appear very doubtful of the estimates as yet pub lished, but give the following table up to eight atmospheres, as nearly correct: above that they say it was impossible to go without farther experiments. It is advised that no direction should be given for the composition of the fusible plugs or plates, but their preparation entrusted to some competent person who should be responsible for the accuracy of their fusing points. The fittest place for them, all things being considered, is the upper surface of the boiler. Their proper diameter and thickness have not yet been ascertained; they should be such as to bear the force of the vapor without risk of breaking; and, when the plate is fused, to leave an aperture sufficient for the ready escape of the vapor. 473. Mr. Prideaux has furnished a series of ingenious arguments, illustrative of the use of high pressure steam, that may be here quoted. He states, i. That the caloric of steam, in contact with water, is a constant quantity at all temperatures. ii. That every elastic fluid, at a given density, has its expansive force in proportion to its temperature, increasing for each ascending degree of Fahrenheit. iii. That every elastic fluid, at a given temperature, has its expansive force directly as its density. 474. From these premises it follows that the force of steam is directly as its density multiplied by 1 for each degree of increased temperature, the caloric corresponding with the density alone. For instance: steam at 212° has an elastic force 30 inches of mercury; and at 300° = nearly 140 inches, neglecting fractions. 475. By the second law, steam of the density due to 212° raised 88° with a geometrical increase of for each degree, shall gain about 5-6 inches; or possess at 300° a force = 35.6 inches of mercury. 476. And by the third the density due to 300° shall be as 35'6 inches to the force found=140 inches, or about 3.9 times greater than at 212o. But the caloric being constant is in simple proportion to this density; and the fuel consumed must be expected to correspond with the caloric. Then 30 inches x 39 117 inches, the force due to the density at 300°, deducted from 140 inches, the force found by experiment, gives 23 477. If this example be just, the weight of 478. It is plain, from the nature of geometrical progression, that this profit will increase as the temperature is more elevated: if we work at 600° the force of each pound of steam will be double of that at 212°; and if we go up to 960° or 980°, it will be quadruple; the caloric, and consequently the fuel, remaining a constant quantity. It is easy to illustrate this from the reports of working engines; but the effects in these cases are dependent on such mixed causes that no uniform conclusion can be drawn from them. In taking the caloric contained in the steam as a measure of the fuel consumed, there is not exact precision: radiation will of course increase with temperature; but I thought this might probably be more than compensated by the diminished surface of the vessels; and that, in the rapid action of a steam engine, it could hardly make an appreciable difference. 479. A collateral advantage of not less importance, well known to engineers, and which did not escape the sagacity of Mr. Watt, is gained in allowing high pressure steam to expand in the cylinder. Mr. Watt has given a formula for calculating the profit on this proceeding; but for a much more perspicuous demonstration we are indebted to Mr. Perkins. I II III IV V VI 480. Suppose we have to work at a pressure of ten pounds on the inch. Let the steam be raised to a force of eighty pounds on the inch, and let in one-eighth of the stroke; then stop the communication, the piston being at I. We have thus oneeighth at eighty pounds. When the steam has expanded to II the volume is doubled, and the force reduced to forty pounds (supposing the cylinder to keep the temperature constant), the mean from I to II being sixty pounds. Hence we have one-eighth at sixty pounds. When the piston reaches IV the volume is again doubled, and the force reduced to twenty pounds, the mean from II to IV being thirty pounds. This gives one-fourth stroke at thirty pounds. On reaching VIII the volume will again double itself, and the force will be reduced to one-half; thus becoming ten pounds on the inch as proposed; but the mean, from IV to the bottom, is fifteen pounds. Which makes half a stroke at fifteen pounds. Adding these quantities together, we have I. at 80 lbs. = 10lbs. I to II. = 7.5 II to IV. at 30 lbs. 7.5 VII VIII 32.5 lbs. on the inch. for the mean impetus communicated to the 179 fly-wheel by each stroke of the piston: and, as ten pounds on the inch, the power thus gained the cylinder full of steam is at a density of only appears at first view enormous. But against this must be set the irregularity of the impulse communicated to the fly, and of the temperature weight and friction of the machinery, and other supplied to the cylinder; beside the additional considerations, involving too many theoretical principles to allow of a satisfactory estimate from calculation without direct and repeated experiment. low pressure steam have however been most fully 481. The comparative advantages of high and discussed by a Committee of the French Institute, and an analysis of their labors is well worth perusal. engines, that of occupying the least possible 482. Amongst the advantages of high pressure space must be enumerated, and will be the more important as the space for their erection is more confined, or the ground more valuable: where manufactories, and private houses, are so crowded together that each establishment can obtain but a very limited space, and great power is at the larly felt; and it is no less important in the insame time necessary, this advantage is particuterior of mines, for the same reason. gines, and one that is even greater than the 483. A second advantage of high pressure enformer, is the economy of fuel which results will be readily granted, when it is stated that the from the effects of a high temperature. This repairs and expenses of the steam engines employed in draining a single large coal-mine in England amount annually to the sum of £25,500. 484. On this account several large proprietors of copper and tin mines, in Cornwall, adapted machinery to their engines, in 1811, by which an account is regularly kept of the work which they perform; and, from the results of these experiments, conducted on the largest scale, the comparative effect of the different kinds of engines has been ascertained for more than ten years. 485. In the month of August, 1818, the Cornish steam engines raised 15,760,000 lbs. one foot high, for each bushel of coals consumed. From December, of the same year, the improvements were so material, either in the management of the engines, or in some of their parts, that the mean total product was increased to 17,075,000 lbs. by the construction of new and more perfect en- In December, 1812, 18,200,000 lbs. and, since 1815, the product is even still larger, 487. By the side of this augmentation we must place that which results from the employment of Woolf's steam engines, which, as is well known, are condensing engines, and work with a pressure intermediate between that of the high and the low pressure engines. 488. Such a machine, with a double cylinder, has been constructed for the mine Whealvor, in Cornwall; the diameter of the large cylinder is fifty-three inches, and that of the small one 5.3 inches. This engine has raised 49,980,822 lbs. one foot high, by one bushel of coals, whilst the mean product of the other engines was only 20,479,350 lbs. raised to the same height. In 1815 the mean product of two of Woolf's engines was 46,255,250 lbs. 489. One of the inconveniences attending engines of mean and high pressure, is loss of power by the wear of the more delicate parts of their structure, and the consequent loss of steam; at the same time it must be admitted that the improvements in the construction of the steam vessels have materially lessened this serious evil. 490. Experiments made in France support the truth of these reports. MM. Girard and Prony have made separate comparative experiments on the power of low pressure engines, and the condensing engines of mean pressure on Woolf's system, as improved by Edwards. They find that the latter deserves the preference, as to economy of fuel, though their results do not exactly agree as to the extent of the saving in this respect; their conclusions, however, tend to the same end, and their discrepancies are referrible to particular circumstances. 491. Instead of estimating the power of a steam engine by assuming the vague and ill-defined power of a horse as unity, it would be better to assume a given weight, raised to a given height in a given time, as one hundred-weight raised one yard in a second, which might be called a power. The working force of the engine would thus be indicated by the number of powers it is equal to, which may easily be ascertained by loading the piston with a sufficient determinate weight, and marking the space it passes through, so loaded, in one second of time. The tension of the vapor being measured by its relation to the pressure of the atmosphere, taken as unity, it must always be referred to the standard barometrical pressure of thirty inches, and the temperature of 32°. 492. According to the preceding details, it may be assumed as incontestable that it is most economical to employ steam at such a temperature that its tension shall be equal to that of several atmospheres; but it is not so easy to decide to what exact tension it should be raised; or what is the mathematical law which expresses the product of steam engines' powers in the function of the temperature, and the tension resulting from it. 493. We have hitherto, says the report, compared low pressure engines only with those of mean pressure; we now proceed to campare them with high pressure engines, which, as is well known, act without condensation of the vapor. Mr. Trevithick in England, and Mr. Oliver Evans in America, are the persons who first made high pressure engines. 494. In 1814 Mr. Trevithick exported to Peru nine of these engines, for the purpose of clearing the mines of water, from the accumulation of which many of the richest had been abandoned: so effectual were the engines that the treasurer of the province proposed to erect a silver statue to Mr. Trevithick, as a memorial of the gratitude of the new world, for the services he had rendered it. 495. In Philadelphia the saving in fuel, by the substitution of one of Evans's high pressure engines, for the low pressure one previously employed, amounted to about £1250 per annum. This engine raises 20,000 tons (tonneaux) of water, about ninety-eight feet in height, every twenty-four hours, and consumes about 1535 cubic feet of wood per diem. The prime cost of the machine was rather more than £5000; whereas, according to M. Marestier, a low pressure engine, of equal power, would cost considerably more than £8000. 496. Evans's engines work with a pressure of from eight to ten atmospheres; several of them have been constructed in America; and, in 1814, the congress of the United States extended Mr. Evans's patent ten years beyond the usual period, as an acknowledgment, on the part of the republic, of the benefit his invention has conferred on his country. A similar extension was granted in England to Messrs. Boulton and Watt, for their condensing engines. 497. More lately Mr. Perkins, an American, well known by his ingenious processes for employing steel plates, instead of copper, in engraving, has surpassed all his predecessors by the boldness of his conceptions. He employs, for his moving powers, steam under a pressure of more than thirty atmospheres, and apparently with great advantage. 498. With respect to economy of fuel, we must, therefore, consider the high pressure engines, hitherto constructed, as not having attained the maximum. The use of condensed steam is yet in its infancy; and, notwithstanding the services it has already rendered us, we must consider them as far below what may still be expected, when we shall be more capable of availing ourselves of the full benefit of its effects. 499. Habit reconciles us to danger. Hundreds of sailors perish annually. by the power of the wind on the sails of our ships, and we think nothing of it, because we are become familiar to that mode of navigation. But if a steam-boat be blown up, or burnt, the accident is reported in the public prints to every corner of the world; the alarm is given, and that is looked upon as the most dangerous of all mechanical powers which perhaps is the least so in the common course of navigation, and especially on nearing the land. 500. But destruction in some shapes is more appalling to the imagination than in others. Death from explosions, accompanied with noise and confusion, seems more horrible than when it comes in a more tranquil form; and, in all our discussions on the relative dangers of different machines, we should divest them of those accessary circumstances, which frequently produce the greatest effect on the minds of the vulgar and ill STEAM ENGINE. informed. Whenever man accumulates natural powers, to effect certain purposes, they may, by mischance, be diverted from their proper courses, and become the cause of serious accidents; and no machine, by which those powers are concentrated, was ever constructed that has not its peculiar dangers. 501. To wish to employ only such machines as might be secure from the consequences of want of skill, imprudence, and rashness, were to wish to deprive ourselves of the happiest fruits of human skill and industry; at the same time it were a culpable neglect to suffer any man, for the sake of attaining an end of secondary importance, to employ means which might obviously endanger the lives and property of his neighbours. In such a case public authority has a right to interfere, and exercise a beneficial and protecting influence. 502. Does this observation apply to steam engines in general, or only to a particular class? Should the use of high and mean pressure engines be restricted to certain situations? 503. The British parliament has lately taken this subject into serious consideration, and has adopted most of the precautions recommended by a committee of the House of Commons appointed to enquire minutely into it, particularly with the view of obviating the dangers to which steam passage-boats are liable from ill-constructed machinery, carelessness, or mismanagement. The committee particularly recommended that the boilers of the steam engines shall be made of wrought iron or copper, and furnished with safety valves, of proper size and form, one of which shall be so secured as to be inaccessible to the workman who has charge of the engine: it also recommends that this valve shall be loaded only with such a weight that the pressure shall never exceed one-third of that which the boiler has been found, by actual trial, to be capable of supporting without bursting, or one-sixth of its calculated strength; and that any person overloading the valve shall be liable to punishment. 504. Although the British legislature has not forbidden the use of high pressure steam engines, either in passage-boats or manufactories, the preference has been given, especially for boats, to low pressure engines; and much prejudice has been excited against the former from deplorable accidents which have occurred in America, in England, and France. Mr. Evans, however, according to Mr. Marestier, has defied his opponents to produce a single instance of the explosion of one of his engines, although they work with a pressure of ten atmospheres. But serious accidents are not confined to high pressure engines-they have happened with those of low pressure, both in England and America; and more than once explosions occasioned by the latter have been attributed to the former. 505. An account is given by Mr. Stevenson, in the Edinburgh Philosophical Journal, of a dreadful explosion which occurred near Edinburgh, of a high pressure steam-boiler; and, in France, accidents have happened both with low, mean, and high pressure engines, which require our particular attention. Explosions, which have cost many persons their lives, have happened with what are called low pressure engines, but 506. At Paris, the lower part of the boiler of a 507. Since 1815 thirty-two mean pressure engines have been sent to St. Quentin from one manufactory at Paris; and the purchasers are universally well satisfied with the service they perform. 508. It became important to ascertain if the safety of the French engines, from their introduction to the present time, be merely owing to chance, or if it be the necessary consequence of multiplied precautions in their manufacture, and the previous trials to which the boilers are submitted. On this point the following information has been obtained respecting the cast iron boilers, which are considered as the most unsafe. 509. The mean-pressure condensing engines, on Woolf's construction, are those which are made in the principal manufactory in France. With these engines the pressure may be varied from that of one atmosphere to two and a half, or three atmospheres, and is indicated by a mercurial gauge. The true boiler and boiling pipes in Woolf's engines (which must not be confounded) are made of the purest cast iron. The form of the boiler is cylindrical, its axis being horizontal. The thickness of the boilers and boiling pipes of large and small steam engines. |