After the injection had condensed the steam, and the piston was at liberty to descend, if the communication between the boiler and the cylinder was not opened at the precise instant when it had reached the limit of its downward movement, the immense weight on the piston, falling into the vacuum with a great velocity, would shake the apparatus to pieces. All this precision was required too from a mercenary attendant fourteen times every minute, at the hazard of the total destruction of the apparatus." It is obvious, then, that the further and more extended practical introduction of the engine depended on some method being discovered of making its movements self-acting. According to Desaguliers the honour of the invention of the self-acting movements is due to an idle boy of the name of Humphrey Potter, an attendant on the engine. The following is the statement: "It was usual to work with a buoy in the cylinder, enclosed in a pipe, which buoy rose when the steam was strong, and opened the injection-pipe, and made a stroke, whereby they were only able, from this imperfect mechanism, to make six or eight strokes in a minute, till a boy named Humphrey Potter, who attended the engine, added what is called a scoggan, a catch that the beam or lever always opened; and then it would go fifteen or sixteen strokes in a minute." "To scog," says a writer, in explanation of the term, 66 is a verb found in certain vocabularies throughout the north of England, implying to sculk; and this young gentleman, impelled by a love of idleness or play common to boyhood, and having his wits about him, after some meditation, devised this contrivance, by which so important an improvement was effected, and himself allowed the means of scogging for his own diversion.” Whether this is the correct account of the origin of this improvement, it is difficult to ascertain; certainly the statement of Desaguliers has never been proved to be wrong. In the year 1718 an engine was erected having self-acting movements, termed "hand-gear," the invention of Mr. Henry Beighton, an engineer of Newcastle, and which consisted of a series of tappets operated on by the beam, and by which the various cocks were opened and shut as required. The following is a description of the means employed for this purpose: The entrance to the steam-pipe was covered with a sliding-valve placed inside the boiler; this valve was worked by a lever attached to the spindle of the valve which projected through the top of the boiler, as in fig. 15, where a is the steam-pipe, b the sliding-valve, the spindle of which passes through c, and is worked by the bent lever d. To this bent lever a horizontal one e, fig. 16, is connected; the other extremity is formed into a fork at ƒ; a spindle joins the two extremities of the fork; and two stirrups, as at g, connected each side of the fork with an axle rotating on the centre h. This axle was made to move by the pins in the beam nn striking the ends of the spanners km, which were firmly fixed in the axle h. On the axle, at a position between the two prongs, a lever called a tumbling-bob was fixed, having a Y-end, or two projecting arms ii, and a weight at the other. The injection-cock was opened and shut by the mechanism shown in fig. 17. Let a a be the beam corresponding to n n, in fig. 16, having projecting-pins which strike the end of the lever d, terminated with a toothed quadrant taking into a second quadrant, fixed on the spindle of the injection-cock. The following is the operation of the apparatus: on the beam n n falling, a tappet, or projectingpin, strikes one of the spanners k; this turns the axle vibrating at h, and causes the tumbling-bob to fall over with considerable force, one of the

arms i striking the bar which joins the two prongs of the fork ef; this pulls forward the lever e towards the beam, and opens the steam-cock b,

fig. 16. Simultaneously with the striking of the tappet on the spanner k, another tappet strikes the end of the lever d, fig. 17, and operating on the quadrants fg, shuts the injection-cock h. The piston is now drawn upwards by the counterpoise. On arriving within a short distance of the top of the cylinder, a tappet strikes the spanner m, fig. 16, and causes the tumbling-bob to fall over, moving, as before, the lever e from the beam; one steam-valve is thus shut, and by ineans of the tappets, lever, and quadrants, the injection-cock h, fig. 17, is opened. By means of this contrivance the atmospheric engine was rendered self-acting. Thus improved, the machine remained for a considerable period in statu quo. Minor improvements were from time to time introduced; but it was reserved for the celebrated engineer John Smeaton to bring it to as great a state of perfection as was possibly allowed by the nature of its principle. In fig. 18 we give a diagram illustrative of the general arrangements of an atmospheric engine, after the introduction of the self-acting movements of Beighton, and 27. anterior to the improvements of Smeaton.

In 1767, Smeaton was employed to construct an engine for the New River Company, and he availed himself of an opportunity of introducing several improvements. In calculating the proportions, on considering that the stoppage of water occurring at every stroke, and putting the piston, beam, and other appliances, from a state of rest to that of motion twice every stroke, resulted in a great loss of power, he determined to work the engine slower, putting on the piston all the load it would bear, working with larger pumps. In order still further to reduce the velocity of the column of water in the pump-barrel, he made the beam oscillate on a centre out of its true centre; the stroke of the piston being then nine feet, whilst that of the pump, which lifted thirty feet, was only six feet. arrangement necessitated the employment of a long narrow cylinder, eighteen inches in diameter: with these arrangements he increased the load of the piston from seven pounds to ten pounds and a half on the inch. From

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the employment of a long cylinder Smeaton contemplated gaining other advantages, namely, " that every part of the steam being nearer the surface of the cylinder, would be more readily condensed; and, in consequence, that

a less quantity of injection-water would serve the cylinder, which would itself be more heated." Under these advantages Smeaton thought himself quite secure; "but how great," he writes, "was my surprise and mortification to find, that instead of requiring less injection-water than common, although the injection-pump was calculated to afford as much injectionwater as usual, in proportion to the area of the cylinder, with a sufficient overplus to answer all imaginable wants, it was unable to support the engine with injection; and that two men were obliged to assist to raise the injection-water quicker by hand, to keep the engine in motion.

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the same time the cylinder was so cold I could keep my hand upon any part of it, and bear it for a length of time in the hot well. By good fortune the engine performed the work it was appointed to do, as to the raising of water; but the coals by no means answered my calculation. The injection-pump being enlarged, the engine was in a state for doing business; and I tried many smaller experiments, but without any good effect, till I altered the fulcrum of the beam so much as reduced the load upon the piston from 10 lbs. to 84 lbs. per inch. Under this load, though it shortened the stroke at the pump-end, the engine went so much quicker, as not only to raise more water, but to consume less coals; took less injection-water, the cylinder became hot, and the injection-water came out at 180° of Fahrenheit; and the engine, in every respect, not only did its work better, but went more pleasantly. This at once convinced me that a considerable degree of condensation of the steam took place in entering the cylinder, and that I had lost more this way by the coldness of the cylinder than I had gained by the increase of load. In short, this single alteration seemed to have unfettered the engine. But in what degree this condensation took place under different circumstances of heat, and where to strike the medium, so as upon the whole to do the best, was still unknown to me. But resolving, if possible, to make myself master of the subject, I immediately began to build a small fire-engine at home, that I could easily convert into different shapes for experiments, and which engine was set to work in 1769." The result of the experiments conducted with this engine Smeaton carefully tabulated, and took as a guide to regulate his future practice. The engines of a large class which he afterwards erected fully verified by their performance the correctness of his assumptions, and evidenced the practical care with which he had, in this as in other matters, conducted his experiments. In 1772 he was employed to construct an engine at Long Benton Colliery, at Newcastle,-and in this he introduced the several improvements suggested by his experiments,—similar in construction to that introduced by Beighton; it was, however, "distinguished by juster proportions and greater nicety of detail than had yet been realised; and the innovations thus introduced were found to be highly beneficial in practice." The engine erected by Smeaton, and known as the "Chacewater Engine," was the most celebrated of his performances. We give, in fig. 19, a diagram showing the arrangements, derived from a plate in Smeaton's Reports (vol. ii. plate 11; published by Walton and Maberly, Paternoster Row). The diameter of cylinder was 72 inches, length of stroke 9 feet; making 9 strokes per minute with its full load of 51 fathoms of pump; capable of turning out per hour, from working barrels full 16 inches, 800 hogsheads of water, with a consumption of 13 bushels of coal, London measure, per hour. Calculated according to the modern formulæ, the power of this engine may be taken at 76 horses; but from Mr. Smeaton's statement, he estimated it at a higher rate. He says: "This engine, though not the largest that has been built, will be of considerably greater power than any I have seen; and, when worked at its full extent, will work with a power of 150 horses acting together; to keep which power throughout the twenty-four hours would require at least 450 horses to be maintained." Although there is nothing in connexion with the improvement of the atmospheric engine which can be said to be the invention of Smeaton, still the high praise is due to him of "giving the most perfect form and proportion

to those materials supplied by his predecessors and contemporaries." "The improvements"-we quote from the Artizan treatise—“ introduced by Smeaton chiefly resolve themselves into greater care in the construction of the

engines, and a better proportion and arrangement of the boiler; and involve neither the application of any new principle, nor any great expenditure of ingenuity. Before Smeaton's time, the manufacture of the engines was in the hands of very ignorant mechanics, who did not know the difference between power and force; and their perpetual aspiration was to make the piston exert a great force, without taking into account the velocity of the movement necessary to make it operate effectively. It was very rarely the case that the engine was adequately supplied with steam; and when an engine was found incompetent to its work, in consequence of this inadequacy, it was generally provided with a larger cylinder, which only aggravated the evil. Then the cylinders were very ill-bored; and the conden