with explosive mixture through the pipe e, from the preparing box. Just before the piston B reaches the end of its stroke, the inflaming valve g opens, and the flame of the lamp communicating to the vapor causes an explosion which instantly drives out the contents of the cylinder through the perforated arched plate 1, and flexible leathern valve D, which leather valve immediately collapses again to prevent the return

of the air.

225. The steam formed by the explosion is condensed by keeping the cylinder below the pipe E cool, by surrounding it with water in the jacket O O, and by injecting a jet of water through the cock p, (which is also worked by a cam 9, on the crank-shaft).

226. The vacuum being formed under the piston B, it descends by the pressure of the atmosphere, and its connecting rods communicate motion to the crank-shaft, in the same way as in ordinary steam engines; and, a similar operation being carried on in the second cylinder, the power is taken from the crank-shaft, and employed as a moving power.

227. The patent apparatus for generating steam, suggested by Mr. Gurney, promises to be of considerable importance as an adjunct to the high-pressure engine. The specification of his patent contains a description of two sorts of boilers. The first is a tubular vessel consisting of a series of portions, each formed like the figure 8, and these being arranged together, one behind the other successively, will form two cylindrical compartments, in the lower one of which the fire is made, on bars placed across the lower portion of the curved tubes, while the upper one serves as a funnel to convey the smoke to the chimney. Besides the curved tubes, which thus constitute the boiler, two straight tubes, of considerably larger size, are used to connect the others together and form the communication between them, one of which runs across beneath them at bottom and conveys the water to them, and the other passes across them at top, and carries off the steam from them to the working cylinder, apertures being made through them and the bent tubes at their points of contact to form passages for the water and the steam. To make the fire act more powerfully on the upper part of the lower cylindrical compartment, and also to prevent it from passing up between the tubes into the upper compartment, a plate of iron is laid along over the whole of the bottom of this latter, which compels the flame and heated vapor to pass on to its end before it can enter the upper compartment, along which latter they afterwards pass on their way to the chimney, heating in their passage the upper portions of the bent tubes. To confine the heat a double case of iron encloses the whole apparatus, between the two constituent parts of which a space is left, which being filled up with pumice-stone, or some other bad conductor of heat, makes the heat act more powerfully in the direction required. This sort of boiler may be varied by having a greater number of cylindrical compartments formed by additional flexures of the bent tubes on the principle above-mentioned.

228. The second species of boiler is composed

of two horizontal cylinders, one of which being somewhat smaller than the other is placed within it, and united to it at the edges, leaving sufficient space between the two for the reception of water. Above this double cylinder two double semicylinders, formed in a similar manner, are placed one over the other with the hollow part downwards, and the communication between them is effected by bent tubes, which pass from the upper part of the double cylinder to the lower parts of the double semi-cylinder next it at each side, and also from the upper part of this latter to the lower edges at each side of the upper double semi-cylinder. The fire is made within the double cylinder on bars placed on bearers about a third of the height from the bottom, and the flame and smoke pass from its farther end into the space between it and the first semi-cylinder, beneath which they return towards the front and pass round beneath the upper double semi-cylinder, whence they enter the chimney at its further end. The double cylinder, and the double semi-cylinders, are divided into a number of small vertical parallel compartments by bent bars of iron placed within them at regular distances, and fastened by rivets which pass through them and the parts of the cylinders and semi-cylinders with which the bars are in contact. These bent bars are somewhat shorter than the parts within which they lie, so as to leave a free space beneath them for the passage of water, or of water and steam, according to their positions; and a similar space may be left by the same means along the upper ridges of the cylinder and semi-cylinders for the passage of the steam, which may be also conveyed by separate tubes from each of the small compariments. A large straight tube runs beneath the bottom of the cylinder to convey water to it, and another proceeds along the top of the upper semi-cylinder to carry off the steam from it to the working cylinder. The whole of this boiler is to be also enclosed in a case similar to that of the first described boiler.

229. An account is next given of an apparatus for separating the steam from the water when they are forced out intermixed from tubular boilers, which consists of an upright cylinder, into the upper part of which a tube enters from the top of the boiler, and then bends round and descends to near its bottom, whence another tube returns again to the boiler at its lowest projection. Near the bottom of the cylinder a wooden valve is placed, which permits the water separated from the steam to return into the boiler; but, in case of any sudden expansion of the steam in the boiler, it rises up and prevents water from the boiler being forced by this expansion into the cylinder. From the top of this cylinder the steam proceeds to the engine through another tube, after having left behind the water with which it was first intermixed.

230. Another apparatus is after this described, whose office is to convey water to the boiler. This is also an upright cylindrical vessel, from the top of which a pipe goes to the reservoir, and from its bottom another pipe descends to near the bottom of the vessel last described, while a third tube passes down from its upper

part to about the middle of the former vessel. In the pipe that goes to the reservoir is a stopcock near the top of the vessel, and two more in the two descending pipes, which latter may be placed so as to be closed by one plug traversing the two; to these cocks are fixed cranks connected together by a bar worked by the steam engine, and the passages of the cocks are so arranged that when the two lower ones in the descending pipes are closed, that in the upper one leading to the reservoir will be open, and vice versâ. By this arrangement of the pipes the end of the one that passes from the top of the upper vessel to the middle of the lower one will be sometimes above the water and sometimes beneath it, according as its level varies in this vessel; when the first happens the patentee states that the steam will rise up through it into the upper vessel, and will force the water which this contains through the other descending pipe into the lower vessel; and when the latter case occurs, and its lower end is immersed, then the passage of the water will be interrupted. It is mentioned that this latter apparatus may be used with any other sort of steam-boiler as well as those described.

231. Finally, a method is recited for freeing the tubes of the boilers from the sediment and incrustations that are more or less formed in all boilers, which method consists in filling them, if formed of iron, with a mixture of one part of muriatic acid to 100 parts of water, or with a mixture of vinegar and water, in the proportion of a pint of the former to three gallons of the latter; and if the tubes are made of copper, then sulphuric acid is directed to be added to marine salt, in the proportion of half an ounce to a pound, and with a proper quantity of water to be put into them. These mixtures are to be used cold, and a little while after one of them is poured in, a small fire of chips is to be made under the boiler, to make the fluid circulate and bring fresh portions of it in contact with the concretions; and, when these are observed to be sufficiently dissolved, the liquor is to be run off through a cock in the bottom of the boiler along with the undissolved siliceous matter.

232. Mr. M'Curdy's apparatus has excited too much attention to be passed without particular notice. It consists of eleven cylinders, twelve feet long and twelve inches in diameter, placed horizontally in two rows, like gas retorts, in a furnace, six being in the lowest row and five in the upper one over the intervals of these, and a fire-place being made under each three of the cylinders in the lower row. Each cylinder contains another cylinder within it, so much smaller as to leave a space between them of about an inch in extent, which space is to be maintained by bands, or pins, reaching so much beyond the inner cylinders, and placed on them at regular distances asunder.

233. Above the eleven double cylinders, or steam-generators, in the same furnace, a twelfth cylinder of the same length, but somewhat more than thirty inches in diameter, is placed, which Mr. M'Curdy calls a steamometer, and which will hold ten times the contents of all the spaces between the inner and outer cylinders; the eleven

outer cylinders are connected by tubes, which run from one to the other sideways near their tops in each row, and from the lower to the upper row; a tube also passes from the upper row of cylinders to the steamometer, and another goes from the latter to the working cylinder of a steam engine. The supply of water is given by a forcing pump, which draws it from the reservoir and impels it into the space between the inner and outer cylinders, which are placed at the outside of the lowest row, and thence through the tubes and the spaces between all the other inner and outer cylinders up to the steamometer; at the bottom of which latter a cock is placed, by which the ascent of the water to that point may be known; while by another cock, fixed between the forcing pump and the reservoir, the degree of the supply of water is regulated. A safety valve on the steam tube, close to where it passes from the steamometer on its way to the steam engine, completes the apparatus.

234. In pursuing the history of the steam engine, we have already examined the most perfect form of the single-acting apparatus; but an engine of double powers was soon produced by the comprehensive mind of Mr. Watt. He had introduced steam acting against a piston to press it downwards; he now formed a communication between both sides of the piston and the boiler, and also with the condenser, and made the steam act to press the piston upwards as well as downwards.

235. The mechanism was now, as far as the principle went, perfect; and it was freed, for the first time, from the enormous dead weight of counterpoises, which had hung on it from the first attempts of Newcomen; and the equally enormous load which was used in the construction of the various parts, for the purpose of equalising the motion.

236. The cylinder a, plate VII., is enclosed in a jacket or casing like the single engine, having a similar interval, which may be filled with steam or air. The piston b is attached to the lever-beam by the rod x. 1, 2, 3, 4, are the valves which admit steam to the cylinder, or open a communication between the upper and under sides of the piston and the condenser. g is the pipe leading from the valves to the condenser. m, m, the levers or spanners, which are elevated or depressed by the tappets or pins n, z, in the plug-frame, and open or shut the valves to which they may be connected. h is the condenser; L, a pipe connecting it with air-pump i, and a second air-pump E. C the piston-rod of this second pump, attached like the other, . to the lever-beam. F, a pipe from the cold water-pump 9, to supply the reservoir in which the condenser and its pumps are placed. k, a trough or reservoir into which the water heated by the condensation of steam in the condenser, which is raised by the air-pump, is pumped back by м into the boiler. G, a pulley; and H, an endless chain moving over it, also going round a pulley fixed on the upright axis of the conical pendulum or governor z. R, the handle of the lever which regulates the quantity of injection water admitted. P, P, the masonry or wall on which the cylinder, and other parts of

the machine are placed. d, a pipe conveying steam from the boiler to the cylinder. B, a cock or valve, called the throttle-valve or regulator, placed on the pipe conveying the steam from the boiler, and which is moved by the levers shown as supported at D, and connected with the conical pendulum. T,Q, Q, Q, w, are a system of levers, called the parallel motion. z is the axis of the lever-beam y.

been

237. The motion at first is produced in this machine in the same manner as in the single engine,-by filling the condenser and cylinder with steam, and then opening the injection-cock. 238. This process may be considered to have gone through, and that the piston has arrived at the top of the cylinder. At this moment the tappets n, n, and levers m, m, open the valves 1 and 4, and shut 3 and 2. The steam from the boiler now acts on the upper side of the piston, while a vacuum is formed under it by valve 4 opening a communication between that part of the cylinder beneath it and the condenser. The piston is therefore pressed by the elasticity of the steam to the bottom; when it has arrived at the lowest point, the tappets on the plug frame, which also descend with the piston-rod, shut the valves 1 and 4, and open 3 and 2. The steam from the boiler, instead of flowing in at the top of the cylinder, is admitted at the bottom, and a communication is opened between the upper end of the cylinder and the condenser: a vacuum is then produced above the piston, and the elasticity of the steam (instead of the counterpoise in our last figure) forces it upwards. When it is elevated to the required height, the tappets again act on the spanners, and prevent the further flow of steam beneath the piston, and admits it at its upper end, opening at the same moment a communication between the lower end and condensing apparatus. The motion of the piston is then reversed, and this alternation may be continued indefinitely.

239. The mode of pumping out the water from the condenser, being the same as that in the single engine, will be easily understood from an inspection of our figure. In order to show the four valves in section, a pipe placed in the same direction, and opposite to o, has been omitted in the engraving; it connects the top of the cylinder and the condenser together.

240. The power of the condensing engine is easily known by ascertaining the temperature of the steam, which moves the piston, the area of the piston, and the temperature of the vapor which remains in the condenser. We know from experiment that steam of the temperature of 212° will balance the pressure of the atmosphere, or, what is the same thing, will force a piston into a vacuum with a force equal to about fourteen pounds and three quarters weight for every square inch of the area of the piston. The difference between the elasticity of the steam in the condenser, and that issuing from the boiler, will be the measure of the power of the engine. It is however found most expedient to raise the steam to a somewhat higher temperature than 212°, so as to produce a pressure between seventeen and eighteen pounds on each square inch of the piston; yet, in practice, from

the imperfect vacuum which is made in the condenser, and after making allowance for the friction of the piston on the sides of the cylinder, and for the friction of the various parts of the intermediate machinery, this pressure of eighteen pounds on each square inch of the piston, cannot raise more water per inch than would weigh about eight pounds and a half, so that somewhat more than a half of the whole power of the steam is absorbed to give motion to the intermediate mechanism. The height to which, this weight can be elevated depends on the length of the cylinder, and to raise the same weight to twice the height by the same piston requires double the quantity of steam. The double condensing engine will also perform double the work of a single engine in about half the time, but it requires double the quantity of steam.. So that in all cases, under the same circumstances, the work performed will be as the quantity of steam used.

241. When the impulse of the steam impelled the piston only in one direction (downwards) its motion could be imparted to the beam by means of chains; but, when the impulse was to be communicated upwards as well as downwards, some other contrivance for connecting the beam and piston became necessary; and one of the conditions of this contrivance must be, to convert the motion in a curved path of the end of the leverbeam, into a rectilineal motion of the cylinder piston-rod. Mr. Watt, in his earlier engines, used to form the end of the beam as a sector with teeth, which worked into a rack fixed on the end of the piston-rod; this allowed the rod to move perpendicularly upwards or downwards, but it was very inelegant in appearance, worked with a great noise, and was easily deranged, especially at the instant when the direction of the motion was changed.

242. Even after the motion of the piston was equalised, by shutting off the steam sooner or later from the cylinder, another source of irregularity was found to arise from the varying quantity of steam which in different states of the fire under the boiler, was admitted into the cylinder. Several modes of adjustment occurred to Mr. Watt. The one most generally employed, and probably as accurate as any, was, by placing a valve in the pipe connecting the boiler and the cylinder, which could be made to increase or diminish the steam-way. The next improvement was to make this valve, called a throttle-valve, a self-acting one, and to admit of its being so adjusted that, when the piston was moving with too great a velocity, it would admit less steam into the cylinder, aud so diminish its speed, and on the contrary admit a greater quantity when it was moving too slow.

243. A similar irregularity in the motion of corn-mills, from the varying quantity of water or resistance, had early exercised the ingenuity of millers, to obtain some means by which its injurious effects could be obviated. One of the most usual modes was by means of a couple of heavy balls, attached by a jointed rod, which were made to revolve by being connected with the spindle or axis of the mill-stones. When the stones were moving at a great speed, the meal, by the rise of the stones, was too coarse;