the height of the mercurial column passing up the graduated tube.

373. This gauge is just the reverse of the preceding; the barometer showing the pressure of the atmosphere on a given space of the piston, while the steam-gauge indicates the force of elastic vapor entering from the boiler. It is the duty of the fire-man frequently to look at his gauge, that he may know when to increase the fire in the furnace, and thus a sufficient supply of steam will always be secured to the engine.

374. In the early atmospheric engines, the working-beam was composed of a large and almost unhewn tree; but Mr. Smeaton employed a framing of wood for this purpose, which was afterwards much simplified and improved by Mr. Hornblower.

375 In double-acting engines it is usual to have the beam cast in one piece, the extremities being turned in a lathe to form cylindrical pins, upon which are fitted sockets or pieces, having other pins projecting from them to form the points of the parallel-motion and connectingrod. Thus, there is one pin on each side of the socket, the two links of the parallel-motion being fitted to the two projecting pins at one end, while the double joint of the connecting-rod is fitted on the two pins at the other end of the beam. The advantage of this construction is, that the joints at the ends of the beam become universal joints, having liberty of motion in all directions; and, in some of Mr. Murray's best engines, the same contrivance is applied to the crank-pin and connecting-rod.

376. The following table exhibits the proportions of the parallel-motion and as such is important.

377. The two great links of the parallel motion are each composed of a strap or loop of iron, bent so as to form a double link, in the upper bend of which are two sockets for the pivots at the end of the beam, and at the lower end are two others, for the pivots which project on each side of the piston-rod socket. The brasses of this joint are held in by wedges, put through the two links at the lower end, which, on being driven inwards, tighten the fittings at pleasure.

378. The proper length of the stroke for different engines is not at all settled; Mr. Watt's first engines were made much longer than this table, but of late years they have been made

shorter, and without any adequate reason which we can perceive; for it must be an advantage to a machine to make as few reciprocations as is consistent with a practicable length of cylinders. These differences in the length of stroke do not affect the calculation of powers, because, if the length of the stroke is altered, the number per minute is also changed, and the velocity of the piston is the same; at least it will be always nearly the same as the table for those engines that work a crank and fly-wheel. But it must be observed that these engines move with a greater celerity than the engines for pumping water, because it is necessary to accumulate a considerable velocity in the fly-wheel, or it must be immensely heavy if the piston was to move so slowly as the pumping-engine generally does.

379. The cylinder and piston being those parts of the engine in which the effective force is more immediately produced, may now claim attention:

380. The piston of the atmospheric engine is generally made of cast iron nearly fitting the inside of the cylinder, a circular ledge or rim being formed round it to receive the packing, without which the steam would find a passage through the interstices in the cylinder. Mr. Smeaton, who greatly improved the atmospheric engine, coated the under side of the piston with elm or beech planks about two inches and a quarter thick; the wooden bottom being screwed to the iron with a double thickness of flannel and tar, to exclude the air between the iron and the wood. By the adoption of this improvement its property of conducting heat was diminished, and the wood, having been previously jointed with the grain radiating in all directions from the centre, was not liable to expand very materially by the heated steam. This piston was kept airtight by a small stream of water continually falling on its upper surface; but in Mr. Watt's engine he was compelled to effect this by improving the fitting of the piston, the old mode being inadmissible.

381. It is now cast with a projecting rim at bottom, which is fitted as accurately to the cylinder as it can be, to leave it at full liberty to rise and fall through the whole length. The part of the piston above the rim is a little less all round than the cylinder, to leave a circular groove for the hemp which forms the packing. To keep this in its place, a lid or cover is put over the top of the piston, with a ring or projecting part which enters into the circular groove for the packing bb, and pressing upon it the plate is forced down by screws a, a, which work into the body of the piston at cc. By this means the packing is made to fill the diameter of

d

the cylinder with tolerable accuracy, and to prevent for a time any steam passing between the piston and the cylinder. The connexion with the piston-rod is seen at ne. When, however, by continued working the piston became too easy, and so occasioned a waste of steam, it was found

necessary to take off the top of the cylinder to get at the screws, even when fresh hemp or packing was not wanted, and this operation being attended with considerable labor was seldom resorted to by the engine-man till a great waste of steam had taken place. By an improvement on this piston, introduced by Mr. Woolf, this is now effected without taking off the cylinder-cover, except, indeed, when new packing is required. 382. To accom

plish this, Mr. Woolf fastens on the head of each of the screws a small cog-wheel or nut, c, c, c, c, c, and these are all connected together by means of a central wheel d, working loose upon the piston-rod in such a manner, that if

piece, so that the three pieces are thus kept so uniformly in contact with the cylinder, that the longer the machine is worked the better the rings must fit. To prevent steam passing through the cuts in the lower rings, the solid parts in those upon the upper side are made to fall upon the divisions and springs of the under ones, thus interrupting the communication that would otherwise remain open, and forming a perfect break-joint.

any one of the screws be turned, a similar motion is given to the remainder, a cap being provided in the upper end of the cylinder screwed down by bolts to make it steam-tight. In a piston thus constructed, there is little difficulty in drawing down the packing, by applying a key to the square head of the projecting screw employed to communicate with the rest. Another method contrived by Mr. Woolf for the smaller pistons differs but little from the preceding in construction. Instead of having several screws, all worked down by one motion, there is in this but one screw, and that one cut upon the piston-rod itself; on this is placed a wheel, the centre of which is furnished with a female screw, which is forced down by means of a pinion furnished with a square projecting head turned in a similar manner to the preceding.

385. A diagram of an improved piston is annexed and it will be seen that the flexible springs 1, 1, 1, rest on the rod S, and operate on the wedges 0, 0, 0, and as such must expand the remaining portions h, h, h, so as to ensure

the accurate fitting of the piston rm. The interior surface of the cylinder in which the piston works requires to be bored with the greatest exactness, though this was but little attended to in the early atmospheric engines, some of them being composed of timber hooped together in the same manner as barrels are constructed. Mr. Watt, in his first attempts at improving the steam engine, employed this material in the construction of his cylinders, though he afterwards abandoned it for those of bored metal; the operation of boring being performed with the greatest precision, by an apparatus invented by Mr. Wilkinson.

383. For high-pressure engines, however, the metallic piston invented by Mr. Cartwright has the most decided preference. This not only saves the trouble and expense of packing, which must be frequently renewed in all other engines, but also a great deal of steam, on account of the more accurate manner in which it is made to fit the cylinder. This is effected in the following manner:-Two metal rings are accurately ground into the cylinder, so that no steam can pass between their exterior surface and the inside of the cylinder, their upper and under sides are also ground perfectly flat, and applied one upon the .other. On the upper ring is placed a plate of metal, rather smaller in diameter than the cylinder, while a similar flat plate is placed below the under ring, both of which, with the rings between, are attached firmly to each other by means of the piston-rod passing through them.

386. Mr. Murray has also effected considerable improvements in this part of the engine, and the boring machines employed in his manufactory are of considerable value. They are worked by a separate steam engine, which is never stopped during the operation, as in that case a shoulder or ring would be formed, running completely round the cylinder.

387. In small engines, it is common to place the cylinder within the boiler, in which case no artificial mode of retaining the heat is required; but to this arrangement in those of larger dimensions there are several objections, not the least of which is the frequent repairs that are necessary in the boiler; and a similar effect has been produced by the use of a double cylinder. This was first adopted by Messrs. Boulton and Watt, the outer cylinder or steam-jacket keeping the inner cylinder at the temperature of boiling water, by the action of a partition of steam made to pass between the jacket and the working cylinder, as we have already stated.

384. A shell being thus formed, the rings are each of them cut into three pieces, and in cutting them such a portion of the metal is taken away as to leave room to introduce between two of the pieces, a spring in form of the letter V, the open end of which is placed outwards, almost close to the circumference; by which means the two pieces, against which the two sides of the spring act, are pressed in the direction of the circumference, against the ends of the third

388. Before the boiler of a high-pressure engine is set to work, it should be proved effectually, first, by drilling small holes through it at different places, to actually measure the thickness of the metal, and ascertain that it is equal throughout, and then it should be proved by injecting water into it, until the pressure lifts the safety-valve, when loaded considerably more than is intended to be when the engine is set to work, but this proof should not be too severe, because the metal may be weakened, although it is not burst, by the proof; and in consequence may afterwards burst with a much less pressure of steam. At

the same time the engineer, who undertakes to make these engines, should fully inform himself of the real strength of metal boilers of determinate thicknesses, which could be easily done without danger by injecting water into the boilers until they actually burst. We do not know that such experiments have ever been made; and, in those boilers which have been burst by the explosion of steam, the pressure at the moment of the accident has not been known.

389. Mr. Bramah has made several steam engines, in which he employed a four-passaged cock, on a construction somewhat different from the one usually resorted to; the steam from the boiler is made to enter into a hollow at the large end of the cone of the cock, and to pass away to the condenser by a passage at the small end of the cone of the cock, which by this means is always pressed into its seat by the force of the steam acting upon a surface equal to the small end of the cock, from which the pressure is relieved. This keeps the cock always tight; and, to prevent the moveable part from being fixed fast by the pressure, the cone is made much more

obtuse than usual.

390. In joining the flanches of iron cylinders, and other parts of hydraulic and steam engines, a strong and durable cement is required. The following are receipts for cements proper for such purposes:-Mix boiled linseed oil, litharge, red and white lead together, to a proper consistence; this cement is to be applied on each side of a piece of flannel, previously shaped to fit the joint. The quantities of the ingredients may be varied without inconvenience, only taking care not to make the mass too thin with the oil. It is difficult, in some cases, to make a good fitting of large pieces of iron-work at once, and this renders it necessary sometimes to join and separate the pieces repeatedly before a proper adjustment is obtained. When this is expected, the white lead ought to predominate in the mixture, as it dries much slower than the red. A workman, knowing this fact, can exercise his own discretion in regulating the quantities; but it is safest to have too much rather than too little white lead, as the durability of the cement is no way injured thereby, only a longer time is required for it to dry and harden. When the fitting will not admit of so thick a substance as flannel being interposed, linen may be substituted, or even paper, or thin pasteboard, the only reason for employing any thing of the kind being the convenience of handling. This cement answers well also for joining broken stones, however large. Cisterns built of square stones put together with this cement will never leak or want any repairs; in this case the stones need not be entirely bedded in it; for an inch or even less of the edges that are to lie next the water need only be so treated, and the rest of the joints may be filled with good lime.

391. Another cement, which is preferable to the former for withstanding the action of the steam, is compounded as follows:-Take two ounces of sal ammoniac, one ounce of flour of sulphur, and sixteen ounces of cast iron filings or Borings; mix all well together by rubbing them in a mortar, and keep the powder dry. When

the cement is wanted for use, take one part of the above powder, and twenty parts of clean iron borings or filings, and blend them intimately by grinding them in a mortar; wet the compound with water, and, when brought to a convenient consistence, apply it to the joints with a wooden or blunt iron spatula. By considering the affinities of these ingredients, those who are at all acquainted with chemistry, will be at no loss to comprehend that a degree of action and re-action takes place among the ingredients, and between them and the iron surfaces, which at last causes the whole to unite as one mass; in fact, after a time, the mixture and the surfaces of flanches become a species of pyrites, holding a very large portion of iron, all the parts of which strongly cohere together.

392. Another cement of the same kind is made by mixing together two parts of flour of sulphur and one part of sal ammoniac, and making them into a stiff paste with a little water. When the cement is wanted for use, dissolve a portion of the above paste in urine, or in water rendered slightly acidulous; and to this solution add a quantity of turnings or borings sifted, to get rid of the grosser particles. This mixture spread upon or between the flanches of iron pipes, or put into the interstices of other parts of iron work, will in a little time become as hard as stone.

393. Mr. Malam has proposed some important improvements in the construction of rotatory steam engines. Tho first of these consists in the use of a hollow cylinder or drum, in which the pressure of steam is contrived to drive a leaden piston (as the patentee terms it) round the cylinder; but which piston, by its gravity, always remaining near the lowest part, the steam is impelled upwards against the partition of the cylinder, and consequently forces it round upon its axis.

394. Two standards support the arms or axles of the revolving cylinder or drum, one of which arms is hollow, having two passages, the one communicating through the leg of the standard to the steam boiler, the other through the other leg to the condenser.

395. There are three valves in the hollow cylinder or drum, for the purpose of shutting off the steam and forming three distinct compartments; there are also three hollow arms, each leading from the hollow axle into one of these compartments. There is a heavy block of lead formed exactly to fit the curvature of the drum or hollow cylinder, which block or piston slides completely round the cylinder, the valves falling back into recesses as it passes by them. This block is so well fitted as to prevent the passage of steam between it and the cylinder.

396. The steam admitted through the hollow axle and through the hollow arm which leads to the lower compartment, where the block or piston lies, not being able to drive the block far from its place forces against the valve, which divides the chambers, and lifts that side of the revolving cylinder, carrying it round until the next valve has passed the block or piston, by which time the arm that supplied the steam has passed on so as to open a communication with the con

pistons or buckets, and is then condensed or discharged into the atmosphere at the top of the working cylinder.

402. Sir John Paul has a patent for a mode of generating steam. This invention is a mode of generating steam of high pressure, by passing water through a long contorted pipe enclosed within a furnace; by which contrivance a greatly extended surface is exposed to the action of heat, and steam of very high pressure produced with great rapidity. It is proposed to place a long metal pipe of small diameter round the furnace, and to force or inject water into one end of it, by means of a pump: which water is to be allowed to escape at the other end of the pipe, in the form of high pressure steam. The requisite length, capacity, and strength of the pipe, of which the boiler is to be formed, will depend upon the engine it is intended to work. For an engine of two-horse power the pipe should be 150 feet long, having its internal diameter at least three-sixteenths of an inch, and, if of copper, about one-sixteenth of an inch thick. This pipe is to be heated throughout its whole length nearly to redness, when the water injected at one end will escape at the other end in the state of steam, equal in pressure to about 150 lbs. upon the inch.

403. In constructing a boiler upon this principle, it is proposed to make a conical vessel of sheet iron, as an external casing, the broad end of the frustum downwards; round the inside of this vessel, the pipe above described is to be coiled, and supported upon flanges. A portion of the pipe, at the lower part of the vessel, is to be bent, so as to form the grate bars, upon which the fuel for heating the boiler is to be deposited, and the fuel may be introduced from the chimney at top, or otherwise, as may be found most convenient. A reservoir of water must be placed contiguous, and a forcing pump for injecting it into the pipe; and it is proposed to coat or cover the outer case with brick dust or coal ashes in order to prevent the radiation of heat.

404. The fuel being ignited, as in ordinary furnaces, the forcing pump is put in operation, which injects the water at the lower extremity of the pipe, so that as it rises through the conical worm, or contorted pipe, the water may become more and more heated, until it expands into steam, and this steam as it passes through the upper part of the coil, continuing to receive additional heat, ultimately escapes at the end of the pipe with great elastic force, in which state it proceeds to the engine.

405. For constructing the boiler of a steam carriage, where a chimney cannot be conveniently employed, another disposition of the worm pipe is proposed. In this boiler, the pipe is to be coiled in the form of two frustums of cones, of different dimensions, placed concentric; the larger or outer cone having its wider part at top, and the smaller, or inner one, its wider part at bottom. The continuous coil of pipe, thus formed into two cones, is to be fixed firmly upon suitable supports within the casing, which must be double, and the space between filled with coal ashes, or brick dust, or any other imperfect conductor of heat, as in the former instance.

The fire is made at the lower part in contact with the pipes, and the bottom of the casing is to be formed into a funnel shape, for the purpose of receiving the cinders and dust which fall from the fire, and conducting them to an ash pit below. In this funnel an aperture is made, and a pipe inserted, which conducts a gentle current of air to the furnace, in order to keep up the fire, and consume the smoke; the current of air may be produced by bellows or by a wind hole, as may be found most desirable: and the fuel may be supplied from an air-tight box above, or by any other suitable means. The injecting pump is to be applied to one end of the pipe as before described, and the high pressure steam allowed to escape at the other end, for the purpose of working the engine. The carbonic acid, or other vapor, which is not destroyed by passing through the fire, is allowed to descend through the passage to the ash pit and to escape into the open air.

406. The specification concludes by saying that, for supplying engines of greater power than those above contemplated, more than one pipe boiler may be employed, and these may be heated either in the same way as above described, or in separate furnaces; they may be heated either by immediate contact with the ignited fuel, by the radiant heat from it, by the flame emitted, by the burning of gas, by heated air, or by any other fit and convenient means. The size, shape, or construction of the furnace, may also be varied considerably, according to circumstances, as may likewise be the arrangement of the coils of pipe, provided that the boiler consists of one entire or continuous pipe, or several pipes united together, so as to form one continuous passage from the end where the water is injected to that where the high pressure steam rushes out. pipes may be made of any other suitable material besides copper, but their substance must be varied accordingly. The exterior surface of the pipe may be coated with a layer of fire clay, or any other fit material, to guard the pipe against the effects of the fire, and to prevent oxidation.

The

407. An apparatus for changing the water has been suggested by Messrs. Maudslay and Field, which is particularly adapted for the boilers of steam vessels, where salt water is used for the production of steam, as in the ordinary mode, the deposition of salt and other earthy matters, on the bottom and sides of the boilers, render them extremely liable to injury from the action of the fire. The second part of the invention is a mode of arresting the heat contained in the boiling water withdrawn from the boiler, and of conveying it into the water about to be introduced.

408. When sea water is employed in the boilers of steam engines, the evaporation quickly produces a supersaturation of salt in the remaining water, and this, rapidly increasing, causes a concentration of the salt, and a deposition of earthy matters on the internal surface of the boiler, which renders it necessary to change the water almost every fifty or sixty hours. It appears from the experiments of the patentees, that

from twenty to thirty per cent. of the quantity evaporated taken out will keep the water within