The very first mechanical work of difficulty, but of pre-eminent importance, in making an engine, is boring the cylinder, that is, if the same is a casting, and not a piece of tube ready made and smooth on the inside. This is, properly speaking, lathe work, yet may be done in a different way. Suppose you have bought your entire set of castings, which is the best way, and that the cylinder is half an inch diameter inside, which is a manageable size to work upon. Get a half-inch rosebit, which is very like the countersinks sold with the carpenter’s brace and bits. Mount it in the lathe in a chuck, A, Fig. 65. Unscrew the point of the back poppit, and slip over the spindle a boring-flange, B, which is merely a flat plate like a surface chuck, only the socket is not screwed but bored out, generally large enough to slip over the spindle. Sometimes there is, however, a screw at the back, to screw into the spindle, the same as the points or centres. On the face of this lay a piece of board of equal thickness, but it is as well if not planed, as its object is partly to prevent the cylinder from slipping about during the operation, as it is sometimes inclined to do upon the smooth metal flange, and partly to prevent the borer or rosebit from coming in contact with the flange when it has passed through the cylinder. Grasp the latter in the left hand, and you can easily prevent it from revolving with the drill, which will go through rapidly, and leave the hole beautifully finished and quite true from end to end,—indeed, I have bored iron also, rapidly and with great ease, with this tool.

It is absolutely necessary, remember, that this hole bored in the cylinder should be at right angles to the ends of the same, and to secure this you must now make use of it to mount the cylinder in the lathe to turn these ends or flanges. I will show you a simple and easy way to do this. C is a bar of iron or steel, preferably of the latter, about 6 inches long, and three-eighths diameter, filed into six sides. It is a good plan to have three or four sizes of such bars, with centre holes drilled carefully into each end, so that you can mount them with a carrier-chuck, as you would if you were going to turn them. Taking one of about the size named, mount upon it a piece of wood, and turn this down until your cylinder will just go tightly upon it. Being a six-sided bar, it is easy to mount the wood upon it by boring the latter with a gimlet and then driving the bar into it. It will hold tightly, and not turn round upon the metal. The cylinder being fixed in this way, you must turn the two flanges with a graver if the cylinder is of iron, but with a flat tool or the four-sided brass tool if of the latter metal; and also turn the edges of the flanges. The rest of the cylinder will be left in the rough, and may be painted green or black. I should advise you always to bore the cylinder first when possible, and then to mount it as described and turn it on the ends, which are thus sure to be correctly at right angles to the bore. Some cylinders, however, especially short ones, may be squared up first, and then mounted on a face-plate and bored. Unless, however, you have either a grip-chuck, which is self-centring, or some clamps properly constructed for this particular work, you will find the first method the easiest, especially for small light work.

You should now make the ports for steam and exhaust. Mark them upon the flat part of the casting, after you have filed this as level as you can, and do not mark them so long as not to leave you room beyond the ends of the ports for the steam-box or case which has to be placed here. The upper and lower ports are to be the same size, but the middle one may be a trifle larger with advantage. In larger engines these are cast in the metal, and have only to be trimmed and faced; but in the small models you have to drill them out in the boss cast on the cylinder. Drill down from the top, as shown at D by the dotted lines, but take great care not to go farther than the outer ports, which are to be therefore first made, so that you can tell when the drill has gone far enough. If you pierce the middle port from either end, the cylinder is spoiled. To cut the middle one, you merely drill a hole straight in towards the cylinder, and meet it by another drilled from the side, into which the pipe for the exhaust is to be screwed. You also drill straight through into the cylinder at a b, and you then plug the end of f, and that at the other end of the cylinder. Your port faces, however, are generally oblong, and not round. Make a row of holes with the drill, and then, with a little narrow steel chisel and light hammer, chip out the superfluous metal, and finish with a small file. You can always make narrow channels with squared sides by thus drilling two or more holes, and throwing them into one with a file; but in reality, for these small engines, it is very little matter whether the ports are round in section or square.

The bottom and top of the cylinder demand our next attention. E and F show these. They are easily and instantly mounted in a self-centring chuck, but can be held very well in one of wood carefully bored with a recess of the right size and depth. You must here, nevertheless, be very particular, else you will get your work untrue at this point, and then your piston-rod will stand awry, and all your subsequent fitting will be badly done. I therefore give you at G a section of the chuck bored to take the cover truly. Recess the part down to the line a b, to fit the cover exactly, taking care to level very carefully the bottom of the recess. Below this cut a deeper hole, to allow the flange in which the stuffing-box will be to go into it. It need not, however, fit the flange. The rough casting will hold very well in a chuck like this, even if it is of iron. You now carefully face the bottom of the cover, and turn the slight flange exactly to fit into the cylinder; then reverse it in the chuck, so as to get the stuffing-box outside; and in doing so, take the greatest care that it beds flat upon the bottom of the chuck. Turn off level the top of the flange first at x of fig. E, and then place a drill with its point against the middle of this, and its other end (with a little hole punched in it to keep it steady) against the back poppit centre, and carefully drill a hole down to the level of c, large enough to admit the gland of the stuffing-box or nearly so; but remember that you must not go too far, because the rest of the hole must only just allow the piston-rod to go through it. Therefore, after you have drilled about three-fourths of the distance, replace this drill by a smaller one, and with it bore quite through. The advantage of beginning in this way is, that you can now bring up the back centre of your lathe to steady the cylinder cover while you finish turning it; and as you will have to make a chuck only to take hold of the flange b, while you turn the edge, you will need probably some extra support of this kind. I have, nevertheless, turned an iron cylinder cover 2½ inches diameter without any such support; the actual strain not being very severe, provided you understand how a tool should be made and held.

The above directions apply equally to the cylinder bottom, the great secret in this and all similar work being to take care to bed the work well and truly against the bottom of the recess, turned in the chuck; this being neglected, will result in the two faces not being parallel, which will terribly throw out of truth the rest of your work. Indeed, in all fitting of this kind, it is absolutely necessary to be exact in the squaring and truing of each several piece that has to be turned or filed; otherwise no planning or clumsy arrangement will make your mechanism work as it ought to do. Take a week, if necessary, over any part, and don’t be content until it is well done.

Your cylinder ought now to have a finished appearance when the cover and bottom are placed in position, but the latter have to be permanently attached by small screws, and these I strongly advise you to buy. They cost about 50 cents a dozen, including a tap with which to make a thread in the holes made to receive them; or, if you prefer it, you can buy miniature bolts and nuts at almost as cheap a rate, which would cost you much time and trouble to make for yourself, if, indeed, you succeeded at all. You will want four of these for the top, and the same for the bottom, the holes for which you will make with a small archimedean or other drill.

The mention I have made of this reminds me that I am gradually adding considerably to your list of tools, and it is necessary to do so if you take up model-making. Set down, at any rate, the following:—

• Archimedean Drill-Stock and 6 Drills.
• Table-Vice.
• Hand-Vice or Pin-Vice.
• Small Brass-Back Saws for Metal.
• Pair of Small Pliers.

And for use in the lathe, either two or three sizes of rose-bits, or engineer’s half-round boring bits, of which I shall have to speak presently; and, unless you buy all screws and nuts, you will want screw-plate and taps, or small stock and dies. Files of square, round, and oblong section are matters of course. Remember, too, that after a file has been used on iron and steel, it is useless for brass; so use new ones on the latter metal first, and after such use they will answer for cast iron and then for wrought iron. You will find the cost of files rather heavy unless you attend to this. Have neat handles to all your smaller files, with ferules to prevent splitting.

When you purchase the castings of the engine, you will find a valve-box to enclose the slide and become a steam-chest, as explained. It is like a box with neither top nor bottom, but with a flange, or turned-out edge all round, for the screws by which it is to be attached to the valve-facings of the cylinder. This box must have its flanges filed up bright on their flat sides and edges—the rest may be painted. It will exercise your skill to get the two faces flat and true, to fit upon the cylinder; and at last you will find it expedient to put a brown paper rim or washer between the surfaces, or a bit of very thin sheet lead, to make a steam-tight joint. Do not solder it, if it is possible to use screws, because this is nearly certain to get melted off; and, if not, it is not nearly so neat and workmanlike a way of uniting the parts. You should, indeed, in all models, put them together in such a way as to be able at any time to separate the different pieces again, either for the purpose of cleaning or repair; and, if you solder, you cannot easily do this.

The valve-casing and its back are generally put on together; four screws at the corners passing through the back and both flanges into the flat side of the cylinder. This depends, however, upon the exact shape of these different pieces; and I can give you no special directions for a particular case unless I could see the castings which you have to fit together. The stuffing-box you will make quite separate, both its outer and inner part, and screw or solder the former into place. It is seldom cast upon the valve-casing, because of the difficulty of chucking a cubical object safely so as to turn any part of it.

You are not to screw or solder the valve-box to the cylinder until you have carefully filed up the valve itself to slide upon the port face, without the possibility of any escape of steam taking place. This needs the greatest possible care; and probably, after doing what you can with a flat file, you will have to put a little emery and oil between the surfaces, and grind them to a perfect fit, by rubbing them together. This grinding with emery is an operation frequently required in mechanical engineering. Lathe-mandrels are fitted in this way into the collars; the cylinder is also ground into the back poppit-head. It is not a very long or difficult operation, but whenever you have had to use it, take care to wipe off the emery, or it will keep on grinding. It is indeed very difficult to do this perfectly; and for very fine work, such as fitting the mandrel of a screw-cutting lathe (i.e., a traversing mandrel), oilstone powder and crocus are used, in place of emery. These, however, cut very slowly, making the operation of grinding exceedingly tedious; and in the present instance, emery will answer quite well enough. In very small engines, a stroke or two of a file is all that is needed to fit the valve, which is so small as hardly to be worthy of the name; but in an engine with cylinder of 1 or 2-inch bore, it will be impossible to do with file alone, as well as you can with grinding.

The piston and piston-rod should be turned at the same time, as already suggested in treating of the atmospheric engine of Newcomen. By this, you will avoid getting the piston “out of square” with its rod, as if you had bored the hole for the latter askew—a not unusual occurrence.

I do not mean to say that it is absolutely necessary for you to turn the piston-rod at all, for, in models, it is generally of round iron or steel-wire, which is as cylindrical as you can possibly make it. Knitting-needles are in general use for this, as being well finished and equalised from end to end. But these are rather hard, being tempered only to about the degree of steel-springs; therefore you must never attempt to cut a screw on them until you have first heated the end to be screwed red-hot, and allowed it to cool again very slowly. If you do this, a screw-plate will put a sufficiently good thread to allow you to attach either the piston, or the small piece of brass necessary to form the hinge, upon the other end of the rod—that is to say, the piece marked H in Fig. 64. Leave this for the present, however, not attempting at present to cut either the piston-rod or valve-rod to its intended length. You cannot do this until you have laid down the exact plan of the engine, and marked on the bed-plate the position of all the parts.

I shall now suppose that you have finished the cylinder, with its slide-valve, casing, stuffing-boxes, and piston, so that you have these in exactly the state in which you might buy them at Bateman’s and elsewhere, if you preferred, to spare yourself the trouble of boring the cylinder and fitting it. You can buy them just in this condition, with the rest of the castings in the rough; but I rather hope you may prefer to try and do for yourself the not very heavy or difficult work which I have described.

I suppose you, indeed, to have bought the forked connecting-rod, either arranged for brasses, or with holes drilled (or to be drilled) in the ends, which would probably be the case for a model of the size named, and also the various bearings, guides, and so forth required—some of which would have to be turned, and some filed, but which ought now to present little difficulty to our young mechanic.

Try to keep sharp edges to all your filed work, unless evidently intending to round them; for surfaces pretending to be flat, but partaking of a curved sectional form, characterise the workman as undeniably a bad hand with the file, and not worth his wages. Still I may tell you at once that nothing is so difficult as to use a file well. It has a knack of rounding off edges, which always get more than their proper share of its work. But this being the case, you will know what to try and avoid. Therefore, always endeavour in filing a flat surface to make it slightly hollow in the middle, which it is scarcely possible, however, for you to do; but the endeavour to effect this by filing the middle more than the edges will help you wonderfully in keeping the latter sharp. Those, for instance, on the fork of the connecting-rod, especially the inside ones, should be as straight and sharp as possible; and if you round the outside edge, take care to do it so that it shall be evident you intended it; and so with all edges, whether turned or filed.

The disc of the eccentric can only be turned by letting it into a chuck to something less than half its thickness, and levelling one side and half the edge, and then reversing it; unless you prefer to drill and mount it on a spindle upon its centre. If you do this, you will of course eventually have two holes in it; because this first one is not that by which it will be mounted when in place. This second hole is not, however, of the least importance, and may be left without plugging, and, if preferred, may become in part ornamented by drilling additional holes, and filing them into some pattern; or if it is desired to conceal the one it was turned upon, this can be plugged and faced off, and will then not be the least apparent. If the outer ring, or strap, as it is called, is to be made in two pieces, with projecting lugs, it is evident the outside edge cannot well be turned; and, unless you have that most useful addition to the lathe, a grip or jaw-chuck, you will have some little difficulty in letting the ring into a wooden chuck, so as to turn the inside. The solid ring is, therefore, preferable (if you use the first, however, you turn it up as a single ring, and then saw it across through the lugs), which can be let into a common chuck, with a place chiselled out to allow the boss to project, into which the eccentric rod has to be screwed. This boss also has to be drilled and turned on the outside. There are several modes of chucking it which can be applied, but the simplest is to use the carrier-chuck, and to let the ring become its own carrier by coming against the pin, as shown in Fig. 66, A.

When the ring is very small, I should first drill the hole for the wire rod, and then screw and mount it upon a little wire spindle, as in fig. B, aiding this, if necessary, by the back centre. But the smallest models require to be put into a watchmaker’s lathe or throw, and, except as curiosities, are scarcely worth making.

I have already told you never to undertake engine-making without first laying down a full-sized plan on paper, with centre lines through the principal parts, from which to take all measurements, and to mark these upon the base-plate, as a guide to the perfect adjustment of the various parts. Some of these are capable of a little extra adjustment after being put in place: the eccentric rod, for instance, can be lengthened or shortened by screwing into or out of the eccentric ring; and the piston-rod, too, may be similarly lengthened or shortened slightly; but try to work as near as you can to precise measure without such adjustment.

To turn the fly-wheel, which is the last operation (including the crank-axle), it is better carefully to drill the boss, if not already done, marking the centre on each side, and working half through from each, so as to insure the squareness of the hole with the side of the wheel, which is very important. Then mount it at once upon its axle, previously turned slightly conical, where the wheel is to be placed, and run both together in the lathe. This will insure the wheel running true when the engine is put together.

In the horizontal engine which I have sketched, the crank is quite separate from the axle; and this is the easiest way to make it. The crank itself is filed up, like C of fig. 66, and drilled for the axle and the pin upon which the brasses on the connecting-rod work. Turn down the end of the crank-shaft very slightly conical, until the crank will almost go over it. Then heat the crank, which will expand it and enable you to slip it on the shaft. Dip it in cold water, and it will be as firm as if made in one piece with the axle. This is called shrinking it on, and the operation will often stand you in good stead, and save the trouble of filing key-ways and making the small wedges called keys. The pin D can in this case be turned up separately, and screwed in, which will complete the work.

The eccentric must evidently be placed in position before the crank is added, and this, too, might be shrunk on, were it not that it cannot easily be fixed in a model until the engine is set up. The best way, therefore, is, in this case, to turn the eccentric with a little projecting boss to take a set screw, E, Fig. 66.

Where the axle has to pass through bearings, it must be turned down at these parts, so that the whole will be like F. First on the right is the journal, e, then the place for the fly-wheel, d, very slightly conical—the smallest part being towards e—then the second journal, and then another slightly conical part, the smallest end towards a, to take the eccentric and crank. The fly-wheel you will key on shaft, thus:—G represents the boss or centre of the wheel bored for the axle, and a key-way or slot filed on one side at a. There is a flat place filed on the axle, and the wheel is turned round to bring this opposite to the key-way. A wedge or key, b, is then driven in, which keeps the wheel secure, and prevents it from turning round or working loose on the axle. If inconvenient to turn a boss and add a set-screw to the eccentric, this also may be keyed in its place after its position has been found; but, for the latter purpose, it should fit rather tightly on the axle, so that it can be just moved round with the finger stiffly until its position with respect to the crank is ascertained.

This position I shall now endeavour to explain, using a diagram from an American work, in which this generally supposed difficult point is thus ably and satisfactorily explained. First, put your engine together as if for work, and having cut the eccentric rod to about the length you seem to require, judging from your plan drawn upon the bed-plate, turn round the eccentric, with your fingers upon the crank-shaft, and, having removed the cover of the valve-box, so that you can see the action on the valve, watch the motion of the latter. Doubtless, the result will be that one of the steam-ports will be opened clear to the exhaust-port, while the other is nearly or entirely shut. The rod is then too long or too short. If in a horizontal engine the port nearest to the crank is wide open and the other shut, the rod is too long, and must be shortened half the difference only (you will do this by screwing it farther into the eccentric hoop). When the valve “runs square,” or opens and shuts the ports correctly, set the eccentric as in the diagram, H, in respect to the crank, i.e., with its widest part at right angles to it. By running square is meant that when the eccentric is turned round as described, the valve opens the ports equally, and does not affect one more than the other. The line a of the diagram shows that the position of the eccentric may advantageously be a little beyond the right angle to the crank, to give what is called “lead,” i.e., to open the valve a little before the piston commences its return-stroke.

The boilers of model engines are made of tin, sheet-brass, or copper; seldom of the latter, which is, nevertheless, by far the best material, and one that you can braze, rivet, or solder satisfactorily, or bend into any shape with a hammer or wooden mallet. When polished, too, its rich red colour is very handsome. Brass is chiefly used from the facility of obtaining tubes of it ready brazed or soldered, from which any desired length can be cut. A brazed copper boiler will stand a great deal of pressure; will tear, and not fly into pieces when it bursts; and may be heated after the water has boiled away without suffering any injury. It would certainly not be worth while to make one for a model engine with a half-inch cylinder, but for one of 1 inch diameter and 2½ stroke; and for larger sizes, it will amply repay the trouble; and I will show you how to make one, with a tube or flue inside to add to the heating surface.

I shall endeavour presently to give the proper dimensions of boilers to work cylinders of given diameters, but the general directions here subjoined apply to all boilers of models, whether large or small. The main body of the boiler is generally cylindrical, and is, in fact, a tube of sheet-metal, with riveted, brazed, or soldered seams, the last greatly predominating in the toy engines; the result of which is, that the first time the water gets too low, out drops the bottom, or, at the least, divers leaky places appear, and the boiler is obliged to go to the tinman’s for repair, its beauty being ever after a thing of the past. It is difficult to braze in an ordinary fire; because even if, by blowing it with a pair of bellows, you get sufficient heat, you cannot always manage to apply your work in a good position, as you can over the hot coals of a forge fire, where there are no bars, hobs, or other parts of the grate standing in the way. Moreover, you often want both hands free just as the solder commences to “run,” and forge-bellows will keep up the blast for a few seconds after your hand is taken from the staff or handle of them. Still, if you have no forge, which is probable, you should make a fire of cinders or coke (the latter if possible); and if you can contrive a grate by putting together a few bricks in some out-house, with a bar or two of hoop-iron below for the coke to rest upon, you will have a far more convenient fire to work at than can possibly be obtained in any ordinary household grate or stove. You will require a pair of light tongs, which ought to be something like A, Fig. 67; but it is quite possible to do without these if you can hold your work in any other way; as, for instance, with a loop of iron wire twisted round it and left long enough to form a handle.

The first thing to do is to cut a strip of copper large enough to make the required tube. A piece 6 inches wide will roll up into a cylinder of about 2 inches diameter (the circumference of a circle being nearly equal in all cases to three times its diameter, or measure through the centre). If, therefore, you want one 6 inches across, which is the smallest size that can be advantageously fitted with a flue or internal tube, you must cut it out 18 inches wide, and if it is 8 in length to the bottom of the steam dome, it will be a large and serviceable boiler, fit to work an engine with a cylinder of 1½ bore by 2½ or 3 inch stroke, which would drive a small lathe. But observe that if you really have pluck and skill enough to try your hand upon an engine that will give you real power, you must take care to remember that “the strength of anything is the strength of its weakest part.” So don’t make the very common mistake of having a good boiler and ample cylinder, and then fit the engine with piston-rod, valve-rod, and such like, too small to bear the strain which you propose to put upon the engine. Remember that every screw and nut and pin upon which strain is liable to fall, must be of sufficient size and strength to bear it safely: if not, your engine will not only come to grief in the heavy trial, but it is quite possible that you also may become subjected to a bad scald or other disagreeable consequence of your error.

Whatever sized strips of copper you use for a boiler, the edges have to come together to form what is called a butt-joint; i.e., they do not overlap like the ordinary joints you see made in tin. Before you coil up the strip into a tubular shape, you have to cut out holes for any boiler fittings you may wish to add, such as safety-valve, steam-dome, and gauges to ascertain the level of the water. These, however, do not all come into the cylindrical part of our present boiler; the gauge-taps and glass water-gauge alone having to be provided for. The man-hole, too, which is added to all large boilers, may be dispensed with, its object being to enable one to get at the inside, which will scarcely be necessary if our work is well done at first. A boiler of the proposed size should be heated with charcoal, as it would require a very large lamp; but where gas can be obtained, it may be preferably used, a ring gas-burner being placed below within the furnace. The object of a steam-dome, which, in a horizontal boiler, would have to be placed somewhere on the tube itself, is to prevent what is called priming, i.e., the carrying into the cylinder water as well as steam, which arises from the spurting caused by the violent boiling of the water. The dome merely provides a chamber for dry steam above the general level of the boiler, the steam-pipe passing from it direct to the cylinders. Our present boiler will be vertical like the last, but with a flue up the middle, and a grate fitted below. It is shown complete in Fig. 67, B, with all the fittings usually attached.

Having coiled up the tube by hammering it over a cylinder of wood turned for the purpose, a little smaller than the intended size of the boiler (the edges having been previously filed up bright, and a width of a quarter of an inch of the upper being similarly cleaned on the inside all along the seam), a few loops of iron wire are tied round it, at intervals of 1 inch or 1½ inches; there being a short piece put round, and twisted together at the ends by a pair of pliers. The object of these is to prevent the seam from opening on the application of heat, which it is otherwise certain to do by the expansion of the metal. Some borax, pounded in a mortar, and heated to drive off the water of crystallisation, is next mixed with a little water to form a creamy paste, and smeared along the inside of the tube, upon the brightened part, the full length of the seam. It is generally better to heat this salt first sufficiently to dry it (or rather fuse it), because it swells prodigiously by the first application of heat, and if the spelter is laid on it, it often carries it off; after once fusing, it only melts quietly.

Before applying the little lumps of spelter, turn over the tube to heat the part opposite to the seam, so as to equalise the expansion. Then hold it in a pair of light tongs, lay the spelter all along upon the borax, and expose it without actually touching the coals to the heat of the fire, urged by a strong blast. Continue this until a blue flame arises, which shows that the spelter has melted; this blue flame being, in fact, that caused by the burning of the zinc in the solder—spelter being copper and zinc fused together, or, if required softer, brass, tin, and zinc. The former is generally used, however, on copper. When the blue flame arises, the solder runs into the joint, and the work is done. With the hardest of these spelters, a red heat will not seriously affect the joint, and, therefore, if at any time the water should get below the line of this seam, so that it becomes exposed to the heat, no harm will be done. Nevertheless, this ought never to occur, as a gauge should be attached to every boiler to show the exact position of the water at any given time.

The inside tube of this boiler will be seen, from the section, to be conical up to the level of the lower part of the chimney. This is of copper, brazed like the cylindrical part, and is 2 inches wide below, and 1 inch above; consequently, the strips to make it must be 6 inches wide at one end, and taper to 3 inches at the other. If the dome rises 2 inches from the level of the top of the cylinder, it will be sufficient; and as this is a difficult piece of work for a boy to manage, a coppersmith should be asked to hammer the dome into the required form, as he will know from experience the best size of circular disc to use for the purpose. This part is so far removed from the action of the fire that it may safely be soldered, but it is, nevertheless, as well to rivet it, turning out both the edge of the cylinder and that of the dome. Use copper rivets, and make the holes half an inch apart. If you find any leakage, you can run a little solder into the joint on the inside. The bottom of the boiler may be quite flat and brazed, a few rivets being first put in to hold the parts accurately together. The same may be said of the tube which passes through both this and the dome. There is nothing equal to riveting and brazing for this kind of work.

I may as well state however here, that as such a boiler as I have now described is worth very good work, it would be a great pity to spoil it; and it will be better to content yourself with smaller boilers and engines soldered, where necessary, until you have had some practice in brazing. This indeed is not difficult in reality, but, at the same time, requires great care, because sometimes the solder and the work melt at so nearly the same temperature, that, like a bad tinker, you will sometimes make two holes instead of mending one. The brass, for instance, used for beer-taps is very soft, and contains lead, and to a certainty would itself melt before ordinary spelter, and could not therefore be brazed; but the best Bristol brass, or yellow metal, will braze easily. A blacksmith, brazing a key or other iron article, will braze it in a different way, using brass wire, with which he will envelop the parts thickly which are to be united, after securing their position with iron binding-wire. He then sprinkles with borax, and heats the work until the wire runs into the joint; after which he files and cleans off level. This makes a very good medium.

I have spoken of riveting in this place. There is no difficulty in this work. You can buy copper rivets of all sizes, and have only to punch holes, put a rivet in place, and hammer it so as to spread the metal to form a second head. If the rivets are heated before being applied, they will draw the parts closer together, because they shrink in cooling. All large boilers are made in this way, but smaller ones of iron are often welded, where such a mode of junction is possible. When you can rivet boilers water and steam tight, you will find no difficulty in constructing them, for you can make riveted joints where brazing would be difficult or impossible.

Fig. 67, B, is a half-section of such a boiler as I have just described. Fig. 68, A, is the lower part, which is separate, and forms the furnace in which the boiler stands, fitting it closely. This is drawn to scale, and is half the real size. a is the steam-pipe, fitted high up in the dome, the tap, b, serving to turn on or off the supply of steam for the cylinder; c is the safety-valve shown in section, and care must be always taken to make the conical part short and of a large angle, or it may stick fast, and cause an explosion; d is the glass gauge, to show the exact height of the water in the boiler. Its construction will be understood from the other which is attached, where the boiler is seen in section. There is no need to have two, and this is added solely to explain the nature of glass-gauges. The top and bottom are of brass, being tubes screwing into the boiler, or fastened by a nut inside; a tube, g, of thick glass, connects these two, so as to form a continuous tube, one end of which opens into that part of the boiler which is full of steam, the other opening below the water-level. Thus the tube forms practically part of the boiler, and the level of the water is clearly seen. The lower tap is used for blowing off water, to insure the communication being kept open, as it might get stopped up with sediment.

Gauge-cocks, e, f, are generally added, even where the glass water-gauge is used. One of these should always give steam, the other water,—the level of the latter being between the two. If the upper one gives water, the boiler is too full; if both give steam, the boiler needs to have water added. With these fittings, even a soldered boiler ought never to get burnt, and will last a long time with care.

The lower part, Fig. 67, is made like that before described, except that, being intended for charcoal, a circular grate is used, which simply rests upon little brackets fixed by rivets for this purpose. The flame and heat play upon the bottom of the boiler, and also pass up the central tube—the latter adding greatly to the quantity of steam produced. This furnace, when lighted, may be fed with bits of coke as well as charcoal, about the size of filberts, and will give plenty of heat. If the draught, however, is deficient, turn the waste steam into the tube, so as to form a jet at each stroke, and it will greatly increase it. It is in this way that the locomotive engines are always fitted, George Stephenson having first suggested the arrangement. Previously to this a fan had been fitted below the grate, which was put in rapid motion by the engine, and thus a sufficient draught was obtained.

THE SAFETY-VALVE.

To find out what pressure is exerted by the safety-valve, it must be clearly understood upon what principle it acts. I have in a previous chapter told you that the atmospheric pressure equals 15 lbs. on each square inch, so that if the surface of the valve which is exposed to the air is 1 inch in area or surface, it is pressed down with a force of 15 lbs. The steam, therefore, inside the boiler will not raise it until its elasticity exceeds this atmospheric pressure. If, therefore, we desire to have only just 15 lbs. per square inch pressing against the inside of the boiler (i.e., a pressure of “one atmosphere,” as it is called), we have only to load the valve so that, inclusive of its own weight, it shall equal 15 lbs. But it is plain that we must not load it at all in reality; for a flat plate, 1 inch square, of no weight, is all that is needed, the atmosphere itself being the load. Suppose, then, that we do load it with 15 lbs. in addition to the 15 lbs. with which nature has loaded it, we shall not find the steam escape until it presses with a force of 30 lbs. on the square inch, or two atmospheres (which, however, is not 30 lbs. of useful pressure upon one side of the piston, if the cylinder is open as in an atmospheric engine, but only 15 lbs.) This is not the strain which the boiler has to stand, because the atmosphere is pressing upon it and counteracting it up to the 15 lbs., so that this strain tending to burst it is but 15 lbs. The number of pounds, therefore, which is straining the boiler can readily be seen; being always that with which the safety-valve is loaded, and this is also the useful pressure for doing any required work. Unfortunately, however, even in the best constructed engines, a pressure of 15 lbs. upon the boiler by no means represents that in the cylinder. Now it would be inconvenient to place weights upon the safety-valve itself, and therefore a lever is added, as seen in the sketch, with a weight hung at one end of it. This is shown at B, Fig. 68, where a section of the valve is given with its stem passing through a guide to insure the correct motion of the valve. The lever is hinged at one end; and the rule of the pressure or weight which is brought to bear upon the valve is, that it is multiplied by the distance at which the weight hangs from the valve, compared with its distance from the hinge or fulcrum. If a weight of 7 lbs. is hung at 1, i.e., at a distance as far on that side of the valve as the fulcrum is on the other side of it, 7 lbs. will be the actual power exerted; at 2, where it is twice the distance, it will be doubled, and, as shown in the drawing, a pressure of 14 lbs. will be brought to bear upon the valve; while, if the weight is hung at 3, it will exercise a force of 21 lbs. This is very easy to understand and to remember. Sometimes (always in locomotives) the weight is removed and a spring balance is attached at the long end. Upon this is marked the actual pressure exerted; there being a nut to screw down, and thus bring any desired strain upon the spring. Mind, however, in case you should try this in any of your models, that the scale marked on the balance when you buy it must be multiplied, as before, according to the length of your lever. Thus, if I attach such a balance at 3 of the drawing, a real weight of 5 lbs. shown by the balance will be 3 × 5, or 15 lbs. upon the valve, and a balance made for such engine would be marked 15 lbs., to prevent the possibility of dangerous error.

ENGINES WITHOUT SLIDE-VALVES EASY TO MAKE.

Having been led on from the atmospheric engine to that of Watt’s, and to slide-valve engines generally, I am now going backward a little to a class easier to make, because they have no slide-valves, nor even four-way cocks; and then I shall have done with engines. But I dare say some of my readers will wonder why I have said so little about condensers and condensing engines. I am sure they will wonder at it if they understood what I explained of the advantage of a vacuum under the piston; so that 15 lbs. pressure upon the piston means 15 lbs. of useful work, instead of 30 lbs. being required for that purpose. But condensing engines are utterly beyond a boy’s power. They require not only a vessel into which the steam is injected at each stroke, but there must be a pump to raise and inject cold water to condense the steam, and a pump to extract from the vessel again this water, after it has been used, and a cistern, and cold and hot wells; and all this is difficult to make so as to act; and I am sure no boy cares for a steam engine that will not work. Moreover, I have given you difficult work as it is—work that many of my readers will no doubt be afraid to try—yet I did it on purpose; because if small boys are unequal to some of it, their big brothers are not, or ought not to be; and mechanical boys must look at difficulties as a trained hunter looks at a hedge—viz., with a strong desire to go over it, or through it, or any how and some how to get to the other side of it. Indeed, you must ride your mechanical hobby very boldly and with great pluck, or you won’t half enjoy the ride. However, I am quite aware that I have led you into several difficulties, and therefore now I propose to set before you some easy work as a kind of holiday task which will send you with fresh vigour to what is not so easy.

The engines without slide-valves have also no eccentrics and no connecting-rods. There is just a boiler, a cylinder, piston, piston-rod, and crank, and you have the sum total, save and except the fly-wheel. These are direct-action engines, the cylinders of which oscillate like a pendulum, and the piston-rod itself is connected to the crank, doing away with the necessity for guides.

Fig. 69, A, shows one of these engines, and you see that the cylinder leans to the left when the crank is turned to that side; and if you turn the wheel to the right, the crank will presently cause it to lean the other way; and thus, as it turns on a pin, or “trunnion,” as it is called, it keeps on swinging from side to side as the wheel goes round.

Now, when it is in its first position, the piston is at the bottom of the cylinder, and it then needs to have the steam admitted below it to drive up the piston; but when this has passed its highest position, and the cylinder is turned a little to the right, the piston must be allowed to descend, and, therefore, we must let out the steam below it. We ought, at the same time, to admit steam above the piston to force it down; but, in the simplest models, which are called single-action engines, this is not done. The fly-wheel, having been set in motion, keeps on revolving, and, by its impetus, sends down the piston quite powerfully enough to overcome the slight resistance which is offered by the friction of the parts.

Now, you can, I daresay, easily understand that it is possible to make this to-and-fro motion of the oscillating cylinder open first a steam-port to allow steam to raise the piston, and then an exhaust-port to let it blow off into the air. This is exactly what is done in practice, and it is managed in the following manner:—

Fig. 69.