I have already told you that Watt suggested the use of steam alternately on each side of the piston; and carried it out by closing the top of the cylinder, and allowing the rod of the piston to pass through a stuffing-box or gland. I now have to explain to you how this alternate admission of the steam may be effected.

You evidently require first an opening at the top and bottom of the cylinder, communicating with the boiler, one only being open at a time; but in this case, where is the steam to escape that was on one side of the piston when the opposite side was being acted upon? It must go somewhere, but evidently must not return to the boiler. Hence, some method has to be contrived by which, when one end of the cylinder is open to the boiler, the other may be open to the air or to the condenser (in which the steam is cooled under Watt’s plan). Fig. 61 will, I think, render clear one or two of these arrangements.

The first is the four-way cock, a very simple contrivance, easily and frequently used in models. You must first understand how a common water or beer tap is made. Fig. 61, A, represents one in section, turned so as to open the passage along the pipe to which it is attached; C is the pipe in which is the tap, a conical tube of brass set upright, and with a hole right and left made through it, fixed into a short horizontal tube (generally cast with it in one piece). Into this fits very exactly the conical plug B, also with a hole through it sideways. When this is put into place, no water or other liquid can pass, unless the hole in the plug is in the same direction with the hollow tube forming an open passage. If a key is put on the square part of the plug, and it is turned half round, the passage through the pipe will be closed. A steam tap would be made in a similar manner, if its only office were to open and close a passage in a tube. But we now want two passages closed and two opened, and then the alternate pair closed and opened. This is cleverly effected by a four-way cock.

At D is shown a section of the steam cylinder and piston, with the stuffing-box and all complete. A pipe enters this at the top and bottom, and another crosses it in the middle, making four passages. Shaded black is the four-way cock, the white places showing the open channels through the plug. When this plug stands as at D, steam can pass from the boiler to the top of the cylinder only, above the piston, which it drives downward; the steam below the piston escapes through the other open-curved channel into the air, or to the condenser. Just as the piston reaches the bottom of the cylinder, the tap is turned, and the passage stands as seen at E. Steam now passes to the bottom below the piston, driving it upward, and the steam above it, which has done its work, passes outward through the other open channel of the tap.

You must understand that when Newcomen first set up his engine, a man had to turn the taps at the proper moment; and it is said that one Humphrey Potter, a boy, being left in charge, and getting tired of this work, first devised means to make the engine itself do this, by connecting strings tied to the handles of the taps to the beam that moved up and down above his head. Beighton and others improved on this, and very soon it became unnecessary for the attendant to do anything but keep up a good fire, and attend to the quantity of water in the boiler, and the pressure of the steam.

In the model I gave you of Newcomen’s engine, I purposely left the taps to be moved by hand; but F of the present figure shows how, by bringing them near together, and adding cogged wheels or pulleys, you would make one handle answer for both; and I shall leave you to devise an easy method of making the engine work this one handle for itself. When Watt made his first engine, therefore, this work had been already done, and he only had to improve upon it, and to make it work more accurately to suit the engine designed by himself.

If you should chance to pay a visit to the Museum at South Kensington, you may see, I believe, Watt’s original engine, if not Newcomen’s. The cylinders are so large and cumbrous, that the wonder is they were ever bored by the inefficient means then in use; and the beam is a most unwieldy mass of timber and iron, that looks as if no power of steam could ever have made it oscillate. Yet it was in its day a successful engine, the wonder of the age; and did good work for its inventor and purchaser. I strongly advise my readers to try and visit Kensington, for there are many interesting models there, besides engines and appliances of older days. They will thus learn what rapid progress has been made since the days of Savery, Newcomen, and Watt; not only in the improvement of the arrangement of the parts, but in the workmanship, which last is mainly due to the invention of the slide-rest and planing-machine.

We must now return to the double-acting or real steam engine, and consider a second means whereby the steam can be alternately admitted and exhausted.

The four-way cock, already explained, was found to wear very considerably in practice, and hence work loose, and a new contrivance, called the slide-valve, soon took its place. Of this there are two patterns, the long D-valve and the short one, which latter is used for locomotives. There is also a form called a tappet-valve, often used for large stationary engines, but which is noisy and subject to rapid wear. I shall describe the long D first, in the form in which it would be most easily made for a model engine.

The two ports by which steam passes to the cylinder are shown at d, e, of H, Fig. 61. C is the passage to the boiler, K is that to the condenser. These are openings in a tube smoothly bored within, and having at the top a stuffing-box like that on the cylinder. Within this tube works an inner one, b, having rings or projections at the ends fitting perfectly, and which are packed with india-rubber, hemp (or, in modern days, with metal), to make a close fit. In a model, two bosses of brass, K, soldered on the tube and then turned, make the best packing. These packed portions of the inner tube form the stoppers to the steam ports, e e, alternately, at the top and bottom. The upper part of the inner tube has a cross arm, 3, affixed, from the centre of which rises the valve-rod by which it is moved up and down. In the position 1, the steam can pass from c round the tube to d, and thence to the top of the cylinder to which d is attached. The exhaust steam passes from e below the piston by k to the condenser. In the second position, 2, the steam is evidently shut off from d, but can pass out at e e below the cylinder, while the communication is still open to the condenser from d, through the middle of the tube to K. This is a very good form of valve, because the exhaust is always open, and the motion is smooth and equal.

There are many modifications of the long D-valve, but the principle of all is the same; I shall therefore describe the short slide-valve which is nearly always used in the models which are purchased at the shops. This, too, is the usual form of valve in locomotives, traction-engines, and the majority of those in use for agricultural and similar purposes. A, Fig. 62, is the cylinder as before in section with piston. A thick piece is cast with the cylinder, on one side of it, having steam ports also cast in it, which are here left white. The two as before go to the top and bottom of the cylinder, and have no communication with the central one, which is bored straight into the boss, and generally is turned at right angles and connected with the condenser, or with a pipe opening into the chimney of the engine to increase the draught by means of the jets of steam, as is the case always in locomotives, or into the air, which is less usual. Seen from behind, these ports are like B, being cast and cut rectangular; and the face, B, is planed quite level, which is absolutely necessary to the proper action of the slide-valve which has to work upon it. This valve is a box of iron, C, with a wide flange or rim, this flange being of sufficient width to close either port. If this valve is placed as it stands when the engine is at rest, b covers the upper steam port, and a the lower; while the exhaust or middle port is open to the hollow part of the box. Now, if we slide the valve downwards until the upper port is open, the other two will be in communication, being united by being both together in the inside of this box or valve. Suppose the valve then cased in, and that steam is admitted from the boiler into the case, it is evident that such steam could freely pass to the top of the cylinder above the piston to force it downwards, while that which was below would escape by the lower port into the box, and thence pass to the condenser. If, instead of pushing down the valve, we had drawn it upwards, the lower port would have been opened, and the upper and middle would have been brought into communication inside the valve, and the contrary effect would have been produced upon the piston.

This is the arrangement adopted, and which will be clearly understood from the following sectional drawing, D. a, a, is the thick casting upon the cylinder, with the upper and lower steam ports, which end towards the middle of the cylinder, with the third port lying between; then b is a section of the valve, in such a position that the flange of it no longer covers the lower steam port, while the other two are open together on the inside of the valve. The latter is cased in by the valve-box, e e, in the back of which is the steam pipe f coming from the boiler. The valve-rod, which is moved by the engine, passes at o through a stuffing-box. It is evidently necessary that this slide-valve should fit, and work very smoothly and correctly against the face of the ports, so as not to allow any escape of the steam. It is not, however, packed in any way at the back (although springs have been sometimes added), because, as the back is subjected to the full pressure of the steam from the boiler, this keeps it quite close to its seat. The rod, however, by which it is worked, might prevent this close contact of the two surfaces if it was screwed into the valve; it is therefore made with a cross, E, at the end, which falls into a notch in a boss cast upon the back of the valve as seen at F. This allows a certain degree of play in one direction, and permits the steam to press it close even after it has become worn by use.

You will, I think, now clearly understand how steam can be admitted alternately to the top and bottom of a cylinder, and how the exhausted steam that has done its work escapes. I must therefore now tell you how the rod of the slide-valve is moved up and down by the engine, but to do this, I must draw such engine complete.

The cylinder, A, is screwed down on its side upon the bed-plate, R R, out of which are cut two holes, one for the fly-wheel, P, of which part only appears for want of space, the other for the crank, L, on the end of the axle, M M, running through bearings, N N. The slide-valve-box is at B, C being the steam-pipe from the boiler. The piston-rod has necessarily to move only in a straight line in the direction of its length, but the crank which it has to work to turn the fly-wheel must needs move round in a circle. Hence, a poker-and-tongs joint, F O F, is arranged. The connecting-rod, H, which is attached to the crank by brasses at K, divides or is attached to a forked piece, at the lower end of which are a pair of bearings or brasses, F F. The piston-rod carries the piece O, the cross-bar of which is turned, being, in fact, the pin which passes into these bearings at F F. This forms, therefore, a hinge-joint at this place, so that although the piston-rod cannot leave the right line, and can only slide in the guide, E, the rod, H, has an up-and-down motion upon this hinge, allowing the revolution of the crank-pin to take place. D is the valve-rod, in which is a hinge at S, which suffices for the slight movement required in the rod, as it rises and falls by the action of the eccentric, T, the motion and effect of which I now have to explain.

V is a round disc of metal with a recess on its edge, so that it is like an ordinary pulley, but large in proportion to its thickness. A hole for the main crank axle, to which it has to be firmly keyed, is made through it, but not in its centre (hence its name, eccentric—out of the centre). As the axle revolves, it is evident that this disc revolving with it will carry any point, Y, of its surface round in a circle; the centre of which is on the central line or axis of the crank-shaft. I have drawn such circle as described by the point Y, farthest from the axis; but any and all points describe larger or lesser circles round the same centre. The point Y may, therefore, be considered as the centre of a crank-pin; and the eccentric might, so far as its effects are concerned, be replaced by a crank. Now, if you turn the fly-wheel of your lathe by hand, the crank will revolve, but the treadle will rise and fall only in a straight line; and you will presently see how the eccentric, in its revolution, gives just such a to-and-fro motion to the rod D, and consequently also to the slide-valve, which it has to move.

Round the disc V, closely encircling it, is a flat ring, shown separately at X, with a rod, W, attached to and part of it. This ring is generally made in separate halves, united by bolts passing through projecting lugs or ears. The ring also fits into the groove turned on the edge of the disc V, so that it cannot slip off sideways. This outer ring is turned quite smooth and true on the inside, so that the eccentric disc can revolve within it. In doing so, it is plain that the whole ring will rise and fall, and that the rod W will move up and down, or to and fro, like the treadle of the lathe, thereby giving motion to the valve-rod, which is a continuation of the rod W. As the upper end, however, of this rod has an oscillating, or up-and-down motion, this is imparted, in a certain degree, to its other end, at the farthest distance from the eccentric; and hence the necessity for a hinged joint at S, to prevent the valve-rod from partaking of this movement. It is, however, very slight, so that the rod of the valve is not often made to pass through guides like the piston. The whole movement of the valve-rod is very limited, its traverse only being required to be sufficient to shift the valve the width of one of its ports at each stroke. The length of stroke or traverse which can be obtained by the eccentric is always equal to twice the distance between its real centre, and that on which it turns, which will always be a guide to you in making an engine.

Fig. 64.