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.
Fig. 62.