Whenever you have to make an engine, you should draw upon the bed-plate the position of each part, as I have done here, because it will serve you as a guide for measurement of the several pieces. The four small circles at S S show the positions of the legs of the support C, which carries the beam. In the drawing only two are given, but there would be a similar triangular frame upon this side. This may be made very well of stout brass wire, but in a bought engine it would be a casting of brass, painted or filed bright.
The beam itself should be of mahogany, 6 inches long, half an inch wide (on the side), and a quarter of an inch thick. The curved pieces you will turn as a ring 3 inches diameter with a square groove cut in the edge for the chain. You can then saw into four, and use two of these, morticing the strip of mahogany neatly into them. Then finish with four brass wires, as shown, which will keep the curved ends stiff and give a finished appearance. The pin in the centre should be also of brass, as a few bright bars and studs of this metal upon the mahogany give a handsome look to the engine.
The pump will be of brass tube, made like the cylinder, but the bucket may be of boxwood, and so may the lower valve, each being merely a disc with a hole in it, and a leather flap to rise upwards. The bucket, however, should have a groove turned in its edge, to receive a ring of india-rubber, or a light packing of tow. The end of the pump-rod must be split to make a fork like Y, to allow the valve to rise. You can get just such a fork ready to hand out of an umbrella, if you can find an old one; if not, and you cannot split the wire, make the rod rather stouter, and bend it, as shown, so as to form only one side of a fork, which will probably answer the same purpose in so light a pump.
The valve in both of these may be made of a flap of leather—bookbinder’s calf, or something not too thick—and it may be fastened at one edge by any cement that will not be affected by water, or by a small pin,—cut off the head of a pin with half an inch of its shank, and point it up to form a small tack. If the valve-box is of boxwood, you must drill a hole;—you may make it, if preferred, of softer wood.
There is no support shown in the drawing for the cold-water cistern; but you must stand it on four stout wires, or on a wooden (mahogany) frame, which can be attached to the bed-plate. As this last is always of some importance, I shall add it again in this place (Fig. 60), to a scale of three-quarters of an inch to the foot, showing the position of each part.
Fig. 60.
Always begin with a centre line and take each measure from it, and draw another across for the same purpose, at right angles to the first. You will quickly see the use of this. We draw two lines as described A, B, C, D, crossing in o. The longest is the centre line of beam, cylinder, and pump. The beam is to be 6 inches long to the outside of the middle of each arc, whence the chain is to hang. We, therefore, from the centre point, set off 3 inches each way. At the exact 3 inches will be the centres of the cylinder and pump;—set these off, therefore, on the plan. The end of the tank we must have near the cylinder, because we have to bring a pipe from it into the bottom of the cylinder. Set off, therefore, the end of the tank 2½ inches—i.e., 1¼ on each side of the central line, and draw it 4 inches in length. N shows the position of the pipe close to the end and on the line. The centre of the boiler is the same as that of the cylinder, so we draw a circle round it with a radius of 1½ inches, which gives us the 3-inch circle of the boiler. Then we may set off equal distances, N, N, for the extremities of the legs of the frame which is to support the beam, and we complete our plan. M is the waste pipe, and K is the opening for the water to flow into the tank. We now find, therefore, that the bed-plate must be 13 inches long and 6 inches wide to take the engine of the proposed size, and we may, of course, extend this a little, if thought desirable. Mark off on the bed all the lines of the plan as here given, and always start any measurement from one of the two foundation lines, or else, if you make one false measure, you will carry it on, probably increasing the amount of error at every fresh measurement. Let this be with you a rule without exception. It is plain that if you work all parts of your engine to size, you can set it up on the marked bed-plate with perfect accuracy.
The description I have given will not only enable you to make a Newcomen engine with very little difficulty, but will give you an insight generally into this kind of work; and you will learn, too, a practical lesson in soldering, turning, and fitting. I must, nevertheless, help you a little in putting your work together.