A casting of that shape and three-quarters of an inch thick, which is the distance between the inner and outer casing BC, must be procured and drilled with holes every three-eighths of an inch, and firmly riveted in position, as shown in section at D.
Two pins or lugs (FF) should project on either side of the inner surface to support the fire-bars and ash-pan, and the bars should be made of cast-iron, and small enough to be got out easily by tilting up one side, and the bars ought to run lengthways of the engine.
Fig. 33.
You next require some hard-drawn brass tubing three-quarters of an inch diameter, and must cut the pieces slightly over seventeen inches long, then drill ten holes in the inner plate as at E ([Fig. 32]), and in the position and arrangement shown in [Fig. 33]. These tubes should have a wire ring brazed on about a quarter of an inch from either end, and then being placed in their respective holes in tube plate, the projecting portion is to be beaded back with a flange, or you can fit them in as described previously ([Fig. 16]) by each being double-screwed and nutted. These tubes allow the smoke and flame to pass through from the furnace to the smoke-box (M [Fig. 20]), and so away up the chimney, and by the large surface they expose to the fire, help to raise steam very quickly.
Fig. 34.
If you just add together the combined surfaces of these tubes, you will find there is more than two square feet of surface exposed and acted on by the fire, which enables the boiler, although small, to make steam rapidly. In some large engines three hundred tubes are fitted. The steam supply-pipe and regulating lever-handle should now be made and placed in position, and [Fig. 34] shows the shape to make it.
A B are the front and rear plates of boiler, C is the supply pipe, bent with a screw end downwards after passing plate A, and then upwards into steam-dome, where it should be securely fastened by a cross-piece; D is the tap or valve, which can be turned on or off from the foot plate by means of the long rod F, ending in lever-handle G.
The rod must be fitted with a stuffing-box, the same as those used on the cylinders, and packed with cotton to prevent loss of steam by leakage; and when this is all firmly fixed, the forward end of the boiler can be furnished with tube-plate, riveted on and the tubes flanged over.