LOCOMOTIVE MECHANISM AND ENGINEERING. CHAPTER I. THE LOCOMOTIVE BOILER. The best subject with which to start is the lomoco- tive boiler, of which there are two illustrations. Fig. i is a wagon-top boiler. Fig. 2 is a Belpaire boiler, which is now being used on many roads. The principal difference between them is the shape of boiler over fire-box, and the method of staying. In the Belpaire boiler the back end over fire-box is nearly square, there being a radius on the corner for bending. The Belpaire boiler also uses radial stays, thus obviat¬ ing the necessity of using crown-bars and sling-stays ; which seems an advantage, as it not so complicated, and the mud cannot accumulate on the crown-sheet, as it does when using crown-bars. The sheets can adjust themselves to each other better under different pressures and change of temperature. The principal parts of a boiler are the fire-box, flues, dome, steam or dry pipe, throttle-valve, the smoke- arch in which are the steam-pipes leading to each steam-chest, the exhaust-nozzles, and the stack.
2 LOCOMOTIVE MECHANISM AND ENGINEERING. That portion of boiler which is over the fire-box must be very strongly stayed with either crown-bars or radial stays. The Belpaire boiler is strengthened in the latter manner. This staying is necessary from the large area and the exposure to the hottest part of the fire. The dome is for the purpose of providing a reser¬ voir for dry steam and in it the throttle-valve is placed as high as possible above the water-line, to prevent entrained water from being carried over into cylinder, which is detrimental to the engine, and might cause a cylinder-head to be knocked out, if in large quantities. The throttle-valve is a double disk (Fig. 3) having two seats in the steam-pipe. The disks are of nearly the same diameter, in order to balance. The top disk is the larger, so that the throttle will stay closed, due to the greater pressure on top, hold-
