[Illustration: Fig. 16.--Longitudinal Section of a Locomotive Boiler. Fig. 17.--Transverse Section.] A locomotive boiler consists of a rectangular fireplace or fire-box, as shown at _A_, in Figure 16, which is a longitudinal section, and Figure 17 a transverse section through the fire-box. The fire-box is connected with the smoke-box _B_ by a large number of small tubes, _a a_, through which the smoke and products of combustion pass from the fire-box to the smoke-box, and from the latter they escape up the chimney _D_. The fire-box and tubes are all surrounded with water, so that as much surface as possible is exposed to the action of the fire. This is essential on account of the large amount of water which must be evaporated in such boilers. To create a strong draught, the steam which is exhausted from the cylinders is discharged up the chimney through pipes, and escapes at _e_. This produces a partial vacuum in the smoke-box, which causes a current of air to flow through the fire on the grate, into the fire-box, through the tubes, and thence to the smoke-box and up the chimney. Probably many readers have noticed, that of late years the smoke-boxes of locomotives have been extended forward in front of the chimneys. This has been done to give room for deflectors and wire netting inside to arrest sparks and cinders, which are collected in the extended front and are removed by a door or spout, _L_, below. [Illustration: Fig. 18.--Rudimentary Injector.] To get the water into the boiler against the pressure of steam a very curious instrument, called an injector, has been devised. Formerly force-pumps were used, but these are now being abandoned. The illustration (Fig. 18) shows what may be called a rudimentary injector. _B_ is a boiler and _E_ a conical tube open at its lower end--and connected to a water-supply tank by a pipe, _C_. A pipe, _A_, is connected with the steam-space of the boiler and terminates in a contracted mouth, _F_, inside of the cone _E_. If steam is admitted to _A_, it flows through the pipe and escapes at _F_. In doing so it produces a partial vacuum in _E_, and water is consequently drawn up the pipe _C_ from the tank. The current of steam now carries with it the water, and they escape at _G_. After flowing for a few seconds the water has a high velocity and the steam, mingling with the water, is condensed. The momentum of the water soon becomes sufficient to force the valve _H_ down against the pressure below it, and the jet of water then flows continuously into the boiler. A very curious phenomenon of this somewhat mysterious instrument is that if steam of a low pressure is taken from one boiler it will force water into another against a higher pressure. Figure 19 is a section of an actual injector used on locomotives. [Illustration: Fig. 19.--Injector used on Locomotives.]