[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.]
Having explained how the steam is generated, it remains to show how it propels a locomotive. It does this very much as a person on a bicycle propels it--that is, by means of two cranks the wheels are made to revolve, and the latter must then either slip or the vehicle will move. In a locomotive the driving-wheels are turned by means of two cylinders and pistons, which are connected by rods to the cranks attached to the driving-wheels or axles. These cranks are placed at right angles to each other, so that when one of them is at the "dead-point" the piston connected with the other can exert its maximum power to rotate the wheels. This enables the locomotive to start with the pistons in any position; whereas, if one cylinder only was used it would be impossible to turn the wheels if the crank should stop at one of its dead-points.