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. It will probably interest a good many readers to know how the steam gets into the cylinders and moves the pistons and then gets out again, and how a locomotive is made to run either backward or forward at pleasure. Figure 20 (p. 118) shows a section of a cylinder, _A A′_, with the piston _B_ and piston rod _R_. The cylinder has two passages, _c c_ and _d d_, which connect its ends with a box, _U_, called a steam-chest, to which steam is admitted from the boiler by a pipe, _J_. The two passages _c_ and _d_ have another one, _g_, between them, which is connected with the chimney. These passages are covered by a slide-valve, _V_, which moves back and forth in the steam-chest, alternately uncovering the openings _c_ and _d_. When the valve is in the position shown in Figure 20, obviously steam can flow into the front end _A_ of the cylinder through the passage _c_, as indicated by the darts. The valve has a cavity, _H_, underneath it. When this cavity is over the passage _d_ and _g_, it is plain that the steam in the back end _A′_ of the cylinder can flow through _d_ and _g_ and then escape up the chimney. Under these circumstances the steam in the front end _A_ of the cylinder will force the piston _B_ to the back end. When it reaches the back end of the cylinder the valve is moved into the position shown in Figure 21, and steam can then enter _d_ and will fill the back end _A′_ while that in the front end escapes through _c_ and _g_. The piston is then forced to the front end by the pressure of the steam behind it. It will thus be seen that the steam enters and escapes to and from the cylinder through the same openings. [Illustration: Figs. 20 (above) and 21.--Sections of a Locomotive Cylinder.] From what has been said it is obvious, too, that every time the piston moves from one end of the cylinder to the other the valve must also be moved back and forth in the steam-chest. This is done by what is called an eccentric.