An "eccentric" is a disk or wheel (Fig. 22) with a hole, _S_, the size of the axle of the locomotive to which it is attached. The centre _n_ of the outside periphery of the eccentric is some distance from _S_, the centre of the shaft. A metal ring, _K K_ (Fig. 23), made in two halves, embraces the eccentric, and the latter revolves inside of this ring. A rod, _L_, is attached to the strap, and is connected with the valve so that the motion of the eccentric is communicated to it. It is obvious that if the eccentric revolves it will impart a reciprocating motion to the rod _L_, which is communicated to the valve. [Illustration: Fig. 22.--Eccentric. Fig. 23.--Eccentric and Strap.] If properly adjusted on the axle the eccentric will run the engine in one direction. To run the opposite way another eccentric must be provided. Therefore locomotives always have two eccentrics for each cylinder. These, _J_ and _K_, are shown in Figure 24, which represents the "valve-gear" of a locomotive. _S_ is a section of the main driving-axle, to which the eccentrics are attached by keys or screws. _C_ is the eccentric rod of the forward-motion eccentric and _D_ that of the one for running backward. As a locomotive must be run either backward or forward, and, as the one eccentric moves the valve to run forward and the other to run backward, we must be able to connect or disconnect the rods to and from the valve at will. The eccentric rods of the early locomotives had hooks on the ends by which they were attached to or detached from suitable pins connected with the valves. But these hooks were very uncertain in their action and therefore were abandoned, and now what is known as the "link-motion" is almost universally used for the valve-gear of locomotives. It consists of a "link" (_a b_, Fig. 24) which has a curved opening or slot, _k_, in it in which a block, _B_, fits accurately, so that it can slide from end to end of the link. This block has a hole bored in the middle which receives a pin, _c_, which is attached to the end of the arm _N_ of the "rocker" _M O N_. The rocker has a shaft, _O_, which can turn in a suitable bearing, and two arms, _M_ and _N_; the latter, as explained, is connected to the link by the pin _c_ and block _B_. The upper arm _M_ has another pin, _V_, on its end, which is connected by a rod, _v V_, to the main slide-valve _V_. The rocker-arms, as will be seen, can vibrate about the shaft _O_. [Illustration: Fig. 24.--Valve Gear.]