_Answer._ The ends of each pair of eccentric-rods are connected together by a link, _a b_, fig. 103. This link has a curved groove or slot, _a b_, in it, in which a block, _B_, fits accurately, so that it can slide freely from one end to the other. This block is attached to the lower rocker-arm by a pin, _c_, which works freely in the block. The two eccentric-rods _C_ and _D_ are attached to the ends of the link at _e_ and _f_ by pins and knuckle-joints. It is apparent that if the link is down, or in the position shown in fig. 103 and also on a smaller scale in fig. 104, the motion of the upper eccentric-rod, which is usually used for the forward motion, will be imparted to the rocker, and thus to the valve, and when the link is in the position shown in fig. 105, that the valve will be moved by the lower or backward eccentric-rod _B_. In order to reverse the engine, it is then only necessary to provide the means of raising and lowering the links. This is done by a shaft, _A_, fig. 103, called a _lifting-shaft_, which has two horizontal arms, _E_,[50] one for each link, and a vertical arm, _F_. The links are suspended from the ends of the horizontal arms by rods or bars, _g h_, called _link-hangers_, which are connected to the links and to the arms above by pins, which enable the hangers to vibrate freely. The lower pin is attached to a plate, _L d_, called a _link-saddle_, which is bolted to the link. The vertical arm of the lifting-shaft is connected by a rod, _G G_, called the _reverse-rod_, to a lever _O_, _O_, Plate II. in the cab called a _reverse-lever_, the construction of which will be explained hereafter. This lever is worked by the locomotive runner, and by moving the upper end of it forward, the link will be lowered, and the rocker and valve will be moved by the forward eccentric; and if the reverse-lever is moved back, the link will be raised, and the backward eccentric will move the valve. When this is done, the valve-gear is said to be thrown into the _forward_ or _backward motion_, or _forward_ or _back gear_. [50] Only one of these is shown in the engraving. [Illustration: _Fig. 104._ _Fig. 105._ Scale, ³⁄₈ inch = 1 foot.] [Illustration: _Fig. 106._ _Fig. 107._ Scale, ³⁄₈ inch = 1 foot.] QUESTION 188. _How is the travel of the valve changed by the motion of the link?_