QUESTION 185. _How must the eccentrics for each cylinder be set in order that the one may run the engine forward and the other backward?_ _Answer._ This can be best explained by reference to fig. 100, in which the piston, _P_, is represented at the beginning of the backward stroke, and the valve _V_ has the requisite lead and is just about to open the front steam-port. It is obvious that, in order to complete the backward stroke of the piston, the front port must be opened to admit steam into the front end of the cylinder, and therefore the valve must be moved in the direction indicated by the dart _a_. To do this, the upper arm of the rocker _r_ must move in the same direction, and the lower arm must be moved the reverse way, as indicated by the dart _e_. If the crank is intended to move in the direction indicated by the dart _N_, then the centre of the eccentric must be above the centre of the shaft or axle, in order to move the rocker in the direction indicated by the dart _e_. Supposing, however, it was intended to move the crank the reverse direction, as shown by the dart _N_ in fig. 101; it is evident in that case that the valve must be moved in the same direction as before, in order to open the front steam-port and thus admit steam to force the piston back. But if the crank turns in the direction shown by the dart _N_, fig. 101, then the centre of the eccentric must be placed _below_ the centre of the axle in order to move the lower rocker arm in the direction of the dart _e_ and the valve in that indicated by _a_. It will thus be seen that the centres of the eccentric for running forward and that of the one for running backward must be placed, the one above and the other below the centre of the axle at the beginning of the stroke of the piston, as shown in figure 101. [Illustration: _Fig. 100._ _Fig. 101._ _Fig. 102._ Scale ³⁄₈ in. = 1 foot.] QUESTION 186. _Why is it that the centres of the eccentrics are not placed opposite to each other on the axle?_