_Answer._ The steam is released earlier in the stroke in proportion as the amount of outside lap and lead is increased, but the steam-port is also closed to the exhaust, or _compression_, as it is called, begins earlier with lap and lead than without. Thus, in fig. 25, it will be seen that at the beginning of the stroke both ports are entirely closed; in fig. 26, however, in which the valve has both lap and lead, the port _d_ is nearly wide open at the beginning of the stroke, and by following the motion-curve _r s t u v w x_, which represents the position of the exhaust edge of the valve, it will be seen that the steam was released from the port _c_ before the piston had completed its stroke, or when it had still nearly 3¹⁄₂ inches to move. In fig. 25 the port _c_ is not opened to the exhaust until the commencement of the stroke, but it remains open to its completion, whereas in fig. 26 it is closed, or compression begins, at 18 inches of the return stroke, as shown by the dotted motion-curve. QUESTION 54. _How does the action of the connecting-rod influence the motion of the valve in relation to the piston?_ _Answer._ By delaying the movement of the crank in the backward stroke of the piston, and accelerating it in the forward stroke. This will be best explained by reference to fig. 14, in which the piston is represented in the center of the cylinder, or the middle of the backward stroke. If now we take a pair of dividers set to a length equal to that of the connecting-rod, and from the center, _f_, describe an arc of a circle, _a b_, from the center of the shaft, and through the lower half of the circle which represents the path of the crank-pin, we will find that the point of intersection, _a_, falls short of the vertical line, _c d_, and that the crank-pin has not made quite one-quarter of a revolution while the piston was moving through the first half of the backward stroke. By referring to fig. 21, in which the piston is again in the middle of its stroke, but is moving forward, and by describing another arc of a circle, _b a_, from the center of the shaft and intersecting the path of the crank-pin, it will be seen that the latter has moved _more_ than a quarter revolution, while the piston has made the first half of the forward stroke. Owing to this _angularity_, as it is called, of the connecting-rod, the crank-pin is behind the piston during its backward stroke and ahead of it during the forward stroke. As the valve is moved by the eccentric, and it in turn by the shaft and crank, any irregularities of the latter are of course communicated to the valve. We therefore find, by referring to fig. 26, that the point of cut-off occurs during the backward stroke at 14¹⁄₂ inches, and during the forward stroke at 12 inches. A similar inequality is observable in the points of release for the front and back strokes. It is not, however, a matter of very great practical importance with stationary engines which run