_Answer._ Because before the beginning of the stroke of the piston it is necessary to move the valve from its middle position a distance equal to the lap before the steam-port begins to open. If we have a valve like that shown in fig. 10--that is, without any lap--the centres of the eccentrics could be placed at right angles, or, as mechanics say, “square” with the crank, as was shown in fig. 11, and exactly opposite to each other, because such a valve begins to take steam as soon as it moves from the middle of the valve-face. If, however, we have a valve like that shown in fig. 27, it is plain that before it will admit or _take_ steam, as it is called, in either of the steam-ports, it must be moved from the centre of the valve-face, or its _middle position_, a distance equal to the lap, L. For this reason, therefore, the eccentric, instead of being placed at _half-throw_,[49] as it is called, must be so far ahead of the middle position as to have moved the valve a distance equal to the lap, and if any lead is given to the valve, equal to the lap and lead together. In figs. 100 and 101, _f g_ is a vertical line at right angles to the crank at the beginning of the stroke. It will be seen that the centre of each of the eccentrics is set far enough ahead of this line to give the valve the required lead. When the piston reaches the back end of the cylinder, the two eccentrics will occupy the position shown in fig. 102, in which position the lower one would move the valve so as to turn the crank in the direction of the dart _N_, and the upper one in the reverse direction. It will be seen that in this position both of the eccentrics are again ahead of half-throw, when the piston is at that end of its stroke. [49] This would be at right angles to the crank when the piston is at the end of the stroke. [Illustration: _Fig. 103._ Scale ³⁄₄ in. = 1 foot.] QUESTION 187. _How is the motion of either eccentric communicated to the valve?_