_Answer._ As already explained, the effect is to increase the period of admission, or in other words to cut off later in the stroke, and also to increase the width of the opening of the steam-port or the distance which the valve throws over the port. This has an important influence upon the admission, when the link-motion is used. QUESTION 231. _What is meant by the angular advance of the eccentrics?_ _Answer._ It is the angle which a line, _e f_, fig. 141, drawn through the centre of the axle and the centre of the eccentric makes with a vertical line _a b_, when the crank is on one of the dead-points or centres. Thus in fig. 141 the crank _A_ is represented on the front centre. In order to give the valve the necessary lead the eccentric must be moved ahead of the vertical line _a b_. The angle _c_ which the line _e f_ (drawn through the centre of _g_ of the eccentric and _f_ of the axle) makes with the vertical line is called the _angular advance_. QUESTION 232. _What is meant by linear advance?_ _Answer._ By linear advance is meant the distance which the valve has moved from its middle position at the beginning of the stroke of the piston. This, when the two rocker arms are the same length, is the same as the distance of the centre of the eccentric _g_ from the vertical line _a b_, fig. 141. QUESTION 233. _Why does the cut-off occur earlier with an eccentric having a short throw than with one which gives more travel to the valve?_ _Answer._ Because it is necessary to give the eccentric with the short throw more angular advance in order to give the valve the required lead. This is illustrated in fig. 142, in which a section of a valve, _V_, and ports _c_, _g_, and _d_, are represented. In order to simplify the diagram as much as possible the rocker is left out and the valve is supposed to be moved by the rod _R_ directly from the centre _a_ of the eccentric.[63] The effect of the angularity of the connecting rod and eccentric rod is also neglected. The circle _a b e f_ represents the path of the centre of an eccentric having 5 in. throw, and _h i j_ the path of one having 3¹⁄₂ in. throw. In order to give the valve the required lead, which is supposed to be just line-and-line at the beginning of the stroke, the linear advance of the valve must be equal to the lap, or ⁷⁄₈ in. If therefore we draw a line, _p a_, parallel to the vertical centre line, _e k_, and ⁷⁄₈ in. from it, the intersection of _p a_ at _a_ and _h_ with the paths of the eccentric will be the centres of the eccentrics. If through these centres and the centre of the circle, lines, _o a_ and _o p_, be drawn, the angles which they make with the vertical _e k_ will be the angular advance. It will be seen from these lines that in order to give the valve the required lead it is necessary to give the eccentric with the small travel more angular advance than is necessary for the one with the larger throw. It is obvious, too, that when the centre of the larger eccentric has