_Answer._ It delays the release of the steam. Thus in fig. 26 the valve has ¹⁄₈ in. inside lap. The motion-curve _r s t u v w x_ shows that the release takes place during the back stroke at 21³⁄₄ in. If now there was no inside lap, the dotted line _y_, _x_ would represent the exhaust edges of the valve, and the release would then occur somewhat earlier, or at 21 in. For this reason no inside lap is usually given to valves for engines which run at high rates of speed, as it allows too little time for the steam to escape. In fact, in some cases, what is called _inside clearance_ is given to the valve; that is, the valve as shown in fig. 29, when it is in the middle of the valve-face, does not entirely cover the steam-ports. The effect of this is just the reverse of that produced by inside lap; that is, it causes the release to occur earlier in the stroke. [Illustration: _Fig. 29._ Scale ³⁄₁₆ in. = 1 inch.] QUESTION 52. _With the same outside lap, what is the effect of changing the travel of the valve?_ _Answer._ By increasing the travel, the period of admission is increased and that for expansion lessened; and by reducing it, the admission is lessened, and the degree of expansion is increased. This is shown by the motion-curves in fig. 28, in which the same valve and ports are represented as are shown in fig. 26, but the valve has a travel of 5 instead of 3 inches. The valve also has the same lead. By following the motion-curve _h i j k l m n_, it will be seen that the steam is thus admitted up to 20¹⁄₂ inches of the stroke of the piston, and the period of expansion, as compared with that in fig. 26, is correspondingly lessened. It will also be seen by comparing fig. 26 with fig. 28 that with the short travel of the valve the ports are not opened so wide as they are when the travel is increased. This evil is practically obviated, however, by making the ports so long that with a comparatively small opening they will still have area sufficient to admit enough steam to fill the cylinders, and it is known that an opening less than the whole area of the steam-ports is sufficient to facilitate the passage of steam into the cylinder. [Illustration: _Fig. 28._ Scale ³⁄₁₆ in. = 1 inch.] QUESTION 53. _How is the exhaust affected by lap and lead?_