In the simple form of valve that appears in Fig. 24, the valve faces are just wide enough to cover the steam ports. If the eccentric is not _square_ with the crank, the admission of steam lasts until the very end of the stroke; if set a little in advance--that is, given _lead_--the steam is cut off before the piston has travelled quite along the cylinder, and readmitted before the back stroke is accomplished. Even with this lead the working is very uneconomical, as the steam goes to the exhaust at practically the same pressure as that at which it entered the cylinder. Its property of _expansion_ has been neglected. But supposing that steam at 100 lbs. pressure were admitted till half-stroke, and then suddenly cut off, the expansive nature of the steam would then continue to push the piston out until the pressure had decreased to 50 lbs. per square inch, at which pressure it would go to the exhaust. Now, observe that all the work done by the steam after the cut-off is so much power saved. The _average_ pressure on the piston is not so high as in the first case; still, from a given volume of 100 lbs. pressure steam we get much more _work_. HOW THE CUT-OFF IS MANAGED. [Illustration: FIG. 27.--A slide-valve with "lap."] [Illustration: FIG. 28.] Look at Fig. 27. Here we have a slide-valve, with faces much wider than the steam ports. The parts marked black, P P, are those corresponding to the faces of the valves shown in previous diagrams (p. 54). The shaded parts, L L, are called the _lap_. By increasing the length of the lap we increase the range of expansive working. Fig. 28 shows the piston full to the left; the valve is just on the point of opening to admit steam behind the piston. The eccentric has a throw equal to the breadth of a port + the lap of the valve. That this must be so is obvious from a consideration of Fig. 27, where the valve is at its central position. Hence the very simple formula:--Travel of valve = 2 x (lap + breadth of port). The path of the eccentric's centre round the centre of the shaft is indicated by the usual dotted line (Fig. 28). You will notice that the "angle of advance," denoted by the arrow A, is now very considerable. By the time that the crank C has assumed the position of the line S, the eccentric has passed its dead point, and the valve begins to travel backwards, eventually returning to the position shown in Fig. 28, and cutting off the steam supply while the piston has still a considerable part of its stroke to make. The steam then begins to work expansively, and continues to do so until the valve assumes the position shown in Fig. 27. If the valve has to have "lead" to admit steam _before_ the end of the stroke to the other side of the piston, the _angle of advance_ must be increased, and the eccentric centre line would lie on the line E^2. Therefore--total angle of advance = angle for _lap_ and angle for _lead_. LIMIT OF EXPANSIVE WORKING.