Similar motion-curves, such as _h′ i′ j′ k′ l′ m′ n′_, (represented in fig. 25) can be drawn to represent the position of the other edges of the valve, and also for the return stroke. The latter are shown in dotted lines. If we follow the curve _h′ i′ j′ k′ l′ m′ n′_, which represents the position of the edge of the valve _h′_ which governs the exhaust from the back end of the cylinder, we see that the port _d_ is opened and closed to the exhaust simultaneously with the opening and closing of the port _c_ for the admission of steam to the front end of the cylinder, and that they both remain open until the completion of the stroke. The width that the ports are opened by the valve is thus ascertainable from these diagrams, for any point of the stroke, and in fact can be seen at a glance. By the aid of such motion-curves, the movement of slide-valves can therefore be analyzed more perfectly than is possible without them. QUESTION 45. _What were some of the disadvantages of valves, like that shown in fig. 10, and which are shown by the motion-curves in fig. 25._ _Answer._ The free admission of the steam until the completion of the stroke by the piston was hurtful to the machinery, as it co-operated with the momentum of the piston and its connections in producing undue strains in the working parts. The steam then escaped from the cylinder without expansion, so that much of its useful energy was lost. The steam was not allowed to escape from one end of the cylinder until it was admitted at the opposite end, and as the process of exhausting it occupies some time, there was always more or less back pressure until all the exhaust steam was expelled from the cylinder. In practice, the imperfections of the valve-gear frequently delayed the opening of the ports, both for admitting and exhausting steam, until after the commencement of the stroke of the piston.[13] [13] D. K. Clark’s “Railway Machinery.” QUESTION 46. _How may some of these evils be overcome?_ _Answer._ By moving the eccentric forward on the axle so that the motion of the valve is advanced to the same extent, and the admission and exhaust of the steam will occur a little before the completion of the stroke of the piston. In this way the steam is admitted into the cylinder so as to act as a cushion to receive the momentum of the piston, and some time is given to the exhaust steam to escape, before the return stroke. QUESTION 47. _What is meant by lead?_ _Answer._ By _lead_ is meant the width of the opening of the steam-ports at the beginning of the stroke of the piston. On the steam side of the valve it is called _outside lead_; on the exhaust side _inside lead_. In fig. 26 the opening _h_ of the steam-port is the outside lead and _h′_ the inside lead. [Illustration: _Fig. 26._ Scale ³⁄₁₆ in. = 1 inch.] QUESTION 48. _What is meant by the travel of a valve?_