If now we want to represent the motion and relative position of the back steam edge of the valve in relation to its port, it is only necessary to assume that the line _t_ represents that edge, and that the curve _h i j k l m n o h_ represents its motion, and to draw the back steam-port in its proper relation to it. When the valve is in its middle position, as shown in fig. 27, the outside edge of the port _h_ is ⁷⁄₈ in., or a distance equal to the lap, from the steam edge _q_ of the valve. As the center line _a b_, fig. 127, represents the middle position of the edge of the valve, it is only necessary to draw a line, _n n′_, ⁷⁄₈ in., or the same distance from the centre line _a b_ that the outer edge of the port _d_ is from _q_ in fig. 28, to represent this edge of the port in fig. 127, and another _b b′_, at a distance from the former equal to the width of the port, to represent its inner edge. A line, _q_, below the line 0 24, will represent the edge of the valve at the beginning of the forward stroke. The curves in relation to the port _d_ will then show the motion of the valve in relation to this port, in the same way that the dotted curve _d f_ does in fig. 28. [Illustration: Fig. 127. Scale ³⁄₁₆ inch = 1 inch.] If it is desired to represent the motion of the exhaust edge _h′_, fig. 28, of the valve, it is only necessary to imagine that the line _t_, fig. 127, represents that edge, and then draw in the port _d_ in the same relation to it that it bears to the edge _h′_, in fig. 28. This has been done in dotted lines, _c c′_ and _e e′_, in fig. 127. If the reader will cut a paper section of a valve like that shown in fig. 27 and place the different edges, _h_, _i_, _h′_ and _q_, so that they will successively correspond with the line _t_ in fig. 127, the diagram will perhaps be more clear. If, for example, the paper section be placed to the right of the line _t_, so that the edge _h_ will correspond with _t_, then it will be seen that the port _c_ occupies the same relation to it that it does in fig. 28. If the valve be placed to the left, so that the edge _q_ corresponds with _t_, then the port _d_ will be in the same relation to it that it has in fig. 28. If the edges _i_ and _h′_ be made to correspond with _t_, then the ports drawn in dotted lines in fig. 127 will represent the ports _c_ and _d_ in fig. 28.