We will suppose, in the first place, that the line _f f_, fig. 25, represents the valve-face, _c_ and _d_ the steam and _g_ the exhaust-port, drawn to a larger scale than in the preceding figures. We will now draw the valve _H_ in the position represented in fig. 11, where the piston is at the beginning of the stroke. In order to show the valve in the position represented in fig. 12, where the piston has moved 4 in. of the stroke, we will draw a line 4-20, four inches below and parallel to _f f_, and extend the lines, representing the edges of the ports _c_, _d_ and _g_, downward. On the horizontal line 4-20 we will now draw the edges _i_, _r_, _i′_, _r_, of the valve in the same position in relation to the port _c_ that it has in fig. 12. We will then draw another horizontal line, 8-16, eight inches below _f f_, and parallel to it, and on this represent the valve in the position shown in fig. 13. In the same way we will draw lines 12, 16, 20 and 24 in. below _f f_, and draw the valve on each one respectively in the positions shown in figs. 14, 15, 16 and 17. The distance between the lower line 24-0, and _f f_, will then represent the stroke of the piston, or 24 in. If now we begin from the edge _h_ of the valve on the line _f f_, and draw a curve, _h i j k l m n_, through the same edge of the valve, represented on each of the parallel lines below, the curve will indicate the position of the valve in relation to the steam-port _c_ at each point of the stroke. To illustrate this, suppose we draw lines 1-23, 2-22, and 3-21, one inch apart and parallel to _f f_, and between it and 4-20. They will then represent the position of the piston after it has moved 1, 2 and 3 in. from the beginning of the stroke, and where they intersect the curved line will be the position of the edge of the valve when the piston has moved 1, 2 and 3 in. of the stroke. The curved line will in fact represent the position of the valve at any point of the stroke between these lines. Other horizontal lines, 5-19, 6-18, etc., can be drawn to represent every inch of the rest of the stroke. The curve line _h i j k l m n_, or _motion-curve_ as it is called, will then show the exact position of the edge of the valve and of the width of the opening of the steam-port during the whole stroke. From it we see that the valve opens the port _c_ for the admission of steam simultaneously with the movement of the piston, and when the latter has made one inch of its stroke the port _c_ is half open. At 4¹⁄₂ inches of the stroke the port is wide open,[12] and at 19¹⁄₂ inches it begins to be closed, but is not completely closed until the end of the stroke. [12] By cutting a paper section of the valve and placing it on the diagram in each position named, it will probably help the reader to understand the movement of the valve more and the nature of the motion-curves. [Illustration: _Fig. 25._ Scale ³⁄₁₆ in. = 1 inch.]