QUESTION 43. _How was the admission and escape of the steam effected by this valve?_ _Answer._ In order to explain this clearly, a series of diagrams will be necessary. Before referring to them, however, it should be explained first that the motion of an eccentric is exactly the same as that of a small crank. It is in fact a crank with a crank-pin whose diameter is very much enlarged. In the diagrams, figs. 11 to 25, the eccentrics will therefore be represented as small cranks, and most of the other parts by their centre-lines and points only, so as to make the diagrams as simple as possible. The dimensions selected for these illustrations are for the cylinder 16 in. diameter and 24 in. stroke, and a connecting-rod 7 ft. long. The steam-ports are 1¹⁄₄ in., the exhaust-port 2¹⁄₂ in., and the metal or bars between them, which are called _bridges_, are 1¹⁄₈ in. wide. The eccentric produces a lateral movement of 3 in., which is called its _throw_. In fig. 11 the piston is at the beginning of the backward stroke. The valve is then in the centre of the valve-face, and the eccentric is consequently at half-throw. The slightest movement of the crank in the direction of the dart _N_ will move the eccentric enough to open the front steam-port to the steam and the back one to the exhaust. In fig. 12 the piston is represented as having moved 4 in. of its stroke; the valve has then partly opened the front steam-port, and the other one is open to the exhaust. In fig. 13 the piston has moved 8 in. of its stroke, and the ports are now wide open, the front one to the steam and the back one to the exhaust. In fig. 14 the piston has moved 12 in., or is at half-stroke, and the valve has then moved as far as it will in that direction. In fig. 15 the piston has moved 16 in. and the valve has begun to return. In fig. 16 the piston has moved 20 in., and the valve has nearly closed the front port to the steam and the other to the exhaust. In fig. 17 the forward stroke is completed, and both ports are closed by the valve. Figs. 18, 19, 20, 21, 22 and 23 represent the piston and the valve on the return stroke in the positions corresponding with those described for the backward stroke. [Illustration: _Fig. 11._ _Fig. 12._ _Fig. 13._ _Fig. 14._ _Fig. 15._ _Fig. 16._ _Fig. 17._ Scale ¹⁄₄ in. = 1 foot. _Fig. 24._ _Fig. 23._ _Fig. 22._ _Fig. 21._ _Fig. 20._ _Fig. 19._ _Fig. 18._ Scale ¹⁄₄ in. = 1 foot.] QUESTION 44. _Is there any other method by which the motion of a valve can be represented by a drawing?_ _Answer._ Yes, by what are called _motion-curves_. It is, however, difficult to explain these clearly, and as they are purely imaginary, it is difficult to understand their nature and purpose. Close attention will therefore be required to the following description: