Assume that the piston is in position shown, Fig. 253, and that both the main and auxiliary valves cover their respective steam ports. By means of a starting bar, operating through a stuffing-box in the valve chest, the piston valve, A, is moved toward the head of the steam chest, D, thus opening the ports, E and L, and admitting live steam through L, from the cavities, S, of the valve piston to the housing end of the main steam cylinder, through the port, F, Fig. 255, forcing the main piston, P, toward the opposite end of the stroke, or toward the left in the figure. The port, E, Fig. 253, being open, the exhaust steam escapes from front of the main piston through the port, F, Fig. 255, into the main exhaust port, G, through the port, E. The piston, P, travels to its extreme left position and the auxiliary slide valve has been drawn to such a position in the direction indicated by the arrow in the smaller drawing in Fig. 252, as to bring valve piston, A, toward the opposite end; the exhaust steam from the steam chest escapes from before it, through the exhaust port, K, the opening of which into the chest is at such a distance from the head as will permit sufficient exhaust steam to remain to afford a cushion to the valve piston. [Illustration: FIG. 253.] [Illustration: FIG. 254.] [Illustration: FIG. 255.] [Illustration: FIG. 256.] With the auxiliary slide valve in position to bring the hole, H, over the port, J, Fig. 256, it is plain that the exhaust through the port, K, will pass into the main exhaust through the port, L. With the main piston at its extreme travel toward the right, the ports, E and L, which correspond to F and F, respectively, in Fig. 255, are opened in such a manner as to exhaust steam to the atmosphere from the housing end of the steam cylinder through the port, F, and live steam from the chest to the head end of the main cylinder, through the port, F, thus driving the main piston, P, toward the housing end of the cylinder, or toward the right. The piston and reciprocating parts traveling in this direction move the auxiliary slide valve to its maximum point of travel in the opposite direction, thus opening the opposite auxiliary steam and exhaust ports and again driving the valve piston toward the head, D, of the steam chest, whence a new stroke begins. Lost motion in the valve gear is taken up by adjustable links, on all sizes above 7 inches diameter by 10 inches stroke and on some smaller sizes. Cushioning of the steam pistons in the larger sizes and upwards is accomplished by means of suitable valves called cushion valves. In the smaller sizes sufficient cushioning is done by exhaust steam passing from the clearance space next the head through a small hole drilled into the main steam port.