As the main piston nears the right-hand port, the valve lever H, attached to and moving with piston rod brings the dog, I, on plate, J, in contact with the valve arm, K. This moves the supplemental valve, B, to the right and supplies live steam to the right of the valve piston, C, and exhausts the steam from the left-hand end through the ports, L. The main valve, being enclosed by the valve piston, moves with it to the left. As the steam enters the right hand main port and exhausts from the left hand port, the main piston commences its return stroke and the operation just described becomes practically continuous. As the main piston closes the main port, F, to the right, it cushions on compressed exhaust steam, because the main valve, A, has then closed the auxiliary port, M, leading to that end of the main cylinder. The steam in this case is supplied through the main and auxiliary ports, but exhausts through the main port only. [Illustration: FIG. 266.] [Illustration: FIG. 267.] _The slow movement of the main piston near the ends of the stroke_ allows the pump valves to seat quietly, and gives the pump cylinder time to fill with water, thus effectually preventing shock and noise. The valve piston, C, is cushioned by cushion valves, which are simply loose valves inserted near the ends in the valve piston. By removing the steam valve chamber, the main and supplemental valves may be readily examined or repaired. These valves are the ordinary plain flat slide with which all engineers are familiar. _To set the valve of this pump._ Place the piston in the center of its travel with the valve lever, H, plumb and also valve arm, K, plumb; adjust the supplemental valve so as to cover equally all the posts, which is done by lengthening or shortening its valve stem. THE NATIONAL. [Illustration: FIG. 268.] [Illustration: FIG. 269.] The main valve in this pump is moved by steam acting upon a valve piston. The steam admitted to and released from the auxiliary cylinder is controlled by a small slide valve operated by the valve gear. This small slide valve is in every essential feature identical with the slide valve of a steam engine, admitting and releasing the steam in precisely the same manner. In this pump the usual collars and tappets on the valve stem are dispensed with, the stem receiving motion by means of a roller carried by a slide block to which the valve stem is attached. The roller is given a lateral motion by set screws in the forked ends of the rocker arm, as shown in the engraving, Fig. 269. By adjusting these set screws the travel of the small slide valve is made to suit the speed of the pump, thus preventing the main piston from striking the cylinder heads. A sectional elevation of the cylinder and main valve is also shown; the connection of the valve piston to the main valve, also the construction of the small slide valve. The valve gear is provided with roller bearings to reduce the friction.