The port valves provide a simple means of contracting these passages and regulating the velocity of the up and down strokes to meet the requirements of the water ends of the pump so that uniform strokes are obtained regardless of the resistance against the main piston. When adjusting the auxiliary valve, place the rocker bar parallel to the piston rod and so that the arm to which the valve rod is connected will stand at right angles to the piston rod. The auxiliary valve must then cover all the ports equally. The upper or outer end of the arm on the rocker bar is slotted, hence moving the valve rod pin toward the end of the arm lengthens the stroke and moving it toward the piston rod shortens it. _To set the valve_ of this pump it is necessary only to square the levers and equalize the travel. THE GUILD AND GARRISON. [Illustration: FIG. 273.] [Illustration: FIG. 274.] The steam chest of this pump differs somewhat from other pumps in which valve pistons are employed to operate the main valve. See accompanying engraving, Fig. 272. This pump has a steam chest with a cover for inspection and repairs. This chest is bored at each end to form suitable cylinders to receive the valve piston, E. By the side of the valve piston, E, in the steam chest is an auxiliary slide valve, G, Figs. 273 and 274, which admits and releases the steam from the ends of the valve piston. The valve piston, E, has two slots at the center, the lower one receiving the lug on the back of the main valve and the upper one the toe on the rocker shaft, D. The rocker shaft has two toes, the larger one, F, engaging with the valve piston, and the smaller one with the auxiliary slide valve, G, as shown in Fig. 274. The auxiliary, as well as the main, valves are plain slides designed to take up the wear automatically. The pendulum lever, J, Fig. 273, is connected with the piston rod as shown and rotates the shaft, D, and by means of the toes the valves move together. The auxiliary slide valve, G, admits and exhausts the steam to and from the ends of the valve piston. Steam being admitted, the piston moves toward one end. The two valves also move in the same direction by means of the rocker shaft and the toes. This movement continues until the piston has nearly completed its stroke, when the auxiliary valve opens one of the small ports leading to the end of the valve piston, thus admitting steam at one end and exhausting it from the other. The valve piston moving from one end of the steam chest to the other, shifts the position of the main valve and reverses the motion of the main piston. The valve piston is moved the greater part of its travel by the toe on the rocker shaft, thus reversing the steam distribution, by steam pressure, which brings the opposite end of the slot in the driver to a second engagement by the toe on the rocker shaft which begins the return stroke.