_To set the valve._ The main piston must be moved to the end of the stroke, so that the face of the piston and the edge of the steam port are flush, as a preliminary step to set the small slide valve, then the set screw in the rocker arm nearest the roller on the valve stem should be adjusted so that when the roller is in contact with the set screw the small slide valve will have opened the steam port corresponding to the position of the main piston. Now push the main piston to a corresponding position at the opposite end of the stroke and adjust the other set screw nearest the roller on the valve stem in the same manner as the first one. Mark the striking points of piston on the piston rod close to the stuffing-boxes. Start the pump slowly, and if the parts work smoothly, gradually increase the speed, keeping close watch of the striking points to see that the piston has ample clearance. If the stroke is too short, the set screws should be backed out until the stroke is found to be right. Screwing the set screws in shortens the stroke and increases the clearance at the ends of the stroke. A full stroke should always be required of every pump. THE HILL. [Illustration: FIG. 270.] This pump is for deep wells; it has a valve piston and two slide valves operated by valve stem, also supplemental port valves to regulate and control the up and down strokes of the main piston. The main and auxiliary valves are both flat slides, each covering five ports, as shown in Fig. 271. The main valve is actuated by means of a valve piston, the steam being admitted to and exhausted from the ends of the valve piston in precisely the same manner that the main valve controls the admission and release of steam in the main pump cylinder. [Illustration: FIG. 271.] One of the auxiliary ports enters the end of the main steam chest, and the other enters at a point nearer the middle of the chest. The end ports are admission only and the inner ones exhaust. When the auxiliary valve reaches the upper end of its travel, as shown in the engraving, the admission port opens at the bottom of the steam chest, while the one at the top closes. At the same time, the upper end of the main cylinder is open to the exhaust. The admission of steam drives the valve piston upward, but before reaching the end of its stroke it closes the exhaust port, thus entrapping a portion of the exhaust steam, which cushions the valve piston at the ends of the stroke. The auxiliary slide valve is operated by means of a double-cone tappet on the piston rod, which strikes a rocker bar pivoted to the frame of the pump. The rocker bar carries an arm or lever at right angles to it and to this arm the valve rod is connected. A short shaft runs across and through the upper part of the steam chest and is provided with a toe, as shown, by means of which the valve piston may be moved by hand when necessary and without disconnecting any part of the pump or valve gear. [Illustration: FIG. 272.]