In the Knowles pump a valve piston, G, Fig. 277, in the steam chest moves the main valve. This valve piston is driven alternately backward and forward by the pressure of steam, carrying with it the main valve, which admits steam to the main steam piston that operates the pump. The main valve is a plain slide whose section is of ^B^ form, working on a flat seat. [Illustration: FIG. 276.] [Illustration: FIG. 277.] The valve piston is slightly rotated back and forth by the rocker bar, H; this rotative movement places the small steam ports, D E F, Fig. 276, which are located in the under side of the valve piston in proper position with reference to the corresponding ports, A B, cut in the steam chest. Steam enters through the port at one end and fills the space between the valve piston and the head, drives the valve piston to the end of its stroke and carries the main slide valve with it. When the valve piston has traveled a certain distance, a corresponding port in the opposite end is uncovered and steam enters, stopping its progress by giving it the necessary cushion. In other words, when the reciprocating rotative motion is given to the valve piston through the outside mechanism, it opens the port to steam admission on one end, and at the same time opens the port on the other end to the exhaust. There is no point in the stroke at which either the valve piston or the main piston is not open to direct steam pressure, hence the immunity from any dead position or dead center. The operation of the pump is as follows: The piston rod with its tappet arm, J, Fig. 277, moves backward and forward with the piston. At the lower part of this tappet arm is attached a stud or bolt, K, on which is a friction roller, I. This friction roller, lowered or raised, adjusts the pump for a longer or shorter stroke. This roller coming in contact with the rocker bar at the end of each stroke, and this motion is transmitted to the valve stem, causing the valve to roll slightly. This action opens the ports, admits steam and moves the valve piston, which carries with it the main slide valve which admits steam to the main piston. The upper end of the tappet arm does not come in contact with the tappets, L M, on the valve rod, unless the steam pressure from any cause should fail to move the valve piston, in which case the tappet arm moves it mechanically. _To set the valve_, loosen the set screws in the tappets on the valve stem. Then place the piston at mid-stroke, and have the rocker bar, H, in a horizontal position, as shown in the engraving. The valve piston should then occupy the position shown at C, Fig. 276. The valve piston may be rotated slightly in order to obtain this position by adjusting the length of connection between the rocker bar, H, and the valve stem. Then turn the valve piston, G, one way or the other to its extreme position, put on the chest cover, and start the pump slowly.