On the admission of steam to the cylinder the main port, F, the main piston, cam and valve will move in the direction indicated by the arrows. The first movement of the cam oscillates the valve, preparatory to bringing it into a proper position for the opening of the auxiliary steam ports, E, to live steam, and E, to exhaust also to close the valve mechanically just before the main piston reaches the end of its stroke. This causes a slight cut-off and compression, and fully opens the auxiliary ports, E, to steam, and E_{1}, to exhaust. By the admission of steam to one end, the other being open to the exhaust, the valve pistons move the valve to allow the admission and exhaust of steam from the cylinder for the return stroke. This main valve is as much under the control of the piston rod as is the valve of an ordinary steam engine worked by an eccentric which insures a positive action, the pump being capable of starting from all positions and maintaining a uniform and full stroke. _To set the valve piston_, push the main pistons to the end of the stroke until the inner edge of the port and the piston coincide, then loosen the side lever, turn the cam, C, until the valve piston uncovers the auxiliary steam port, E, leading to the same end of the steam chest occupied by the main piston. [Illustration: FIG. 251.] After setting, secure the cam and then connect the side lever to the connecting rod. The side lever and cam occupy correct relative positions, therefore, the lever should be secured to the cam shaft while in this position. The stroke may be regulated by raising or lowering the end of the connecting rod in the slotted end of the slide lever. Raising the connecting rod shortens the stroke and lowering it lengthens the stroke. When making the foregoing adjustments it is well to have the connecting rod at or near the bottom of the slot as shown in the engravings. LAIDLAW-DUNN-GORDON. The single cylinder pumps of this make are equipped with the gear illustrated in Fig. 252, in sizes varying from 4 inches in diameter by 5 inches stroke to 28 inches in diameter by 24 inches stroke. The arrangement of valves and ports is shown in the engravings, Figs. 253 and 254. The admission of live steam to the cylinder and of exhaust steam to the atmosphere is controlled by a valve piston, A, shown in Fig. 252. [Illustration: FIG. 252.]