[Illustration: FIG. 262.] The main valve and preliminary valve travel on the same seat, and receive their motion from the yoke, _E_, Fig. 263, which surrounds them. This yoke fits the preliminary valve neatly, allowing an independent movement of the main valve and valve piston. The preliminary valve is the ordinary ^D^ type and controls three ports, as shown in Fig. 263, the two steam ports are connected with either end of the auxiliary cylinder and the exhaust, with the small port, _A_, in the main valve seat. The duty of the preliminary valve is to alternately connect each end of the auxiliary cylinder with this port. The main valve controls four ports, and is also of the ^D^ type, having an extended cavity connecting the exhaust from the preliminary valve with the main exhaust. The main valve also controls the main steam and exhaust ports of the pump cylinder. Steam enters the chest through the steam pipe, passes around the central portion of the valve piston and through the passages in the piston to both ends of the valve cylinder, thus balancing the valve piston. Suppose that the yoke and preliminary valve have been carried to the back end of the chest, and have moved the main valve to bring its auxiliary cavity over the auxiliary port, _A_, Fig. 263, in the main valve seat, which connects the back end of the auxiliary cylinder with the main exhaust. The steam from the end of the chest passes up through port, _B_, down through port, _C_, and up through port, _A_, into the exhaust port, _D_, in the main valve. This unbalances the auxiliary piston, which is driven back by pressure on the opposite end, and carries the main valve independently of the yoke. The travel of the main valve causes it to cut off the exhaust from the auxiliary cylinder, and the remaining steam cushions the valve piston. [Illustration: FIG. 263.] The travel of the main valve also opens the main steam port to the forward end of the pump cylinder, and connects the back end of the cylinder with the main exhaust, thus reversing the motion of the piston. This action is repeated at the end of each stroke. Whatever position of the piston, the pump will start when steam is turned on, as there is always a connection either directly from the steam inlet to one of the steam ports, or, if the main valve has covered both steam ports, it is in a position to connect the auxiliary cavity in the main valve seat with the main exhaust, which at once releases the steam from one end of the valve piston, and the pressure on the other end drives the piston, thus moving the main valve and giving direct communication between the steam inlet and one of the main steam ports. Thus it is seen that the piston cannot get into a position where it is impossible for it to respond to the steam pressure.