The valve motion (see Figs. 302, 303) of one cylinder is communicated or produced by the piston of the other through the medium of rocker arms and links. By means of the small lost motion of the levers the pistons have a slight pause at the end of each stroke, which allows the water valves to seat quietly, thus preventing any slam or jar. With this arrangement, as one of the steam valves must always be open, there can be no dead point, thus removing the liability of the pump to stick. The simplicity of the duplex movement is at once evident, each valve is dependent upon its counter part, and both directly control the action of the steam, which is supplied through one simple throttle valve. NOTE.—Of the effect produced by the steam-moved direct-acting pump of much greater capacity it may be said there are now in use pumps of this class, exerting over 250 horse-power, delivering five million gallons of water in twenty-four hours through main pipes, say thirty inches diameter and fourteen miles long, without the use of an air chamber, and which do their work so quietly, steadily, and gently, that a nickel coin set on edge on the extreme end of the pump would not be overthrown by any jar or motion of the pump while it was doing this work. [Illustration: FIG. 302.] [Illustration: FIG. 303.] _The adoption of the Worthington design of duplex pumps_, has been well nigh universal, especially so since the expiration of the earlier patents. Nearly all leading manufacturers now make “duplex pumps.” Single, compound and triple expansion. Compounding consists of adding a second steam cylinder on the end of the high pressure in use, both using the same piston rod, the steam from the boiler being first used in the smaller cylinder, and at the end of the stroke of the piston being exhausted behind the piston of the larger cylinder on its return stroke. In this way the measure of the expansion of the steam used, was the relation of one cylinder to the other. By this arrangement a much smaller steam cylinder, for using the high pressure steam, could be adapted to do the same work, for in addition to the pressure of steam working full stroke in the small cylinder, was to be added the pressure of the steam being expanded in the large cylinder. In addition to this, for large compound and triple expansion engines was added the further economy realized by attaching a condenser to form a vacuum in the large steam cylinders. Adapting these newer improvements in the marine engine coupled with what had previously been accomplished in the direct-acting steam pump, they were at once brought up in size, capacity, and economy alongside the previously constructed rotative pumping engines.