Two pumps of similar construction are placed side by side, a lever attached to the piston rod of each pump connects to the slide valve of the opposite steam cylinder; thus the movement of each piston, instead of operating its own slide valve as in the single pump, operates the slide valve of the opposite cylinder. The effect of this arrangement is, that as the piston or plunger of one pump arrives near the end of its stroke, the plunger or piston of the other begins its movement, thus alternately taking up the load of the water column, producing a regular, steady, onward flow of water, without the unusual strains induced by such a column when suddenly arrested or started in motion. While the “duplex steam pump” overcame one of the greatest objections to the former single pump, there still remained in this class of pumping machinery one other difficulty. It did not use steam expansively. This not only debarred it from competing with other engines where a large quantity of water was required to be raised, and where the cost of fuel was an item of importance, but as well prevented the pump from taking rank among the hydraulic appliances required in supplying towns and cities. This objection was one which seemed insurmountable, steam in them could not be used economically. Applied to the propulsion of the plunger or piston of this pump it must be of sufficient quantity, and pressure, to overcome the height of the column of water on the pump, together with its friction through the pump and its connections, _at the very beginning of the stroke_; and it must be maintained, both as to its volume, and its pressure up to the very last part of the stroke. Any diminution, either of volume or pressure, during any part of the stroke would simply bring the pump to a stop. This apparent inability to cut off the steam in the steam cylinder, and to complete the stroke of the pump by the aid of the steam remaining in the cylinder, and by its expansive force, had debarred this pump from coming into general use for large water works. How this, the only remaining objection to their use for such purposes, was overcome, forms an interesting chapter in the history of the “_Direct-Acting Steam Pump_.” It was when this question had assumed a most formidable, importance, that the principle of _using steam in compound steam engines_ had engaged the careful consideration of the most eminent engineers of this and other countries; _its adjustment to the Duplex pumps was made_, and while it was easily done, owing to their peculiar construction, its application produced a most wonderful result in their working, and their speedy introduction for water works use.