This is an interesting little pump to a design by Elmer Verburg.  On the
right is a double-acting steam engine with D-valve visible through the
plastic cover on the steam chest.  It directly drives the piston of the
double-acting steam pump on the left.  The exterior isn't much to look
at it but inside are four working ball check valves.

Directly connecting the engine to the pump poses a problem for attaching
a flywheel.  Elmer solved this dilemma by driving the flywheel with a
ball joint on the end of the arm attached to the connecting rod.  A side
effect of this technique is the fact that the pistons rotate slightly as
they move back and forth.

In operation, pumping mineral oil, the pump produces a nice steady
stream due to the fact that it pumps on both sides of the stroke.