This is a reversible, double-acting engine built along the lines of a typical mill engine. It was built to serve as a teaching aid for a series of impromptu lectures I gave at local middle schools. It incorporates a transparent window over the valve chamber so the motion of the slide valve can be seen when the engine is running. [This plastic window is replaced with a proper brass cover when the engine is run on steam.]
To reverse the engine, the phasing of the motion of the valve relative to the position of the piston must be changed. This is accomplished via an ingenious rotatable channel for the slider that drives the slide valve. Rotating the channel changes the phase so as to make the engine run in the opposite direction, as shown in the video.