The large piston E moves horizontally. The smaller piston F moves vertically inside E.
T is the cover plate for the engine, incorporating a circular valve-box I, closed by a plate screwed over it. Steam is supplied via the valve K at the top, and passes to the valve-box by a passage not visible here. It enters the "cylinders" via ports G when they are uncovered by the valve disk X. The exhaust leaves through the same ports and enters the annular channel M and escapes through the passages shown dotted, which feed into a common exhaust duct.
The valve-plate X is fitted to an eccentric L on centre-pin D, and is carried round by a pin S on the end of the crank, so that it rolls around the inside of the circular valve-box, opening and closing the ports as it passes.
From Knights American Mechanical Dictionary, 1880.