D slide valve

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The D Slide Valve was a form of rectilinear slide valve for use in rotative steam engines invented by William Murdoch and patented in 1799. It was named for the hollow central piston and was in the shape of a D.

This valve worked by "connecting the upper and lower valves so as to be worked by one rod or spindle, and in making the stem or tube which connects them hollow, so as to serve for an induction pipe to the upper end of the cylinder." This allowed two valves to do the work of four.