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"A valve for opening and closing the induction and eduction passages of a steam-engine cylinder: so called from its plan resembling the letter D. The usual form of the D-valve is shown in fig. 1, where it is seen detached." -Whitney, 1911

D Valve

"A valve for opening and closing the induction and eduction passages of a steam-engine cylinder: so…

"A valve for opening and closing the induction and eduction passages of a steam-engine cylinder: so called from its plan resembling the letter D. fig. 2 ... represents a section of a steam-cylinder and nozles." -Whitney, 1911

D Valve

"A valve for opening and closing the induction and eduction passages of a steam-engine cylinder: so…