Slide valve - A double-acting slide valve cylinder. Steam enters via the steam port SP, and is admitted by the slide valve SV  through the upper passage S  to push down the piston P. At the same time, exhaust steam from below the piston passes back up the lower passage S, via the valve cavity, to exhaust E. As the piston descends, the valve moves upwards to admit steam below the piston and release exhaust from above.
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