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In its motor and only cylinder, which is horizontal and open-ended, works a long trunk piston, the front end of which carries the crosshead pin.

An open-ended "trunk" piston, similar in principle to that used in gas engines, is employed; and the valve is of the piston type, which is less complicated than the box form of valve, though less easily made steam-tight in small sizes.

The trunk piston, C, is made hollow, and formed with a shield covered by refractory material to protect the packing of the piston and the surface of the lower part of the cylinder from heat.

The cylinder has a trunk piston working in the lower part, and on its upper side a shield that almost fills the hot part of the cylinder when the piston is at the extreme of its upstroke.