Instead of the common semi-cylindrical slide valve, cylindrical, or piston slides are sometimes made use of, and possess this advantage over the former, that the packing being metallic, requires no attention, and that the friction is much less. The annexed cut shows a valve of this description, a is the cylinder, with c the slide case; dd eduction passages; ff the slide pistons, which are fitted with metallic packings. That portion of each end of the slide case in which these pistons work, is bored out very truly; and in order to allow the segments of the packings to pass over the passages into the steam cylinder, bridges are cast in the passages, which prevent the segments from flying out of their places.

Piston Slide Valves 575Piston Slide Valves 576