what the
form of steam engine, or the purposes to which it is applied, whether
stationary, locomotive, or marine, some form of cut-off, by which expansion
of the steam can be availed of, is considered indispensable. Many varieties
are in use, but those engines are most popular in which the cut-off is
applied directly to the valves on the cylinder, opening them quickly and
shutting off almost instantly, avoiding all wire drawing of the steam at
the ports, and regulating the speed of the engine promptly. Of this class
of engines, those manufactured by the Corliss Steam Engine Company, of
Providence, R.I., are perhaps the widest known, not only for their
extensive introduction, but also from having, by a long and successful
litigation, established the claims of the patentee, Mr. George H. Corliss.

[Illustration: Fig. 70.]

Fig. 70 is a section of the cylinder and valve chests of a horizontal
Corliss engine. _S_ is the steam connection, and _E_ the exhaust; there are
two distinct sets of valves, the steam _s, s'_, and the exhaust _e, e'_,
operated independently of each other. In their construction the valves may
be considered cylindrical plugs, of which portions near the ports are cut
away to admit the steam and reduce the bearing surface; the valves are
fitted on the lathe and the seats by boring. The motion given to the valves
is rocking, but it will be observed that the valves are not firmly
connected to the rocking shaft or cylinder; in the figure the valves are
shown shade lined, and the shaft or stem plain; in this way the valves are
not affected by the packing of the valve stem, but always rest upon the
face of the ports. In the figure the piston is just about to commence its
outstroke, the movement of the steam is supposed to be represented by the
arrows; the inner steam valve _s_, and the outer exhaust _e'_, are just
beginning to open. It will be observed that the outer steam _s'_ is fully
closed, whilst the inner exhaust valve _e_ is but barely so, showing that
there has been a cut-off on the steam valve, but no lead to the exhaust,
that it was left fully open till the completion of the stroke.

[Illustration: Fig. 71.]

Fig. 71 is a side elevation of the cylinder, with the valve connections
with the governor. _S_ is the steam pipe; _s, s'_ handles to the steam
valves, and _e, e'_ to the exhaust valves, shown in dotted line in fig. 70.
The handles to the exhaust valves are connected directly to a rocking plate