sed. When running at high speed the engineer "links up"
his reversing gear, causing his valves to cut off early in the stroke,
and the steam to work more expansively than it could with the lever at
_full_, or _end_, gear; so that this device not only renders an engine
reversible, but also gives the engineer an absolute command over the
expansion ratio of the steam admitted to the cylinder, and furnishes a
method of cutting off the steam altogether. In Figs. 30, 31, 32, the
valve has no lap and the eccentrics are set square. In actual practice
the valve faces would have "lap" and the eccentric "lead" to correspond;
but for the sake of simplicity neither is shown.

OTHER GEARS.

In the Gooch gear for reversing locomotives the link does not shift, but
the valve rod and its block is raised or lowered. The Allan gear is so
arranged that when the link is raised the block is lowered, and _vice
versa_. These are really only modifications of Stephenson's
principle--namely, the employment of _two_ eccentrics set at equal
angles to and on opposite sides of the crank. There are three other
forms of link-reversing gear, and nearly a dozen types of _radial_
reversing devices; but as we have already described the three most
commonly used on locomotives and ships, there is no need to give
particulars of these.

Before the introduction of Stephenson's gear a single eccentric was used
for each cylinder, and to reverse the engine this eccentric had to be
loose on the axle. "A lever and gear worked by a treadle on the
footplate controlled the position of the eccentrics. When starting the
engine, the driver put the eccentrics out of gear by the treadle; then,
by means of a lever he raised the small-ends[5] of the eccentric rods,
and, noting the position of the cranks, or, if more convenient, the
balance weight in the wheels, he, by means of another handle, moved the
valves to open the necessary ports to steam and worked them by hand
until the engine was moving; then, with the treadle, he threw the
eccentrics over to engage the studs, at the same time dropping the
small-ends of the rods to engage pins upon the valve spindles, so that
they continued to keep up the movement of the valve."[6] One would
imagine that in modern shunting yards such a device would somewhat delay
operations!

PISTON VALVES.

In marine engines, and on many locomotives and some stationary engines,
the D-valve (shown in Figs. 30-32) is replaced by a piston val