ing for admission of steam.

[Illustration: JOY'S REVERSING AND EXPANDING VALVE GEAR.]

The former of these is constant and unalterable, the latter is
controllable by the angle at which the curved slide, J, may be set with
the vertical.

It will further be evident that if the lever, E, were pinned direct to
the connecting rod at the point, A, which passes through a practically
true ellipse, it would vibrate its fulcrum, F, unequally on either side of
the center of the curved slide, J, by the amount of the versed sine of the
arc of the lever, E, from F D; it is to correct this error that the lever,
E, is pinned at the point, D, to a parallel motion formed by the parts, B
and C. The point, D, performing a figure which is equal to an ellipse,
with the error to be eliminated added, so neutralizing its effect on the
motion of the fulcrum, F.

The "lap" and "lead" are opened by the action of the valve lever acting as
a lever, and the port opening is given by the incline of the curved slide
in which the center of that lever slides, and the amount of this opening
depends upon the angle given to that incline. When these two actions are
in unison, the motion of the valve is very rapid, and this occurs when the
steam is being admitted. Then follows a period of opposition of these
motions, during which time the valve pauses momentarily, this
corresponding to the time when the port is fully open. Further periods of
unison follow, at which time the sharp "cut-off" is obtained.

The "compression" resulting with this gear is also reduced to a minimum,
owing to the peculiar movement given to the valves (_i. e._, the series of
accelerations and retardations referred to), as, while the "lead" is
obtained later and quicker, the port is also shut for "compression" later
and quicker, doing away with the necessity for a special expansion valve,
with its complicated and expensive machinery, and allowing the main valve
to be used for expansion, as the "compression" is not of an injurious
amount, even with a "cut-off" reduced to 15 per cent., or about 1/6 of the
stroke.

Thus, so far as the distribution of the steam and its treatment in the
cylinder is concerned, a marked advantage is shown in favor of this valve
gear. But next in its favor, as before said, is that the above advantages
are not gained at the cost of added complication of parts or increased
cost of machinery, but the reverse, as this gear can be built at a less
cost than lin