shaped, and circular to fit its
circular-seat. The valve rod of the expansion valve has a sector upon
it and operated by a gear upon the governor stem, which rotates the
valve rod, and the edge of the valve rod is brought farther over the
steam port, thus practically adding lap to the valve. Little movement
is found necessary to make the ordinary change in cut-off, and it is
found to be much easier to move the riding valve across the valve than
in a direction directly opposite. It would require considerable force
to move the upper valve by the governor faster than the lower, or in a
direction opposite to that in which it is moving, but very little
force applied sideways at the same time it is moving forward will give
it a sideways motion. In this device the governor has only to exert
this side pressure and therefore has less to do than if it were called
upon to move the upper valve directly against the movement of the
lower.

Something similar is the valve of the Woodbury engine, of Rochester,
N.Y. The cut-off valve is cylindrical, covering diagonal ports
directly opposite, and is caused to be rotated by the action of the
governor that operates a rack in mesh with a segment. Very little
movement will effect a considerable change in the lappage of the
valve, the valve turning about one-quarter a revolution for the
extremes of cut off. The cut off valve rod works through a bracket and
its end terminates in a ball in a socket on the end of the eccentric
rod. In this case the governor has not as much to do as in other
instances.

[Illustration: FIG. 4.]

Still another method of effecting this change in cut off, but hardly
by increasing the lap of the valve, is shown in the next drawing, Fig.
4. The cut off valve is held upon the main valve by the pressure of
steam upon its back and rides with it until it comes in contact with
the cut off wedge-shaped blocks, when its motion is arrested, and the
main valve continuing its movement the steam port is closed by the
main valve passing beneath the cut off valve. Thus the main valve
travels and carries the cut off valve upon its back again until the
cut off valve strikes the wedge on the other end and the cut off is
effected. The relative positions of the blocks are determined by the
governor, that will raise or lower them so that the cut off valve will
engage with them earlier or later as desired. This device was designed
specially as an inexpensive method of changing the comm