ustration: FIG. 1.]

There are three ways of effecting the cut off in such engines, the
main valve meanwhile being undisturbed, its eccentric fastened
securely so as not to disturb the points of lead, release, and
compression. All that is required is to cause the edge of the
expansion valve to cover the steam port earlier in the stroke, and
this can be done, first, by increasing the angular advance of the cut
off eccentric; second, by adding lap to the cut off valve; and third
by changing the throw of the eccentric. In all these instances the
riding valve is caused to reach the edge of the steam port earlier in
the stroke. We will take first, as the simplest, those methods by
which the lap of the cut off valve is increased.

It will be noted that there is but one edge of this valve that is
required to do any work, and that is to close the valve. The
eccentrics are so placed that the passage in the main valve is opened
long before the main valve itself is ready to admit steam to the
cylinder, so that only the outer edges are the ones to be considered,
and it will be readily seen that the two valves traveling in opposite
directions, any lap added to the working edge of the cut off valve
will cause it to reach the edge and therefore close the port earlier
than it would if there was less lap. And we might carry it to the
extreme that we could add lap enough that the steam passage would not
be opened at all.

In Fig. 2 is shown the method by which this is accomplished, in what
is called Meyer's valve, and such as is used in the Kendall & Roberts
engine. We have only one point to look after, the cut off, so we can
add all the lap we wish without disturbing anything else. In this
engine the lap is changed by hand by means of a little hand wheel on a
stem that extends out of the rear of the steam chest. The valve is in
two sections, and when it is desired to cut off earlier, the hand
wheel is turned in such a direction that the right and left hand
screws controlling the cut off valve move one valve portion back and
the other forward, which would, if they were one valve and they should
be so considered, have the effect of lengthening them, or adding lap
to them. The result would be that the riding valve would reach the
edge of the steam port earlier in the stroke, bringing about an
earlier cut off. If the cut off is desired to be later, the hand wheel
is so turned that the right and left hand screws will bring the valve
sect