supplied by a passage through the main valve which operates exactly as
an ordinary slide valve would. That is, the inside edges of the steam
passage are the same as the ordinary valve, the additional piece on
each end, if I may so term it, being merely to provide a passage for
the steam which can be closed, instead of allowing the steam to pass
the edge. The eccentric of the main valve is fastened to the shaft to
give the proper amount of lead, and the desired release and
compression, and the expansion valve is operated by a separate
eccentric fastened in line with or 180° ahead of the crank. When the
piston, therefore, commences to move from the crank end to open the
port, D, the expansion valve is forced by its eccentric in the
opposite direction, and is closing the steam port and would have
closed it before the piston reached quarter stroke, thus allowing the
steam then in the cylinder to do work by expansion. The eccentric
operating this expansion valve may be set to close this steam port at
any point in the stroke that is desired, the closing occurring when
the expansion valve has covered the steam port. Continuing the
movements of the valves, the two would move together until one or the
other reached its dead center, when the movements would be in opposite
directions.

[Illustration: 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