right and left handed screw, the turning of which alters the distance
between the plates and the point of cutting off.

The details of this mechanism, however, present several novel features,
of which the following description will be understood by reference to
the detached cuts, which are drawn upon a larger scale than the general
plan shown in Figs. 1 and 2.

[Illustration: Figs. 1-2 IMPROVED STEAM ENGINE.--BY PROF MACCORD.]

The first of these relates to the arrangement of the right and left
handed screw, above mentioned, and of the device by which it is rotated.

Usually, the threads, both right handed and left handed, are cut upon
the cut-off valve stem itself, which must be so connected with the
eccentric rod as to admit of being turned; and in most cases the valve
stem extends through both ends of the steam chest, so that it must both
slide endwise and turn upon its axis in two stuffing boxes, necessarily
of comparatively large size.

All this involves considerable friction, and in the engine under
consideration an attempt has been made to reduce the amount of this
friction, and to make the whole of this part of the gear neater and more
compact, in the following manner:

Two small valve stems are used, which are connected at their lower ends
by a crosstail actuated directly by the eccentric rod, and at their
upper ends by a transverse yoke. This yoke, filling snugly between two
collars formed upon a sleeve which it embraces, imparts a longitudinal
motion to the latter, while at the same time leaving it free to rotate.