[Illustration: THE CLARKE COMPOUND TWIN-SCREW OPERATING ENGINE.]

In the Clarke engine, the exhaust pipe from the high pressure cylinder
leads to the steam chest of the low pressure cylinder, while the
piston in the upper cylinder is secured on a piston rod extending
downward and connected with a piston operating in the lower cylinder,
the exhaust pipe from the latter leading to the outside. On the piston
rod common to both cylinders is secured a crosshead pivotally
connected by two pitmen with opposite crank arms on crank shafts
mounted to turn in suitable bearings on the base, which also supports
a frame carrying the low pressure cylinder, on top of which is a frame
supporting the high pressure cylinder. The valves in the two steam
chests are connected with each other by a valve rod connected at its
lower end in the usual manner with the reversing link, operated from
eccentrics secured on one of the crank shafts.

The crank arms stand at angles to each other, so that the crank shafts
are turned in opposite directions, and the position of the link is
such that it can be readily changed by the reversing lever to
simultaneously reverse the motion of the crank shafts. On the crank
shafts are also formed two other crank arms pivotally connected by
opposite pitmen with a slide mounted in vertical guideways, supported
on a frame erected on the base, the motion of the crank shafts causing
the vertical sliding motion of the slide traveling loosely in the
guideways, and thus serving as a governor, as, in case one of the
propellers becomes disabled, the power of the shaft carrying the
disabled propeller is directly transferred to the other shaft through
the crank arms, pitmen, and slide, and the other propeller is caused
to do all the work. All the parts of the engine are within easy reach