206          LECTURE XX.
wherein is shown the piston, P, and piston-rod, P R. Precisely
the same arrangement is carried out with respect to the low-
pressure cylinder and slide valve. The double (curved) guide,
C G, is free to move to tbe right or to the left in bearings
carried by the main framing brackets, F B (see also the per-
spective and general views of the engine). The reversing or
starting wheel, S W, is keyed at the upper end of the reversing
screw, R S, which screw has a nut, N, fixed to the reversing
levers, R L.

It will thus be seen, that the combined to-and-fro, up-and-down
motion of the connecting-rod, is converted and transmitted to
the valve spindle, in the form of a merely to-and-fro motion.
The eccentricity of the connecting-rod's motion being duly
corrected by the relative positions and lengths of the several
links, L<sub>1</sub> L<sub>2</sub>, L<sub>3</sub>, &c., and that by elevating the nut, N, by the
reversing screw, R S, the curved guides, C G, are turned
backwards or from the cylinder; this action draws forward the
valve rod with valve spindle and slide valve, admitting steam
behind the piston, and causing the engine to move forward, or
in other words putting it in forward gear, while the depressing
of the nut, N, effects precisely the opposite, and causes the
engine to revolve backwards. All the joints are made very sub-
stantial and t!hould be case-hardened, while their pins should
be of steel to prevent wear and obviate rattling.

<i>Alley's Patent flexible Coupling</i>. - The engines being described
were fitted with this invention, which allows the paddle shaft to

[drawings of Face and Longitudinal Section of Coupling here]

FACE OF COUPLING. LONGITUDINAL SECTION.

accommodate itself to the yielding of the paddle boxes and the
hull of the vessel as it vibrates and changes form when working
in a sea-way. It is equally applicable to the shafting of screw
steamers. The hull of a vessel cannot be made absolutely
rigid, and therefore it is wrong to make the shaft rigid. With