of this lever. If the brass
block be set in the middle of the link, no motion is communicated to it,
and the valve being consequently kept stationary and covering both ports,
the engine stops. If the link be lowered until the brass block comes to the
upper end of the link, the valve receives the motion of the eccentric for
going ahead, and the engine moves ahead; whereas if the link be raised
until the brass block comes to the lower end of the link, the valve
receives the motion of the backing eccentric, and the engine moves astern.
Instead of eccentrics, however, pins at the end of the shaft are employed
in this engine, the arrangement partaking of the nature of a double crank;
but the backing pin has less throw than the going ahead pin, whereby the
efficient length of the link for going ahead is increased; and the
operation of backing, which does not require to be performed at the highest
rate of speed, is sufficiently accommodated by about half the throw being
given to the valve that is given in going ahead. A valve shaft extends
across the end of the cylinder with two levers standing up, which engage
horizontal side rods extending from a small cross head on the end of the
valve rod. A lever extends downwards from the end of the valve shaft, which
is connected by a pin to the brass block within the link; and the link is
moved up or down by the starting handle, which, by means of a spring bolt
shooting into a quadrant, holds the starting handle at any position in
which it may be set.

654. _Q._--What is the diameter and pitch of the screw propeller?

_A._--The diameter is 7 feet and the pitch 14 feet. The propeller is Holm's
conchoidal propeller. Its diameter is smaller than is advisable, being
limited by the draught of water of the vessel; and the vessel was required
to have a small draught of water to go over a bar. This engine makes, under
favorable circumstances, 100 strokes per minute. The speed of piston with
this number of strokes is 700 feet per minute, and the engine works
steadily at this speed, the shock and tremor arising from the arrested
momentum of the moving parts being taken away by the counterbalance applied
at the discs.

LOCOMOTIVE ENGINE.

655. _Q._--Will you describe the principal features of a modern locomotive
engine?

_A._--I will take for this purpose the locomotive Snake, constructed by
John V. Gooch for the London and South Western Railway, as an example of a
modern locomotive of g