pelled by this single engine at the rate of fourteen miles an
hour.

641. _Q._--Is not a single engine liable to stick upon the centre so that
it cannot be started or reversed with facility?

_A._--A single engine is no doubt more liable to stick upon the centre than
two engines, the cranks of which are set at right angles with one another;
but numerous paddle vessels are plying successfully that are propelled by a
single engine, and the screw offers still greater facility than paddles for
such a mode of construction. In the screw engine referred to, as the
cylinder is laid upon its side, there is no unbalanced weight to be lifted
up every stroke, and the crank, whereby the screw shaft is turned round,
consists of two discs with a heavy side intended to balance the momentum of
the piston and its connections; but these counter-weights by their
gravitation also prevent the connecting rod and crank from continuing in
the same line when the engine is stopped, and in fact they place the crank
in the most advantageous position for starting again when it has to be set
on.

642. _Q._--Will you explain the general arrangement of the parts of this
engine?

_A._--The cylinder lies on its side near one side of the vessel, and from
the end of the cylinder two piston rods extend to a cross head sliding
athwartships, in guides, near the other side of the vessel. To this cross
head the connecting rod is attached, and one end of it partakes of the
motion of the cross head or piston, while the other end is free to follow
the revolution of the crank on the screw shaft.

643. _Q._--What is the advantage of two discs entering into the composition
of the crank instead of one?

_A._--A double crank, such as two discs form with the crank pin, is a much
steadier combination than would result if only one disc were employed with
an over-hung pin. Then the friction on the neck of the shaft is made one
half less by being divided between the two bearings, and the short
prolongation of the shaft beyond the journal is convenient for the
attachment of the eccentrics to work the valves.

644. _Q._--Will you enumerate some of the principal dimensions of this
engine?

_A._--The bottom frame, on which also the condenser is cast, forms the base
of the engine: on one end of it the cylinder is set; on the other end are
the guides for the cross head, and in the middle are the bearings for the
crank shaft. The part where the cylinder stands is two feet