cted by
Messrs. Ravenhill & Salked, for the Holyhead Packets. Fig. 27 is a
longitudinal section of this vessel, showing an engine and boiler; and fig.
28 is a transverse section of one of the engines, showing also one of the
wheels. There are two cylinders in this vessel, and one air pump, which
lies in an inclined position, and is worked by a crank in the shaft which
stretches between the cylinders, and which is called the _intermediate
shaft_. A A, is one of the cylinders, B B the piston rod, and C C the
crank. D is the crank in the intermediate shaft, which works the air pump
E. There are double eccentrics fixed on the shaft, whereby the movement of
the slide valves is regulated. The purpose of the double eccentrics is to
enable an improved arrangement of valve gear to be employed, which is
denominated the _link motion_, and which will be described hereafter. I I
are the steam pipes leading to the steam trunnions K K, on which, and on
the eduction trunnions connected with the pipe M, the cylinders oscillate.

120. _Q._--By what species of mechanism are the positions of the paddle
floats of feathering wheels governed?

_A._--The floats are supported by spurs projecting from the rim of the
wheel, and they may be moved upon the points of the spurs, to which they
are attached by pins, by means of short levers proceeding from the backs of
the floats, and connected to rods which proceed towards the centre of the
wheel. The centre, however, to which these rods proceed is not concentric
with the wheel, and the rods, therefore, are moved in and out as the wheel
revolves, and impart a corresponding motion to the floats. In some
feathering wheels the proper motion is given to the rods by means of an
eccentric on the ship's side. The action of paddle wheels, whether radial
or feathering, will be more fully described in the chapter on Steam
Navigation.

SCREW ENGINES.

121. _Q._--What are the principal varieties of screw engines?

[Illustration: Fig. 27.]

[Illustration: Fig. 28.]

_A._--The engines employed for the propulsion of screw vessels are divided
into two great classes,--geared engines and direct acting engines; and each
of these classes again has many varieties. In screw vessels, the shaft on
which the screw is set requires to revolve at a much greater velocity than
is required in the case of the paddle shaft of a paddle vessel; and in
geared engines this necessary velocity of rotation is obtained by the
int