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
intervention of toothed wheels,--the engines themselves moving with the
usual velocity of paddle engines; whereas in direct acting engines the
required velocity of rotation is obtained by accelerating the speed of the
engines, and which are connected immediately to the screw shaft.

122. _Q._--Will you describe some of the principal varieties of geared
engines?

_A._--A good many of the geared engines for screw vessels are made in the