208             LECTURE XX.

<b>Paddle-Wheels</b>. - Having briefly described the different forms
of engines used for driving paddle-wheels, we now naturally
refer to the wheels themselves, leaving a description of the screw
propeller until after we have noticed the styles of engines more
particularly adapted to driving it.

The efficiency of the paddle-wheel falls off when the wheel is
too deeply immersed, consequently for long voyages, where the
draught of the vessel decreases as it proceeds, due to consumption
of coal, &c., if the wheels are to be immersed to the proper
depth at the end of the voyage, they must of necessity be too
deep at the beginning. This variable immersion of paddle-wheels
is the most serious objection to their use for long voyages. Also,
in a heavy sea the rolling of the vessel, besides causing the
engines to race, induces unequal straining of the machinery,
since one wheel lifts out of the water, while the other sinks
more deeply in it. Neither of these disadvantages is found in
the screw propeller, for the screw is immersed considerably
below the surface of the water, and since it is placed in the
centre line of the ship, the rolling motion has no effect on it.
The heaving of the ship in a fore and aft direction causes racing
of the engines, but no unequal straining is set up. For short
voyages, however, and where the draught is practically un-
changed during the voyage, the paddle-wheel still holds its own
with the screw, and for navigation in shallow rivers it is very
valuable; the screw, in such a case, being quite unsuited, on
account of its nearness to the bottom of the vessel. The vibra-
tion set up by the motion of paddle engines also is not so great
as that from the fast-running engines necessary for the screw
propeller.

<i>Radial Paddle-Wheel</i>. - This form of wheel is the simplest,
strongest, least expensive, and least liable to derangement, but 
is also unfortunately the <i>least efficient</i>. It consists of radial arms,
which are attached to a cast-iron boss at the centre, and are
bound at their outer extremities by one or two wrought-iron
rings. Flat boards are fixed rigidly to these radial arms, parallel
to the axis of the wheel, and are known as "floats," and it is
the thrust or push which these boards or floats exercise upon
the water as the wheel rotates, which propels the vessel. The
floats of a wheel of this kind, of necessity enter and leave the
water in an oblique manner, and are only perpendicular to the
surface of the water when they come immediately below the
centre of the wheel. Therefore, since the pressure which a float
produces is perpendicular to its surface (i.e., perpendicular to
the radius of the wheel), it is only when the floats are passing
their lowest point, that the whole pressure they exert is utilised