These are instruments placed at the back part of steam-vessels for
the purpose of propelling them through the water. Fig. 1 will show
the position they occupy, and fig. 2 the shape of the propeller. When
first used, they had one or two entire turns round the axis, but are
now made with two blades, each forming about one-sixth part only of
one turn, and this is found to give more power with less friction.
The propeller is turned rapidly round in the water, from which it
meets with resistance in a direction perpendicular to the surface
of its blades, but as this is oblique to the direction of rotation
the force is exerted in two directions, one directly opposes this
rotation, and is overcome by the power of the steam-engine, the other
is in a direction towards the ship, overcoming the inertia of the
vessel and the friction and resistance of the water, so that the ship
is moved along, and the propeller winds its way through the water in
a spiral direction as an ordinary screw does through the hollow screw
made to fit it, the vessel travelling at a speed proportionate to the
screw’s revolutions.