forward. If the direction of the boat is to be reversed,
the rotation of the paddle-wheels is reversed.

[Illustration: FIG. 33]

[Illustration: FIG. 34]

[Illustration: FIG. 35]

[Illustration: FIG. 36]

[Illustration: FIG. 37]

[Illustration: FIG. 38]

Before passing onto the screw, it may be well to explain just how a
paddle-wheel causes a boat to move. When a man gets into a rowboat, he
generally pushes himself off by placing his oar against the dock or
shore and pushing on it. That is just what the paddle does in the water.
It dips into the water and pushes against it. It must be remembered,
however, that water is unlike a solid substance and it "gives." When a
man places his oar against the bank and pushes it, the bank does not
move, and all of the man's energy is used in starting the boat. Water,
however, does not remain stationary when the paddles push against it,
and therefore all of the power it not utilized in moving the boat--part
is used in moving the water.

The paddle-wheel is not so efficient in moving a boat as the more modern
propeller--or screw, as it is more often called. The screw receives its
name from the ordinary metal screw, because its theory of operation is
exactly the same. A wood screw, when turned, forces itself into wood. A
propeller, when turned, forces itself (and thereby the boat) through the
water. A small propeller is illustrated in Fig. 34. This is an ordinary
three-blade propeller. (The writer prefers the word propeller instead of
screw.)

From the drawing, it will be seen that the propeller-blades are mounted
at an angle. This angle of the blades causes them to force water back as
they cut through it when the propeller is revolving. This forcing of the
water back tends to produce a forward motion of the propeller, and in
this way the boat on which the propeller is mounted moves through the
water. The propeller is caused to revolve by a steam-engine,
steam-turbine, or gasolene-engine, as shown in Fig. 35. Longer boats
have more than one propeller. A boat that has two propellers is called a
twin-screw boat. A boat driven with four propellers is called a
quadruple-screw boat.

When a machine screw is turned around just once, it moves forward a
certain distance, as a glance at Fig. 36 will show. The distance the
screw moves forward will depend entirely upon the distance between the
threads. The distance between the threads is called the pitch of the
thread. If the threads