ulleys and gearing were involved, without
seeming to understand that those mechanical devices are absolutely the
same in principle.

This will be made plain by a few illustrations. In Fig. 128, A is a
pulley four times larger, diametrically, than B, and C is the pivot on
which they turn. The pulleys are, of course, secured to each other. In
this case we have the two weights, one of four pounds on the belt, which
is on the small pulley (B), and a one-pound weight on the belt from the
large pulley (A).

[Illustration: _Fig. 128. The Pulley_]

THE LEVER AND THE PULLEY.--If we should substitute a lever (D) for the
pulleys, the similarity to the lever (Fig. 127) would be apparent at
once. The pivot (C) in this case would act the same as the pivot (C) in
the lever illustration.

In the same manner, and for like reasons, the wedge, the screw and the
incline plane, are different structural applications of the principles
set forth in the lever.

Whenever two gears are connected together, the lever principle is used,
whether they are the same in size, diametrically, or not. If they are
the same size then no change in power results; but instead, thereof, a
change takes place in the direction of the motion.

[Illustration: _Fig. 129. Fig. 130. Change of Direction_]

When one end of the lever (A) goes down, the other end goes up, as shown
in Fig. 129; and in Fig. 130, when the shaft (C) of one wheel turns in
one direction, the shaft of the other wheel turns in the opposite
direction.

It is plain that a gear, like a lever, may change direction as well as
increase or decrease power. It is the thorough knowledge of these facts,
and their application, which enables man to make the wonderful machinery
we see on every hand.

SOURCES OF POWER.--Power is derived from a variety of sources, but what
are called the _prime movers_ are derived from heat, through the various
fuels, from water, from the winds and from the tides and waves of the
ocean. In the case of water the power depends on the head, or height, of
the surface of the water above the discharging orifice.

WATER POWER.--A column of water an inch square and 28 inches high gives
a pressure at the base of one pound; and the pressure at the lower end
is equal in all directions. If a tank of water 28 inches high has a
single orifice in its bottom 1" x 1" in size, the pressure of water
through that opening will be only one pound, and it will be one pound
through every other