s (2 feet) is equivalent to a
force of 4 at a distance of 6 inches.

[Illustration: FIG. 99.--The wheelbarrow lightened labor.]

One of the most useful levers of this type is the wheelbarrow (Fig.
99). The fulcrum is at the wheel, the force is at the handles, the
weight is on the wheelbarrow. If the load is halfway from the fulcrum
to the man's hands, the man will have to lift with a force equal to
one half the load. If the load is one fourth as far from the fulcrum
as the man's hands, he will need to lift with a force only one fourth
as great as that of the load.

[Illustration: FIG. 100.--A modified wheelbarrow.]

This shows that in loading a wheelbarrow, it is important to arrange
the load as near to the wheel as possible.

[Illustration: FIG. 101.--The nutcracker is a lever.]

The nutcracker (Fig. 101) is an illustration of a double lever of the
wheelbarrow kind; the nearer the nut is to the fulcrum, the easier the
cracking.

[Illustration: FIG. 102.--The hand exerts a small force over a long
distance and draws out a nail.]

Hammers (Fig. 102), tack-lifters, scissors, forceps, are important
levers, and if you will notice how many different levers (fig. 103)
are used by all classes of men, you will understand how valuable a
machine this simple device is.

155. The Inclined Plane. A man wishes to load the 600-pound bowlder
on a wagon, and proceeds to do it by means of a plank, as in Figure
93. Such an arrangement is called an inclined plane.

The advantage of an inclined plane can be seen by the following
experiment. Select a smooth board 4 feet long and prop it so that the
end _A_ (Fig. 104) is 1 foot above the level of the table; the length
of the incline is then 4 times as great as its height. Fasten a metal
roller to a spring balance and observe its weight. Then pull the
roller uniformly upward along the plank and notice what the pull is on
the balance, being careful always to hold the balance parallel to the
incline.

When the roller is raised along the incline, the balance registers a
pull only one fourth as great as the actual weight of the roller. That
is, when the roller weighs 12, a force of 3 suffices to raise it to
the height _A_ along the incline; but the smaller force must be
applied throughout the entire length of the incline. In many cases, it
is preferable to exert a force of 30 pounds, for example, over the
distance _CA_ than a force of 120 pounds over the shorter distance
_BA_.

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