hich he is
capable would not suffice to accomplish any one of these tasks. How
then does he help himself and perform the impossible? Simply, by the
use of some of the machine types mentioned above, illustrations of
which are known in a general way to every schoolboy. The very knife
with which a stick is whittled is a machine.

[Illustration: FIG. 96.--Separating rice grains by flailing.]

[Illustration: FIG. 97.--The principle of the lever.]

152. The Lever. Balance a foot rule, containing a hole at its middle
point _F_, as shown in Figure 97. If now a weight of 1 pound is
suspended from the bar at some point, say 12, the balance is
disturbed, and the bar swings about the point _F_ as a center. The
balance can be regained by suspending an equivalent weight at the
opposite end of the bar, or by applying a 2-pound weight at a point 3
inches to the left of _F_. In the latter case a force of 1 pound
actually balances a force of 2 pounds, but the 1-pound weight is twice
as far from the point of suspension as is the 2-pound weight. The
small weight makes up in distance what it lacks in magnitude.

Such an arrangement of a rod or bar is called a lever. In any form of
lever there are only three things to be considered: the point where
the weight rests, the point where the force acts, and the point called
the fulcrum about which the rod rotates.

The distance from the force to the fulcrum is called the force arm.
The distance from the weight to the fulcrum is called the weight arm;
and it is a law of levers, as well as of all other machines, that the
force multiplied by the length of the force arm must equal the weight
multiplied by the length of the weight arm.

Force x force arm = weight x weight arm.

A force of 1 pound at a distance of 6, or with a force arm 6, will
balance a weight of 2 pounds with a weight arm 3; that is,

1 x 6 = 2 x 3.

Similarly a force of 10 pounds may be made to sustain a weight of 100
pounds, providing the force arm is 10 times longer than the weight
arm; and a force arm of 800 pounds, at a distance of 10 feet from the
fulcrum, may be made to sustain a weight of 8000 pounds, providing the
weight is 1 foot from the fulcrum.

153. Applications of the Lever. By means of a lever, a 600-pound
bowlder can be easily pried out of the ground. Let the lever, any
strong metal bar, be supported on a stone which serves as fulcrum;
then if a man exerts his force at the end of the rod somewhat as in