[Illustration: FIG. 95.--Primitive method of grinding corn.]
Suppose a 600-pound bowlder which is embedded in the ground is needed
for the tower of a building. The problem of the builder is to get the
heavy bowlder out of the ground, to load it on a wagon for
transportation, and finally to raise it to the tower. Obviously, he
cannot do this alone; the greatest amount of force of which 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.
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