ve swiftly a long
distance._

Why can you go so much faster on a bicycle than on foot?

How can a man lift up a heavy automobile by using a jack?

Why can you crack a hard nut with a nutcracker when you cannot
crack it by squeezing it between two pieces of iron?

"Give me a lever, long enough and strong enough, and something to rest
it on, and I can lift the whole world," said an old Greek philosopher.
And as a philosopher he was right; theoretically it would be possible.
But since he needed a lever that would have been as long as from here
to the farthest star whose distance has ever been measured, and
since he would have had to push his end of the lever something like
a quintillion (1,000,000,000,000,000,000) miles to lift the earth one
inch, his proposition was hardly a practical one.

But levers are practical. Without them there would be none of our
modern machines. No locomotives could speed across the continents; no
derricks could lift great weights; no automobiles or bicycles would
quicken our travel; our very bodies would be completely paralyzed. Yet
the law back of all these things is really simple.

You have often noticed on the see-saw that a small child at one end
can be balanced by a larger child at the other end, provided that the
larger child sits nearer the middle. Why should it matter where the
larger child sits? He is always heavier--why doesn't he overbalance
the small child? It is because when the small child moves up and down
he goes a longer distance than the large child does. In Figure 26 the
large boy moves up and down only half as far as the little girl does.
She weighs only half as much as he, yet she balances him.

[Illustration: FIG. 26. The little girl raises the big boy, but in
doing it she moves twice as far as he does.]

You will begin to get a general understanding of levers and how they
work by doing the following experiment:

EXPERIMENT 18. For this experiment there will be needed a
small pail filled with something heavy (sand or stones will
do), a yardstick with a hole through the middle and another
hole near one end and with notches cut here and there along
the edge, and a post or table corner with a heavy nail driven
into it to within an inch of the head. The holes in the
yardstick must be large enough to let the head of this nail
through.

Put the middle hole of the yardstick over the nail, as is
shown in Figure 2