ATER DOES NOT FLOW OFF THE EARTH. It was because people did not
know about gravitation that they laughed at Columbus when he said the
earth was round. "Why, if the earth were round," they argued, "the
water would all flow off on the other side." They did not know that
water flows downhill because the earth is pulling it toward its center
by gravitation, and that it does not make the slightest difference on
which side of the earth water is, since it is still pulled toward the
center.

WHY THE WORLD DOES NOT FALL DOWN. And people used to wonder "what held
the earth up." The answer, as you can see, is easy. There simply is no
up or down in space. The earth cannot fall down, because there is no
down to fall to. "Down" merely means toward the earth, and the earth
cannot very well fall toward itself, can it? The sun is pulling on it,
though; so the earth could fall into the sun, and it would do so, if
it were not swinging around the sun so fast. You will see how this
keeps it from falling into the sun when you come to the section on
centrifugal force.

WHY THERE IS A PLACE WHERE THINGS WEIGH NOTHING. Now about the place
where gravitation has no effect. Since an object near the sun is
pulled more by the sun than it is by the earth, and since down here
near the earth an object is pulled harder by the earth than by the
sun, it is clear that there must be a place between the sun and the
earth where their pulls just balance; and where the sun pulls just as
hard one way as the earth pulls the other way, things will not fall
either way, but will float. The place where the pulls of the sun and
the earth are equal is not halfway between the earth and the sun,
because the sun is so much larger and pulls so much more powerfully
than the earth, that the place where their pulls balance is much
nearer the earth than it is to the sun. As a matter of fact, it can be
easily calculated that this spot is somewhere near 160,000 miles from
the earth.

There are other spots like it between every two stars, and in the
center of the earth, and in the center of every other body. You see,
in the center of the earth there is just as much of the earth pulling
one way as there is pulling the other, so again there is no up or
down.

_APPLICATION 1._ Explain why the people on the other side of
the earth do not fall off; why you have weight; why rivers run
downhill; why the world does not fall down.

SECTION 2. "_Water seeks its own lev