swiftly forward, up, and on around, bringing it down back of
you (Fig. 36). Swing it around this way swiftly and evenly
several times, finally stopping at the beginning of the up
swing.

It is centrifugal force that keeps the water in the pail. It depends
entirely on inertia. You see, while the pail is swinging upward
rapidly, the water is moving up and tends by its inertia to keep right
on moving in the same upward direction. Before you get it over your
head, the tendency of the water to keep on going up is so strong that
it pulls on your arm and hand and presses against the bottom of the
pail above it. Its tendency to go on up is stronger than the downward
pull of gravity. As you swing the pail on backward, the water of
course has to move backward, too; so now it tends to keep on moving
backward; and when the pail is starting down behind you, the water is
tending to fly out in the backward direction in which it has just been
going. Therefore it still pushes against the bottom of the pail and
pulls away from your shoulder, which is in the center of the circle
about which the pail is moving. By the time you have swung the pail
on down, the water in it tends to keep going down, and it is still
pulling away from your shoulder and pressing against the bottom of the
pail.

[Illustration: FIG. 36. Why doesn't the water spill out?]

In this way, during every instant the water tends to keep going in the
direction in which it was going just the instant before. The result
is that the water keeps pulling away from your shoulder as long as you
keep swinging it around.

_All whirling things tend to fly away from the center about which they
are turning._ This is the law of centrifugal force. The earth, for
example, as it swings around the sun, tends to fly away from the
center of its orbit. This tendency of the earth--its centrifugal
force--keeps it from being drawn into the sun by the powerful pull of
the sun's gravitation. At the same time it is this gravitation of the
sun that keeps the earth from flying off into space, where we should
all be frozen to icicles and lost in everlasting night. For if the
sun's pull stopped, the earth would fly off as does a stone whirled
from the end of a string, when you let go of the string.

The moon, in like manner, would fly away from the earth and sun if
_gravitation_ stopped pulling it, but it would crash into us if its
_centrifugal force_ did not keep it at a safe dis