that fights against the moving thing and tries to stop it,
whether it be sent along the ground or thrown up in the air. You know
what friction is, of course. If you rub your hands along any rough
substance you will quickly feel it, but on a smooth substance you feel
it less. That is why if you send a stone spinning along a carpet or a
rough road it stops comparatively soon, whereas if you use the same
amount of force and send it along a sheet of ice it goes on moving much
longer. This kind of resistance, which we call friction, is one of the
causes which is at work to bring things to a standstill; and another
cause is the resistance of the air, which is friction in another form.
It may be a perfectly still day, yet if you are bicycling you are
breaking through the air all the time, just as you would be through
water in swimming, only the resistance of the air is less than that of
water. As the friction or the resistance of the air, or both combined,
gradually lessens the pace of the stone you sent off with such force,
the gravitation of the earth begins to be felt. When the stone first
started the force you gave to it was enough to overcome the gravitation
force, but as the stone moves more slowly the earth-pull asserts itself,
and the stone drops down to the ground and lies still upon the surface.
Now, if there were no friction, and therefore no resistance, there would
be no reason why anything once set moving should not go on moving for
ever. The force you give to any object you throw is enough to overcome
gravitation; and it is only when the first force has been diminished by
friction that the earth asserts its authority and pulls the moving
object toward it. If it were possible to get outside the air and out of
reach of the pull of the earth, we might fling a ball off into space,
and it would go on in a straight line until something pulled it to
itself by the force of gravity.

Gravitation affects everything connected with the earth; even our air is
held to the earth by gravitation. It grows thinner and thinner as we get
further away from the earth. At the top of a high mountain the air is so
thin that men have difficulty in breathing, and at a certain height they
could not breathe at all. As they cannot breathe in very fine air, it is
impossible for them to tell by personal experiment exactly where the air
ends; but they have tried to find out in other ways, and though
different men have come to different conclusion