s on the subject, it is
safe to say that at about two hundred miles above the earth there is
nothing that could be called air. Thus we can now picture our spinning
earth clothed in a garment of air that clings closely about her, and
grows thinner and thinner until it melts away altogether, for there is
no air in space.

[Illustration: THE EARTH AND MOON HANGING IN SPACE]

Now in the beginning God made the world, and set it off by a first
impulse. We know nothing about the details, though further on you shall
hear what is generally supposed to have taken place; we only know that,
at some remote age, this world, probably very different from what it is
now, together with the other planets, was sent spinning off into space
on its age-long journey. These planets were not sent off at random, but
must have had some particular connection with each other and with the
sun, for they all belong to one system or family, and act and react on
each other. Now, if they had been at rest and not in movement, they
would have fallen right into the sun, drawn by the force of gravitation;
then they would have been burned up, and there would have been an end of
them. But the first force had imparted to them the impulse to go on in a
straight line, so when the sun pulled the result was a movement between
the two: the planets did not continue to move in a straight line,
neither did they fall on to the sun, but they went on a course between
the two--that is, a circle--for the sun never let them get right away
from him, but compelled them to move in circles round him. There is a
very common instance of this kind of thing which we can see, or perhaps
feel, every day. If you try to sit still on a bicycle you tumble off,
because the earth pulls you down to itself; but if, by using the force
of your own muscles, you give the bicycle a forward movement this
resists the earth-pull, and the result is the bicycle runs along the
ground. It does not get right away from the earth, not even two or three
feet above ground; it is held to the earth, but still it goes forward
and does not fall over, for the movement is made up of the earth-pull,
which holds it to the ground, and the forward movement, which propels it
along. Then again, as another instance, if you tie a ball to a string
and whirl it round you, so long as you keep on whirling it will not fall
to the ground, but the moment you stop down it drops, for there is
nothing to fight against the pull of gr