her portions, for instance, the outside portions, will be very much
less dense. It will easily be understood that in all such bodies the
densest or most compressed portions are to be found towards the centre;
while the portions towards the exterior being less pressed upon, will be
less dense.

We now reach a very important point, the question of Gravitation.
_Gravitation_, or _gravity_, as it is often called, is the attractive
force which, for instance, causes objects to fall to the earth. Now it
seems rather strange that one should say that it is owing to a certain
force that things fall towards the earth. All things seem to us to fall
so of their own accord, as if it were quite natural, or rather most
unnatural if they did not. Why then require a "force" to make them fall?

The story goes that the great Sir Isaac Newton was set a-thinking on
this subject by seeing an apple fall from a tree to the earth. He then
carried the train of thought further; and, by studying the movements of
the moon, he reached the conclusion that a body even so far off as our
satellite would be drawn towards the earth in the same manner. This
being the case, one will naturally ask why the moon herself does not
fall in upon the earth. The answer is indeed found to be that the moon
is travelling round and round the earth at a certain rapid pace, and it
is this very same rapid pace which keeps her from falling in upon us.
Any one can test this simple fact for himself. If we tie a stone to the
end of a string, and keep whirling it round and round fast enough, there
will be a strong pull from the stone in an outward direction, and the
string will remain tight all the time that the stone is being whirled.
If, however, we gradually slacken the speed at which we are making the
stone whirl, a moment will come at length when the string will become
limp, and the stone will fall back towards our hand.

It seems, therefore, that there are two causes which maintain the stone
at a regular distance all the time it is being steadily whirled. One of
these is the continual pull inward towards our hand by means of the
string. The other is the continual pull away from us caused by the rate
at which the stone is travelling. When the rate of whirling is so
regulated that these pulls exactly balance each other, the stone travels
comfortably round and round, and shows no tendency either to fall back
upon our hand or to break the string and fly away into the air. It