lls in experiments on the surface, but with a very
much lesser speed. A body dropped down from the distance of the moon
would commence its long journey so slowly that a _minute_, instead of a
_second_, would have elapsed before the distance of sixteen feet had
been accomplished.[11]

It was by pondering on information thus won from the moon that Newton
made his immortal discovery. The gravitation of the earth is a force
which extends far and wide through space. The more distant the body, the
weaker the gravitation becomes; here Newton found the means of
determining the great problem as to the law according to which the
intensity of the gravitation decreased. The information derived from the
moon, that a body 240,000 miles away requires a minute to fall through a
space equal to that through which it would fall in a second down here,
was of paramount importance. In the first place, it shows that the
attractive power of the earth, by which it draws all bodies earthwards,
becomes weaker at a distance. This might, indeed, have been anticipated.
It is as reasonable to suppose that as we retreated further and further
into the depths of space the power of attraction should diminish, as
that the lustre of light should diminish as we recede from it; and it is
remarkable that the law according to which the attraction of gravitation
decreases with the increase of distance is precisely the same as the
law according to which the brilliancy of a light decreases as its
distance increases.

The law of nature, stated in its simplest form, asserts that the
intensity of gravitation varies inversely as the square of the distance.
Let me endeavour to elucidate this somewhat abstract statement by one or
two simple illustrations. Suppose a body were raised above the surface
of the earth to a height of nearly 4,000 miles, so as to be at an
altitude equal to the radius of the earth. In other words, a body so
situated would be twice as far from the centre of the earth as a body
which lay on the surface. The law of gravitation says that the intensity
of the attraction is then to be decreased to one-fourth part, so that
the pull of the earth on a body 4,000 miles high is only one quarter of
the pull of the earth on that body so long as it lies on the ground. We
may imagine the effect of this pull to be shown in different ways. Allow
the body to fall, and in the interval of one second it will only drop
through four feet, a mere quarter of the dista