nce that gravity would
cause near the earth's surface.

We may consider the matter in another way by supposing that the
attraction of the earth is measured by one of those little weighing
machines known as a spring balance. If a weight of four pounds be hung
on such a contrivance, at the earth's surface, the index of course shows
a weight of four pounds; but conceive this balance, still bearing the
weight appended thereto, were to be carried up and up, the _indicated_
strain would become less and less, until by the time the balance reached
4,000 miles high, where it was _twice_ as far away from the earth's
centre as at first, the indicated strain would be reduced to the
_fourth_ part, and the balance would only show one pound. If we could
imagine the instrument to be carried still further into the depths of
space, the indication of the scale would steadily continue to decline.
By the time the apparatus had reached a distance of 8,000 miles high,
being then _three_ times as far from the earth's centre as at first, the
law of gravitation tells us that the attraction must have decreased to
one-ninth part. The strain thus shown on the balance would be only the
ninth part of four pounds, or less than half a pound. But let the voyage
be once again resumed, and let not a halt be made this time until the
balance and its four-pound weight have retreated to that orbit which the
moon traverses in its monthly course around the earth. The distance thus
attained is about sixty times the radius of the earth, and consequently
the attraction of gravitation is diminished in the proportion of one to
the square of sixty; the spring will then only be strained by the
inappreciable fraction of 1-3,600 part of four pounds. It therefore
appears that a weight which on the earth weighed a ton and a half would,
if raised 240,000 miles, weigh less than a pound. But even at this vast
distance we are not to halt; imagine that we retreat still further and
further; the strain shown by the balance will ever decrease, but it will
still exist, no matter how far we go. Astronomy appears to teach us that
the attraction of gravitation can extend, with suitably enfeebled
intensity, across the most profound gulfs of space.

The principle of gravitation is of far wider scope than we have yet
indicated. We have spoken merely of the attraction of the earth, and we
have stated that this force extends throughout space. But the law of
gravitation is not so limited.