f all the individual forces must
also be directed through the centre of the earth. A force of this
character, whatever other potent influence it may have, will be
powerless to affect the rotation of the earth. If the earth be rotating
on an axis, the direction of that axis would be invariably preserved; so
that as the earth revolves around the sun, it would still continue to
rotate around an axis which always remained parallel to itself. Nor
would the attraction of the earth by any other body prove more
efficacious than that of the sun. If the earth really were the
symmetrical globe we have supposed, then the attraction of the sun and
moon, and even the influence of all the planets as well, would never be
competent to make the earth's axis of rotation swerve for a single
second from its original direction.

We have thus narrowed very closely the search for the cause of the
"precession." If the earth were a perfect sphere, precession would be
inexplicable. We are therefore forced to seek for an explanation of
precession in the fact that the earth is not a perfect sphere. This we
have already demonstrated to be the case. We have shown that the
equatorial axis of the earth is longer than the polar axis, so that
there is a protuberant zone girdling the equator. The attraction of
external bodies is able to grasp this protuberance, and thereby force
the earth's axis of rotation to change its direction.

There are only two bodies in the universe which sensibly contribute to
the precessional movement of the earth's axis: these bodies are the sun
and the moon. The shares in which the labour is borne by the sun and the
moon are not what might have been expected from a hasty view of the
subject. This is a point on which it will be desirable to dwell, as it
illustrates a point in the theory of gravitation which is of very
considerable importance.

The law of gravitation asserts that the intensity of the attraction
which a body can exercise is directly proportional to the mass of that
body, and inversely proportional to the square of its distance from the
attracted point. We can thus compare the attraction exerted upon the
earth by the sun and by the moon. The mass of the sun exceeds the mass
of the moon in the proportion of about 26,000,000 to 1. On the other
hand, the moon is at a distance which, on an average, is about one-386th
part of that of the sun. It is thus an easy calculation to show that the
efficiency of the sun's at