e phenomena which are observed. We have invariably found that the
dynamical phenomena of astronomy can be accounted for by the law of
universal gravitation. It is therefore natural to enquire how far
gravitation will render an account of the phenomenon of precession; and
to put the matter in its simplest form, let us consider the effect which
a distant attracting body can have upon the rotation of the earth.

To answer this question, it becomes necessary to define precisely what
we mean by the earth; and as for most purposes of astronomy we regard
the earth as a spherical globe, we shall commence with this assumption.
It seems also certain that the interior of the earth is, on the whole,
heavier than the outer portions. It is therefore reasonable to assume
that the density increases as we descend; nor is there any sufficient
ground for thinking that the earth is much heavier in one part than at
any other part equally remote from the centre. It is therefore usual in
such calculations to assume that the earth is formed of concentric
spherical shells, each one of which is of uniform density; while the
density decreases from each shell to the one exterior thereto.

A globe of this constitution being submitted to the attraction of some
external body, let us examine the effects which that external body can
produce. Suppose, for instance, the sun attracts a globe of this
character, what movements will be the result? The first and most obvious
result is that which we have already so frequently discussed, and which
is expressed by Kepler's laws: the attraction will compel the earth to
revolve around the sun in an elliptic path, of which the sun is in the
focus. With this movement we are, however, not at this moment concerned.
We must enquire how far the sun's attraction can modify the earth's
rotation around its axis. It can be demonstrated that the attraction of
the sun would be powerless to derange the rotation of the earth so
constituted. This is a result which can be formally proved by
mathematical calculation. It is, however, sufficiently obvious that the
force of attraction of any distant point on a symmetrical globe must
pass through the centre of that globe: and as the sun is only an
enormous aggregate of attracting points, it can only produce a
corresponding multitude of attractive forces; each of these forces
passes through the centre of the earth, and consequently the resultant
force which expresses the joint result o