itions which must hold good in reality. Thus with the object of
obtaining some insight into actuality, it is justifiable to discuss an
avowedly ideal problem at some length.

The attraction of gravity alone tends to make a mass of liquid assume
the shape of a sphere, and the effects of rotation, summarised under
the name of centrifugal force, are such that the liquid seeks to spread
itself outwards from the axis of rotation. It is a singular fact that
it is unnecessary to take any account of the size of the mass of liquid
under consideration, because the shape assumed is exactly the same
whether the mass be small or large, and this renders the statement of
results much easier than would otherwise be the case.

A mass of liquid at rest will obviously assume the shape of a sphere,
under the influence of gravitation, and it is a stable form, because
any oscillation of the liquid which might be started would gradually die
away under the influence of friction, however small. If now we impart
to the whole mass of liquid a small speed of rotation about some axis,
which may be called the polar axis, in such a way that there are no
internal currents and so that it spins in the same way as if it were
solid, the shape will become slightly flattened like an orange. Although
the earth and the other planets are not homogeneous they behave in the
same way, and are flattened at the poles and protuberant at the equator.
This shape may therefore conveniently be described as planetary.

If the planetary body be slightly deformed the forces of restitution
are slightly less than they were for the sphere; the shape is stable
but somewhat less so than the sphere. We have then a planetary spheroid,
rotating slowly, slightly flattened at the poles, with a high degree of
stability, and possessing a certain amount of rotational momentum. Let
us suppose this ideal liquid star to be somewhere in stellar space far
removed from all other bodies; then it is subject to no external forces,
and any change which ensues must come from inside. Now the amount
of rotational momentum existing in a system in motion can neither be
created nor destroyed by any internal causes, and therefore, whatever
happens, the amount of rotational momentum possessed by the star must
remain absolutely constant.

A real star radiates heat, and as it cools it shrinks. Let us suppose
then that our ideal star also radiates and shrinks, but let the process
proceed so slowly that