s of
interest to see whether the tidal phenomena may not have a wider
scope; whether they may not, for instance, have determined the
formation of the planets by birth from the sun, just as the moon seems
to have originated by birth from the earth. Our first presumption,
that the cases are analogous, is not however justified when the facts
are carefully inquired into. A principle which I have not hitherto
discussed here assumes prominence, and therefore we shall devote our
attention to it for a few minutes.

Let us understand what we mean by the solar system. There is first the
sun at the centre, which preponderates over all the other bodies so
enormously, as shown in Fig. 4, in which the earth and the sun are
placed side by side for comparison. There is then the retinue of
planets, among the smaller of which our earth takes its place, a view
of the comparative sizes of the planets being shown in Fig. 5.

[Illustration: Fig. 4.--Comparative sizes of Earth and Sun.]

Not to embarrass ourselves with the perplexities of a problem so
complicated as our solar system is in its entirety, we shall for the
sake of clear reasoning assume an ideal system, consisting of a sun
and a large planet--in fact, such as our own system would be if we
could withdraw from it all other bodies, leaving the sun and Jupiter
only remaining. We shall suppose, of course, that the sun is much
larger than the planet, in fact, it will be convenient to keep in mind
the relative masses of the sun and Jupiter, the weight of the planet
being less than one-thousandth part of the sun. We know, of course,
that both of those bodies are rotating upon their axes, and the one
is revolving around the other; and for simplicity we may further
suppose that the axes of rotation are perpendicular to the plane of
revolution. In bodies so constituted tides will be manifested.
Jupiter will raise tides in the sun, the sun will raise tides in
Jupiter. If the rotation of each body be performed in a less period
than that of the revolution (the case which alone concerns us), then
the tides will immediately operate in their habitual manner as a brake
for the checking of rotation. The tides raised by the sun on Jupiter
will tend therefore to lengthen Jupiter's day; the tides raised on the
sun by Jupiter will tend to augment the sun's period of rotation. Both
Jupiter and the sun will therefore lose some moment of momentum. We
cannot, however, repeat too often the dynamical trut