wise tend to confirm it.

Not only the directions, but also the velocities of rotation seem thus
explicable. It might naturally be supposed that the large planets would
revolve on their axes more slowly than the small ones: our terrestrial
experiences of big and little bodies incline us to expect this. It is a
corollary from the Nebular Hypothesis, however, more especially when
interpreted as above, that while large planets will rotate rapidly,
small ones will rotate slowly; and we find that in fact they do so.
Other things equal, a concentrating nebulous mass which is diffused
through a wide space, and whose outer parts have, therefore, to travel
from great distances to the common centre of gravity, will acquire a
high axial velocity in course of its aggregation; and conversely with a
small mass. Still more marked will be the difference where the form of
the genetic ring conspires to increase the rate of rotation. Other
things equal, a genetic ring which is broadest in the direction of its
plane will produce a mass rotating faster than one which is broadest at
right angles to its plane; and if the ring is absolutely as well as
relatively broad, the rotation will be very rapid. These conditions
were, as we saw, fulfilled in the case of Jupiter; and Jupiter turns
round his axis in less than ten hours. Saturn, in whose case, as above
explained, the conditions were less favourable to rapid rotation, takes
nearly ten hours and a half. While Mars, Earth, Venus, and Mercury,
whose rings must have been slender, take more than double that time: the
smallest taking the longest.

* * * * *

From the planets let us now pass to the satellites. Here, beyond the
conspicuous facts commonly adverted to, that they go round their
primaries in the directions in which these turn on their axes, in planes
diverging but little from their equators, and in orbits nearly circular,
there are several significant traits which must not be passed over.

One of them is that each set of satellites repeats in miniature the
relations of the planets to the Sun, both in certain respects above
named and in the order of their sizes. On progressing from the outside
of the Solar System to its centre, we see that there are four large
external planets, and four internal ones which are comparatively small.
A like contrast holds between the outer and inner satellites in every
case. Among the four satellites of Jupiter, the paralle