ion of external forces, it will, if made to rotate with
adequate velocity, form detached rings; and that these rings will break
up into spheroids which turn on their axes in the same direction with
the central mass. Thus, given the original nebula, which, acquiring a
vortical motion in the way indicated, has at length concentrated into a
vast spheroid of aeriform matter moving round its axis--given this, and
mechanical principles explain the rest. The genesis of a Solar System
displaying movements like those observed, may be predicted; and the
reasoning on which the prediction is based is countenanced by
experiment.[17]

But now let us inquire whether, besides these most conspicuous
structural and dynamic peculiarities of the Solar System, sundry minor
ones are not similarly explicable.

* * * * *

Take first the relation between the planes of the planetary orbits and
the plane of the Sun's equator. If, when the nebulous spheroid extended
beyond the orbit of Neptune, all parts of it had been revolving exactly
in the same plane, or rather in parallel planes--if all its parts had
had one axis; then the planes of the successive rings would have been
coincident with each other and with that of the Sun's rotation. But it
needs only to go back to the earlier stages of concentration, to see
that there could exist no such complete uniformity of motion. The
flocculi, already described as precipitated from an irregular and
widely-diffused nebula, and as starting from all points to their common
centre of gravity, must move not in one plane but in innumerable planes,
cutting each other at all angles. The gradual establishment of a
vortical motion such as we at present see indicated in the spiral
nebulae, is the gradual approach towards motion in one plane. But this
plane can but slowly become decided. Flocculi not moving in this plane,
but entering into the aggregation at various inclinations, will tend to
perform their revolutions round its centre in their own planes; and only
in course of time will their motions be partly destroyed by conflicting
ones, and partly resolved into the general motion. Especially will the
outermost portions of the rotating mass retain for a long time their
more or less independent directions. Hence the probabilities are, that
the planes of the rings first detached will differ considerably from the
average plane of the mass; while the planes of those detached latest
wil