have satellites; and, further, that he ought to have a number
intermediate between 1 and 4.]

[Footnote 19: Since this paragraph was first published, the discovery
that Mars has two satellites revolving round him in periods shorter than
that of his rotation, has shown that the implication on which Laplace
here insists is general only, and not absolute. Were it a necessary
assumption that all parts of a concentrating nebulous spheroid revolve
with the same angular velocities, the exception would appear an
inexplicable one; but if, as suggested in a preceding section, it is
inferable from the process of formation of a nebulous spheroid, that its
outer strata will move round the general axis with higher angular
velocities than the inner ones, there follows a possible interpretation.
Though, during the earlier stages of concentration, while the nebulous
matter, and especially its peripheral portions, are very rare, the
effects of fluid-friction will be too small to change greatly such
differences of angular velocities as exist; yet, when concentration has
reached its last stages, and the matter is passing from the gaseous into
the liquid and solid states, and when also the convection-currents have
become common to the whole mass (which they probably at first are not),
the angular velocity of the peripheral portion will gradually be
assimilated to that of the interior; and it becomes comprehensible that
in the case of Mars the peripheral portion, more and more dragged back
by the internal mass, lost part of its velocity during the interval
between the formation of the innermost satellite and the arrival at the
final form.]

[Footnote 20: I was about to suppress part of the above paragraph,
written before the science of solar physics had taken shape, because of
certain physical difficulties which stand in the way of its argument,
when, on looking into recent astronomical works, I found that the
hypothesis it sets forth respecting the Sun's structure has kinships to
the several hypotheses since set forth by Zoellner, Faye, and Young. I
have therefore decided to let it stand as it originally did.

The contemplated partial suppression just named, was prompted by
recognition of the truth that to effect mechanical stability the gaseous
interior of the Sun must have a density at least equal to that of the
molten shell (greater, indeed, at the centre); and this seems to imply a
specific gravity higher than that which he possesses