_shorten_ both periods. Its orbital momentum, however, is so extremely
small in proportion to the rotational momentum of Mars, that any
perceptible inroad upon the latter is attended by a lavish and ruinous
expenditure of the former. It is as if a man owning a single five-pound
note were to play for equal stakes with a man possessing a million. The
bankruptcy sure to ensue is typified by the coming fate of the Martian
inner satellite. The catastrophe of its fall needs to bring it about
only a very feeble reactive pull compared with the friction which the
sun should apply in order to protract the Martian day by one minute. And
from the proportionate strength of the forces at work, it is quite
certain that one result cannot take place without the other. Nor can
things have been materially different in the past; hence the idea must
be abandoned that the primitive time of rotation of Mars survives in the
period of its inner satellite.

The anomalous shortness of the latter may, however, in M. Wolf's
opinion,[1184] be explained by the "trainees elliptiques" with which
Roche supplemented nebular annulation.[1185] These are traced back to
the descent of separating strata from the _shoulders_ of the great
nebulous spheroid towards its equatorial plane. Their rotational
velocity being thus relatively small, they formed "inner rings," very
much nearer to the centre of condensation than would have been possible
on the unmodified theory of Laplace. Phobos might, in this view, be
called a polar offset of Mars; and the rings of Saturn are thought to
own a similar origin.

Outside the orbit of Mars, solar tidal friction can scarcely be said to
possess at present any sensible power. But it is far from certain that
this was always so. It seems not unlikely that its influence was the
overruling one in determining the direction of planetary rotation. M.
Faye, as we have seen, objected to Laplace's scheme that only retrograde
secondary systems could be produced by it. In this he was anticipated by
Kirkwood, who, however, supplied an answer to his own objection.[1186]

Sun-raised tides must have acted with great power on the diffused masses
of the embryo planets. By their means they doubtless very soon came to
turn (in lunar fashion) the same hemisphere always towards their centre
of motion. This amounts to saying that even if they started with
retrograde rotation, it was, by solar tidal friction, quickly rendered
direct.[1187] For it i