n. Hence, if solar tidal friction were strong
enough to keep down the pace below this critical point, the contracting
mass would remain intact--there would be no satellite-production. This,
in all probability, actually occurred in the case both of Mercury and
Venus. They cooled without dividing, because the solar friction-brake
applied to them was too strong to permit acceleration to pass the limit
of equilibrium. The complete destruction of their relative axial
movement has been rendered probable by recent observations; and that the
process went on rapidly is a reasonable further inference. The earth
barely escaped the fate of loneliness incurred by her neighbours. Her
first and only epoch of instability was retarded until she had nearly
reached maturity. The late appearance of the moon accounts for its large
relative size--through the increased cohesion of an already strongly
condensed parent mass--and for the distinctive peculiarities of its
history and influence on the producing globe.

Solar tidal friction, although it did not hinder the formation of two
minute dependents of Mars, has been invoked to explain the anomalously
rapid revolution of one of them. Phobos, we have seen, completes more
than three revolutions while Mars rotates once. But this was probably
not always so. The two periods were originally nearly equal. The
difference, it is alleged, was brought about by tidal waves raised by
the sun on the semi-fluid spheroid of Mars. Rotatory velocity was
thereby destroyed, the Martian day slowly lengthened, and, as a
secondary consequence, the period of the inner satellite, become shorter
than the augmented day, began progressively to diminish. So that Phobos,
unlike our moon, was in the beginning farther from its primary than now.

But here again Mr. Nolan entered a _caveat_. Applying the simple test of
numerical evaluation, he showed that before solar tidal friction could
lengthen the rotation-period of Mars by so much as one minute, Phobos
should have been precipitated upon its surface.[1183] For the enormous
disparity of mass between it and the sun is so far neutralised by the
enormous disparity in their respective distances from Mars that solar
tidal force there is only fifty times that of the little satellite. But
the tidal effects of a satellite circulating quicker than its primary
rotates exactly reverse those of one moving, like our moon,
comparatively slowly, so that the tides raised by Phobos tend to