relations of bending the same face towards
each other; the earth's motion will, in fact, be so far checked, that
the day will become _longer_ than the month.

Thus the doctrine of tidal evolution has conducted us to a prospect of
a condition of things which will some time be reached, when the moon
will have receded to a distance in which the month shall have become
about fifty-seven days, and when the earth around which this moon
revolves shall actually require a still longer period to accomplish
its rotation on its axis. Here is an odd condition for a planet with
its satellite; indeed, until a dozen years ago it would have been
pronounced inconceivable that a moon should whirl round a planet so
quickly that its journey was accomplished in less than one of the
planet's own days. Arguments might be found to show that this was
impossible, or at least unprecedented. There is our own moon, which
now takes twenty-seven days to go round the earth; there is Jupiter,
with four moons, and the nearest of these to the primary goes round
in forty-two and a half hours. No doubt this is a very rapid motion;
but all those matters are much more lively with Jupiter than they are
here. The giant planet himself does not need ten hours for a single
rotation, so that you see his nearest moon still takes between two and
three Jovian days to accomplish a single revolution. The example of
Saturn might have been cited to show that the quickest revolution that
any satellite could perform must still require at least twice as long
as the day in which the planet performed its rotation. Nor could the
rotation of the planets around the sun afford a case which could be
cited. For even Mercury, the nearest of all the planets to the sun of
which the existence is certainly known, and therefore the most rapid
in its revolution, requires eighty-eight days to get round once; and
in the mean time the sun has had time to accomplish between three and
four rotations. Indeed, the analogies would seem to have shown so
great an improbability in the conclusion towards which tidal evolution
points, that they would have contributed a serious obstacle to the
general acceptance of that theory.

But in 1877 an event took place so interesting in astronomical
history, that we have to look back to the memorable discovery of
Uranus in 1781 before we can find a parallel to it in importance. Mars
had always been looked upon as one of the moonless planets, though
grounds we