rface of the earth whence
it arose.

Such a catastrophe is already imminent in a neighbouring planet--Mars.
Mars' principal moon circulates round him at an absurd pace, completing
a revolution in 7-1/2 hours, and it is now only 4,000 miles from his
surface. The planet rotates in twenty-four hours as we do; but its tides
are following its moon more quickly than it rotates after them; they are
therefore tending to increase its rate of spin, and to retard the
revolution of the moon. Mars is therefore slowly but surely pulling its
moon down on to itself, by a reverse action to that which separated our
moon. The day shorter than the month forces a moon further away; the
month shorter than the day tends to draw a satellite nearer.

This moon of Mars is not a large body: it is only twenty or thirty miles
in diameter, but it weighs some forty billion tons, and will ultimately
crash along the surface with a velocity of 8,000 miles an hour. Such a
blow must produce the most astounding effects when it occurs, but I am
unable to tell you its probable date.

So far we have dealt mainly with the earth and its moon; but is the
existence of tides limited to these bodies? By no means. No body in the
solar system is rigid, no body in the stellar universe is rigid. All
must be susceptible of some tidal deformation, and hence, in all of
them, agents like those we have traced in the history of the earth and
moon must be at work: the motion of all must be complicated by the
phenomena of tides. It is Prof. George Darwin who has worked out the
astronomical influence of the tides, on the principles of Sir William
Thomson: it is Sir Robert Ball who has extended Mr. Darwin's results to
the past history of our own and other worlds.[32]

Tides are of course produced in the sun by the action of the
planets, for the sun rotates in twenty-five days or thereabouts,
while the planets revolve in much longer periods than that. The
principal tide-generating bodies will be Venus and Jupiter; the
greater nearness of one rather more than compensating for the
greater mass of the other.

It may be interesting to tabulate the relative tide-producing
powers of the planets on the sun. They are as follows, calling that
of the earth 1,000:--

RELATIVE TIDE-PRODUCING POWERS OF THE PLANETS
ON THE SUN.

Mercury 1,121
Venus 2,339
Earth 1,000
Mars 304
Jupiter