t if the
retention of a halfpenny can be regarded as justifying that assertion.
Saturn, revolving as it does with great rapidity, and having a very
large mass, possesses about L2700, while Uranus and Neptune taken
together would figure for about the same amount.

In conclusion, let us revert again to the two critical conditions of
the earth-moon system. As to what happened before the first critical
period, the tides tell us nothing, and every other line of reasoning
very little; we can to some extent foresee what may happen after the
second critical epoch is reached, at a time so remote that I do not
venture even to express the number of ciphers which ought to follow
the significant digit in the expression for the number of years. I
mentioned, however, that at this time the sun tides will produce the
effect of applying a still further brake to the rotation of the earth,
so that ultimately the month will have become a shorter period than
the day. It is therefore interesting for us to trace out the tidal
history of a system in which the satellite revolves around the primary
in less time than the primary takes to go round on its own axis--such
a system, in fact, as Mars would present at this moment were the outer
satellite to be abstracted. The effect of the tides on the planet
raised by its satellite would then be to accelerate its rotation; for
as the planet, so to speak, lags behind the tides, friction would now
manifest itself by the continuous endeavour to drag the primary round
faster. The gain of speed, however, thus attained would involve the
primary in performing more than its original share of the moment of
momentum; less moment of momentum would therefore remain to be done by
the satellite, and the only way to accomplish this would be for the
satellite to come inwards and revolve in a smaller orbit.

We might indeed have inferred this from the considerations of energy
alone, for whatever happens in the deformation of the orbit, heat is
produced by the friction, and this heat is lost, and the total energy
of the system must consequently decline. Now if it be a consequence of
the tides that the velocity of the primary is accelerated, the energy
corresponding to that velocity is also increased. Hence the primary
has more energy than it had before; this energy must have been
obtained at the expense of the satellite; the satellite must therefore
draw inwards until it has yielded up enough of energy not alone to
acc