the sun-raised tides. This is
obviously only an approximate method of dealing with the question. The
influence of the solar tide is appreciable, and its importance
relatively to the lunar tide will gradually increase as the earth and
moon approach the final critical stage. The solar tides will have the
effect of constantly applying a further brake to the rotation of the
earth. It will therefore follow that, after the day and the month have
become equal, a still further retardation awaits the length of the day.
We thus see that in the remote future we shall find the moon revolving
around the earth in a shorter time than that in which the earth rotates
on its axis.

A most instructive corroboration of these views is afforded by the
discovery of the satellites of Mars. The planet Mars is one of the
smaller members of our system. It has a mass which is only the eighth
part of the mass of the earth. A small planet like Mars has much less
energy of rotation to be destroyed than a larger one like the earth. It
may therefore be expected that the small planet will proceed much more
rapidly in its evolution than the large one; we might, therefore,
anticipate that Mars and his satellites have attained a more advanced
stage of their history than is the case with the earth and her
satellite.

When the discovery of the satellites of Mars startled the world, in
1877, there was no feature which created so much amazement as the
periodic time of the interior satellite. We have already pointed out in
Chapter X. how Phobos revolves around Mars in a period of 7 hours 39
minutes. The period of rotation of Mars himself is 24 hours 37 minutes,
and hence we have the fact, unparalleled in the solar system, that the
satellite is actually revolving three times as rapidly as the planet is
rotating. There can hardly be a doubt that the solar tides on Mars have
abated its velocity of rotation in the manner just suggested.

It has always seemed to me that the matter just referred to is one of
the most interesting and instructive in the whole history of astronomy.
We have, first, a very beautiful telescopic discovery of the minute
satellites of Mars, and we have a determination of the anomalous
movement of one of them. We have then found a satisfactory physical
explanation of the cause of this phenomenon, and we have shown it to be
a striking instance of tidal evolution. Finally, we have seen that the
system of Mars and his satellite is really a for