bodies there
is a reciprocity of tidal-making energy--each of them is competent to
raise tides in the other. As the moon is so small in comparison with
the earth, and as the tides on the moon are of but little significance
in the progress of tidal evolution, it has been permissible for us to
omit them from our former discussion. But it is these tides on the
moon which will afford us a striking illustration of the competency of
tides for stupendous tasks. The moon presents a monument to show what
tides are able to accomplish.

[Illustration: Fig. 3.--The Moon.]

I must first, however, explain a difficulty which is almost sure to
suggest itself when we speak of tides on the moon. I shall be told
that the moon contains no water on its surface, and how then, it will
be said, can tides ebb and flow where there is no sea to be disturbed?
There are two answers to this difficulty; it is no doubt true that the
moon seems at present entirely devoid of water in so far as its
surface is exposed to us, but it is by no means certain that the moon
was always in this destitute condition. There are very large features
marked on its map as "seas"; these regions are of a darker hue than
the rest of the moon's surface, they are large objects often many
hundreds of miles in diameter, and they form, in fact, those dark
patches on the brilliant surface which are conspicuous to the unaided
eye, and are represented in Fig. 3. Viewed in a telescope these
so-called seas, while clearly possessing no water at the present time,
are yet widely different from the general aspect of the moon's
surface. It has often been supposed that great oceans once filled
these basins, and a plausible explanation has even been offered as to
how the waters they once contained could have vanished. It has been
thought that as the mineral substances deep in the interior of our
satellite assumed the crystalline form during the progress of cooling,
the demand for water of crystallization required for incorporation
with the minerals was so great that the oceans of the moon became
entirely absorbed. It is, however, unnecessary for our present
argument that this theory should be correct. Even if there never was a
drop of water found on our satellite, the tides in its molten
materials would be quite sufficient for our purpose; anything that
tides could accomplish would be done more speedily by vast tides of
flowing lava than by merely oceanic tides.

There can be no doubt