earth
and of the moon must have been in those primeval ages when they were
in contact. It is impossible, however, to deny that they must both
have been in a very highly heated state; and everything we know of the
matter inclines us to the belief that the temperature of the
earth-moon system must at this critical epoch have been one of
glowing incandescence and fusion. It is therefore quite possible that
these bodies--the moon especially--may have then been not at all of
the form we see them now. It has been supposed, and there are some
grounds for the supposition, that at this initial stage of earth-moon
history the moon materials did not form a globe, but were disposed in
a ring which surrounded the earth, the ring being in a condition of
rapid rotation. It was at a subsequent period, according to this view,
that the substances in the ring gradually drew together, and then by
their mutual attractions formed a globe which ultimately consolidated
down into the compact moon as we now see it. I must, however,
specially draw your attention to the clearly-marked line which divides
the facts which dynamics have taught us from those notions which are
to be regarded as more or less conjectural. Interpreting the action of
the tides by the principles of dynamics, we are assured that the moon
was once--or rather the materials of the moon--in the immediate
vicinity of the earth. There, however, dynamics leaves us, and
unfortunately withholds its accurate illumination from the events
which immediately preceded that state of things.

The theory of tidal evolution which I am describing in these lectures
is mainly the work of Professor George H. Darwin of Cambridge. Much of
the original parts of the theory of the tides was due to Sir William
Thomson, and I have also mentioned how Professor Purser contributed an
important element to the dynamical theory. It is, however, Darwin who
has persistently deduced from the theory all the various consequences
which can be legitimately drawn from it. Darwin, for instance, pointed
out that as the moon is receding from us, it must, if we only look far
enough back, have been once in practical contact with the earth. It is
to Darwin also that we owe many of the other parts of a fascinating
theory, either in its mathematical or astronomical aspect; but I must
take this opportunity of saying, that I do not propose to make
Professor Darwin or any of the other mathematicians I have named
responsible for