r
contended that the tides offer the only conceivable theory as to the
present condition of things. The argument lies in this wise. A certain
body of facts are patent to our observation. The tides offer an
explanation as to the origin of these facts. The tides are a _vera
causa_, and in the absence of other suggested causes, the tidal theory
holds the field. But much will depend on the volume and the
significance of the group of associated facts of which the doctrine
offers a solution. The facts that it has been in my power to discuss
within the compass of discourses like the present, only give a very
meagre and inadequate notion of the entire phenomena connected with
the moon which the tides will explain. We have not unfrequently, for
the sake of simplicity, spoken of the moon's orbit as circular, and we
have not even alluded to the fact that the plane of that orbit is
inclined to the ecliptic. A comprehensive theory of the moon's origin
should render an account of the eccentricity of the moon's orbit; it
must also involve the obliquity of the ecliptic, the inclination of
the moon's orbit, and the direction of the moon's axis. I have been
perforce compelled to omit the discussion of these attributes of the
earth-moon system, and in doing so I have inflicted what is really an
injustice on the tidal theory. For it is the chief claim of the theory
of tidal evolution, as expounded by Professor Darwin, that it links
together all these various features of the earth-moon system. It
affords a connected explanation, not only of the fact that the moon
always turns the same face to the earth, but also of the eccentricity
of the moon's path around the earth, and the still more difficult
points about the inclinations of the various axes and orbits of the
planets. It is the consideration of these points that forms the
stronghold of the doctrine of tidal evolution. For when we find that a
theory depending upon influences that undoubtedly exist, and are in
ceaseless action around us, can at the same time bring into connection
and offer a common explanation of a number of phenomena which would
otherwise have no common bond of union, it is impossible to refuse to
believe that such a theory does actually correspond to nature.

The greatest of mathematicians have ever found in astronomy problems
which tax, and problems which greatly surpass, the utmost efforts of
which they are capable. The usual way in which the powers of the
mathematic