ount for the increased energy of the primary, but also for the
absolute loss of energy by which the whole operation is characterized.

It therefore appears that in the excessively remote future the retreat
of the moon will not only be checked, but that the moon may actually
return to a point to be determined by the changes in the earth's
rotation. It is, however, extremely difficult to follow up the study
of a case where the problem of three bodies has become even more
complicated than usual.

The importance of tidal evolution in our solar system has also to be
viewed in connection with the celebrated nebular hypothesis of the
origin of the solar system. Of course it would be understood that
tidal evolution is in no sense a rival doctrine to that of the nebular
theory. The nebular origin of the sun and the planets sculptured out
the main features of our system; tidal evolution has merely come into
play as a subsidiary agent, by which a detail here or a feature there
has been chiselled into perfect form. In the nebular theory it is
believed that the planets and the sun have all originated from the
cooling and the contraction of a mighty heated mass of vapours. Of
late years this theory, in its main outlines at all events, has
strengthened its hold on the belief of those who try to interpret
nature in the past by what we see in the present. The fact that our
system at present contains some heat in other bodies as well as in the
sun, and the fact that the laws of heat require continual loss by
radiation, demonstrate that our system, if we look back far enough,
and if the present laws have acted, must have had in part, at all
events, an origin like that which the nebular theory would suppose.

I feel that I have in the progress of these two lectures been only
able to give the merest outline of the theory of tidal evolution in
its application to the earth-moon system. Indeed I have been obliged,
by the nature of the subject, to omit almost entirely any reference to
a large body of the parts of the theory. I cannot bring myself to
close these lectures without just alluding to this omission, and
without giving expression to the fact, that I feel it is impossible
for me to have rendered adequate justice to the strength of the
argument on which we claim that tidal evolution is the most rational
mode of accounting for the present condition in which we find the
earth-moon system. Of course it will be understood that we have neve