particles. It has, however, been suggested that another explanation
of a very interesting kind may be forthcoming, and this we must
endeavour to set forth.

It will be remembered that we have to explain why the period of
revolution of the moon is now shorter than it used to be. If we
imagine the length of the period to be expressed in terms of days and
fractions of a day, that is to say, in terms of the rotations of the
earth around its axis, then the difficulty encountered is, that the
moon now requires for each of its revolutions around the earth rather
a smaller number of rotations of the earth around its axis than used
formerly to be the case. Of course this may be explained by the fact
that the moon is now moving more swiftly than of yore, but it is
obvious that an explanation of quite a different kind might be
conceivable. The moon may be moving just at the same pace as ever,
but the length of the day may be increasing. If the length of the
day is increasing, then, of course, a smaller number of days will be
required for the moon to perform each revolution even though the
moon's period was itself really unchanged. It would, therefore, seem
as if the phenomenon known as the lunar acceleration is the result of
the two causes. The first of these is that discovered by Laplace,
though its value was overestimated by him, in which the perturbations
of the earth by the planets indirectly affect the motion of the
moon. The remaining part of the acceleration of our satellite is
apparent rather than real, it is not that the moon is moving more
quickly, but that our time-piece, the earth, is revolving more
slowly, and is thus actually losing time. It is interesting to note
that we can detect a physical explanation for the apparent checking
of the earth's motion which is thus manifested. The tides which ebb
and flow on the earth exert a brake-like action on the revolving