ween the first critical epoch and the last. We have
already mentioned that the first epoch was one of unstability--it could
not last; but this second state is one of dynamical stability. Once that
state has been acquired, it would be permanent, and would endure for
ever if the earth and the moon could be isolated from all external
interference.

There is one special feature which characterises the movement when the
month is equal to the day. A little reflection will show that when this
is the case the earth must constantly direct the same face towards the
moon. If the day be equal to the month, then the earth and moon must
revolve together, as if bound by invisible bands; and whatever
hemisphere of the earth be directed to the moon when this state of
things commences will remain there so long as the day remains equal to
the month.

At this point it is hardly possible to escape being reminded of that
characteristic feature of the moon's motion which has been observed from
all antiquity. We refer, of course, to the fact that the moon at the
present time constantly turns the same face to the earth.

It is incumbent upon astronomers to provide a physical explanation of
this remarkable fact. The moon revolves around our earth once in a
definite number of seconds. If the moon always turns the same face to
the earth, then it is demonstrated that the moon rotates on its axis
once in the same number of seconds also. Now, this would be a
coincidence wildly improbable unless there were some physical cause to
account for it. We have not far to seek for a cause: the tides on the
moon have produced the phenomenon. We now find the moon has a rugged
surface, which testifies to the existence of intense volcanic activity
in former times. Those volcanoes are now silent--the internal fires in
the moon seem to have become exhausted; but there was a time when the
moon must have been a heated and semi-molten mass. There was a time when
the materials of the moon were so hot as to be soft and yielding, and in
that soft and yielding mass the attraction of our earth excited great
tides. We have no historical record of these tides (they were long
anterior to the existence of telescopes, they were probably long
anterior to the existence of the human race), but we know that these
tides once existed by the work they have accomplished, and that work is
seen to-day in the constant face which the moon turns towards the earth.
The gentle rise and fall