le less than three. The exact amount, however, is not really
very material to us; it would be sufficient for our argument to assert
that there is a certain minimum length of day for which the earth can
hold together. In our retrospect, therefore, through the abyss of time
past our view must be bounded by that state of the earth when it is
revolving in this critical period. With what happened before that we
shall not at present concern ourselves. Thus we look back to a time at
the beginning of the present order of things, when the day was only
some three or four hours long.

Let us now look at the moon, and examine where it must have been
during these past ages. As the moon is gradually getting further and
further from us at present, so, looking back into past time, we find
that the moon was nearer and nearer to the earth the further back our
view extends; in fact, concentrating our attention solely on essential
features, we may say that the path of the moon is a sort of spiral
which winds round and round the earth, gradually getting larger,
though with extreme slowness. Looking back we see this spiral
gradually coiling in and in, until in a retrospect of millions of
years, instead of its distance from the earth being 240,000 miles, it
must have been much less. There was a time when the moon was only
200,000 miles away; there was a time many millions of years ago, when
the moon was only 100,000 miles away. Nor can we here stop our
retrospect; we must look further and further back, and follow the
moon's spiral path as it creeps in and in towards the earth, until at
last it appears actually in contact with that great globe of ours,
from which it is now separated by a quarter of a million of miles.

Surely the tides have thus led us to the knowledge of an astounding
epoch in our earth's past history, when the earth is spinning round in
a few hours, and when the moon is, practically speaking, in contact
with it. Perhaps I should rather say, that the materials of our
present moon were in this situation, for we would hardly be entitled
to assume that the moon then possessed the same globular form in which
we see it now. To form a just apprehension of the true nature of both
bodies at this critical epoch, we must study their concurrent history
as it is disclosed to us by a totally different line of reasoning.

Drop, then, for a moment all thought of tides, and let us bring to our
aid the laws of heat, which will disclose cert