cohere together; it would separate into pieces, just as a grindstone
driven too rapidly is rent asunder with violence. Here, therefore, we
discern in the remote past a barrier which stops the present argument.
There is a certain critical velocity which is the greatest that the
earth could bear without risk of rupture, but the exact amount of that
velocity is a question not very easy to answer. It depends upon the
nature of the materials of the earth; it depends upon the temperature;
it depends upon the effect of pressure, and on other details not
accurately known to us. An estimate of the critical velocity has,
however, been made, and it has been shown mathematically that the
shortest period of rotation which the earth could have, without flying
into pieces, is about three or four hours. The doctrine of tidal
evolution has thus conducted us to the conclusion that, at some
inconceivably remote epoch, the earth was spinning round its axis in a
period approximating to three or four hours.

We thus learn that we are indebted to the moon for the gradual
elongation of the day from its primitive value up to twenty-four hours.
In obedience to one of the most profound laws of nature, the earth has
reacted on the moon, and the reaction of the earth has taken a tangible
form. It has simply consisted in gradually driving the moon away from
the earth. You may observe that this driving away of the moon resembles
a piece of retaliation on the part of the earth. The consequence of the
retreat of the moon is sufficiently remarkable. The path in which the
moon is revolving has at the present time a radius of 240,000 miles.
This radius must be constantly growing larger, in consequence of the
tides. Provided with this fact, let us now glance back into the past
history of the moon. As the moon's distance is increasing when we look
forwards, so we find it decreasing when we look backwards. The moon must
have been nearer the earth yesterday than it is to-day; the difference
is no doubt inappreciable in years, in centuries, or in thousands of
years; but when we come to millions of years, the moon must have been
significantly closer than it is at present, until at length we find that
its distance, instead of 240,000 miles, has dwindled down to 40,000, to
20,000, to 10,000 miles. Nor need we stop--nor can we stop--until we
find the moon actually close to the earth's surface. If the present laws
of nature have operated long enough, and if there