l have been attained. In that state the day is destined
once again to be equal to the month, just as was the case in the
initial stage. The half-way stage will therefore in one sense be that
in which the proportion of the month to the day culminates. This is
the stage which we have but lately passed; and thus it is that at
present we may be said to be almost half-way through the progress of
tidal evolution.

My narrative of the earth-moon evolution must from this point forward
cease to be retrospective. Having begun at that critical moment when
the month and day were first equal, we have traced the progress of
events to the present hour. What we have now to say is therefore a
forecast of events yet to come. So far as we can tell, no agent is
likely to interfere with the gradual evolution caused by the tides,
which dynamical principles have disclosed to us. As the years roll on,
or perhaps, I should rather say, as thousands of years and millions of
years roll on, the day will continue to elongate, or the earth to
rotate more slowly on its axis. But countless ages must elapse before
another critical stage of the history shall be reached. It is needless
for me to ponder over the tedious process by which this interesting
epoch is reached. I shall rather sketch what the actual condition of
our system will be when that moment shall have arrived. The day will
then have expanded from the present familiar twenty-four hours up to a
day more than twice, more than five, even more than fifty times its
present duration. In round numbers, we may say that this great day
will occupy one thousand four hundred of our ordinary hours. To
realize the critical nature of the situation then arrived at, we must
follow the corresponding evolution through which the moon passes. From
its present distance of two hundred and forty thousand miles, the moon
will describe an ever-enlarging orbit; and as it does so the duration
of the month will also increase, until at last a point will be reached
when the month has become more than double its present length, and has
attained the particular value of one thousand four hundred hours. We
are specially to observe that this one-thousand four-hundred-hour
month will be exactly reached when the day has also expanded to one
thousand four hundred hours; and the essence of this critical
condition, which may be regarded as a significant point of tidal
evolution, is that the day and the month have again become equal