f the moon. We can see
how the moon will still follow its outward course. The path in which it
revolves will grow with extreme slowness, but yet it will always grow;
the progress will not be reversed, at all events, before the final stage
of our history has been attained. We shall not now delay to dwell on the
intervening stages; we will rather attempt to sketch the ultimate type
to which our system tends. In the dim future--countless millions of
years to come--this final stage will be approached. The ratio of the
month to the day, whose decline we have already referred to, will
continue to decline. The period of revolution of the moon will grow
longer and longer, but the length of the day will increase much more
rapidly than the increase in the duration of the moon's period. From the
month of twenty-seven days we shall pass to a month of twenty-six days,
and so on, until we shall reach a month of ten days, and, finally, a
month of one day.

Let us clearly understand what we mean by a month of one day. We mean
that the time in which the moon revolves around the earth will be equal
to the time in which the earth rotates around its axis. The length of
this day will, of course, be vastly greater than our day. The only
element of uncertainty in these enquiries arises when we attempt to give
numerical accuracy to the statements. It seems to be as true as the laws
of dynamics that a state of the earth-moon system in which the day and
the month are equal must be ultimately attained; but when we attempt to
state the length of that day we introduce a hazardous element into the
enquiry. In giving any estimate of its length, it must be understood
that the magnitude is stated with great reserve. It may be erroneous to
some extent, though, perhaps, not to any considerable amount. The length
of this great day would seem to be about equal to fifty-seven of our
days. In other words, at some critical time in the excessively distant
future, the earth will take something like 1,400 hours to perform a
rotation, while the moon will complete its journey precisely in the same
time.

We thus see how, in some respects, the first stage of the earth-moon
system and the last stage resemble each other. In each case we have the
day equal to the month. In the first case the day and the month were
only a small fraction of our day; in the last stage the day and the
month are each a large multiple of our day. There is, however, a
profound contrast bet