ent number of 656 hours. The rotation of the earth has,
of course, also been modified, in accordance with the retreat of the
moon. Once the moon had commenced to recede, the earth was released from
the obligation which required it constantly to direct the same face to
the moon. When the moon had receded to a certain distance, the earth
would complete the rotation in less time than that required by the moon
for one revolution. Still the moon gets further and further away, and
the duration of the revolution increases to a corresponding extent,
until three, four, or more days (or rotations of the earth) are
identical with the month (or revolution of the moon). Although the
number of days in the month increases, yet we are not to suppose that
the rate of the earth's rotation is increasing; indeed, the contrary is
the fact. The earth's rotation is getting slower, and so is the
revolution of the moon, but the retardation of the moon is greater than
that of the earth. Even though the period of rotation of the earth has
greatly increased from its primitive value, yet the period of the moon
has increased still more, so that it is several times as large as that
of the rotation of the earth. As ages roll on the moon recedes further
and further, its orbit increases, the duration of the revolution
augments, until at length a very noticeable epoch is attained, which is,
in one sense, a culminating point in the career of the moon. At this
epoch the revolution periods of the moon, when measured in rotation
periods of the earth, attain their greatest value. It would seem that
the month was then twenty-nine days. It is not, of course, meant that
the month and the day at that epoch were the month and the day as our
clocks now measure time. Both were shorter then than now. But what we
mean is, that at this epoch the earth rotated twenty-nine times on its
axis while the moon completed one circuit.

This epoch has now been passed. No attempt can be made at present to
evaluate the date of that epoch in our ordinary units of measurement. At
the same time, however, no doubt can be entertained as to the
immeasurable antiquity of the event, in comparison with all historic
records; but whether it is to be reckoned in hundreds of thousands of
years, in millions of years, or in tens of millions of years, must be
left in great degree to conjecture.

This remarkable epoch once passed, we find that the course of events in
the earth-moon system begins