g, should retard it; and that a
retarding force like friction, if such a force acted, should hasten it,
and make it complete its orbit sooner; but so it precisely is.

Gradually, but very slowly, the moon is receding from us, and the month
is becoming longer. The tides of the earth are pushing it away. This is
not a periodic disturbance, like the temporary acceleration of its
motion discovered by Laplace, which in a few centuries, more or less,
will be reversed; it is a disturbance which always acts one way, and
which is therefore cumulative. It is superposed upon all periodic
changes, and, though it seems smaller than they, it is more inexorable.
In a thousand years it makes scarcely an appreciable change, but in a
million years its persistence tells very distinctly; and so, in the long
run, the month is getting longer and the moon further off. Working
backwards also, we see that in past ages the moon must have been nearer
to us than it is now, and the month shorter.

Now just note what the effect of the increased nearness of the moon was
upon our tides. Remember that the tide-generating force varies inversely
as the cube of distance, wherefore a small change of distance will
produce a great difference in the tide-force.

The moon's present distance is 240 thousand miles. At a time when it was
only 190 thousand miles, the earth's tides would have been twice as high
as they are now. The pushing away action was then a good deal more
violent, and so the process went on quicker. The moon must at some time
have been just half its present distance, and the tides would then have
risen, not 20 or 30 feet, but 160 or 200 feet. A little further back
still, we have the moon at one-third of its present distance from the
earth, and the tides 600 feet high. Now just contemplate the effect of a
600-foot tide. We are here only about 150 feet above the level of the
sea; hence, the tide would sweep right over us and rush far away inland.
At high tide we should have some 200 feet of blue water over our heads.
There would be nothing to stop such a tide as that in this neighbourhood
till it reached the high lands of Derbyshire. Manchester would be a
seaport then with a vengeance!

The day was shorter then, and so the interval between tide and tide was
more like ten than twelve hours. Accordingly, in about five hours, all
that mass of water would have swept back again, and great tracts of sand
between here and Ireland would be left dry.