en behind their respective disks, the whole is spun rapidly round
the centre of gravity, G. The result of a brief spin is to make A and D
fly out by centrifugal force and show, as in the figure; while the moon,
flying out too in its slot, tightens up the string, which causes B and C
to be pulled out too. Thus all four high tides are produced, two on the
earth and two on the moon, A and D being caused by centrifugal force, B
and C by the attraction of gravitation. Each disk has become prolate in
the same sort of fashion as yielding globes do. Of course the fluid
ocean takes this shape more easily and more completely than the solid
earth can, and so here are the very oceanic humps we have been talking
about, and about three feet high (Fig. 112). If there were a sea on the
_moon_, its humps would be a good deal bigger; but there probably is no
sea there, and if there were, the earth's tides are more interesting to
us, at any rate to begin with.

[Illustration: FIG. 112.--Earth and moon (earth's rotation neglected).]

The humps as so far treated are always protruding in the earth-moon
line, and are stationary. But now we have to remember that the earth is
spinning inside them. It is not easy to see what precise effect this
spin will have upon the humps, even if the world were covered with a
uniform ocean; but we can see at any rate that however much they may get
displaced, and they do get displaced a good deal, they cannot possibly
be carried round and round. The whole explanation we have given of their
causes shows that they must maintain some steady aspect with respect to
the moon--in other words, they must remain stationary as the earth spins
round. Not that the same identical water remains stationary, for in that
case it would have to be dragged over the earth's equator at the rate of
1,000 miles an hour, but the hump or wave-crest remains stationary. It
is a true wave, or form only, and consists of continuously changing
individual particles. The same is true of all waves, except breaking
ones.

Given, then, these stationary humps and the earth spinning on its axis,
we see that a given place on the earth will be carried round and round,
now past a hump, and six hours later past a depression: another six
hours and it will be at the antipodal hump, and so on. Thus every six
hours we shall travel from the region in space where the water is high
to the region where it is low; and ignoring our own motion we shall say
that