aken as a verification of the law, that the
efficiency of a body as a tide-producer varies inversely as the cube
of the distance between it and the body on which the tides are being
raised.

For simplicity we may make the assumption that the whole of the earth
is buried beneath the ocean, and that the moon is placed in the plane
of the equator. We may also entirely neglect for the present the tides
produced by the sun, and we shall also make the further assumption
that friction is absent. What friction is capable of doing we shall,
however, refer to later on. The moon will act on the ocean and deform
it, so that there will be high tide along one meridian, and high tide
also on the opposite meridian. This is indeed one of the paradoxes by
which students are frequently puzzled when they begin to learn about
the tides. That the moon should pull the water up in a heap on one
side seems plausible enough. High tide will of course be there; and
the student might naturally think that the water being drawn in this
way into a heap on one side, there will of course be low tide on the
opposite side of the earth. A natural assumption, perhaps, but
nevertheless a very wrong one. There are at every moment two opposite
parts of the earth in a condition of high water; in fact, this will
be obvious if we remember that every day, or, to speak a little more
accurately, in every twenty-four hours and fifty one minutes, we have
on the average two high tides at each locality. Of course this could
not be if the moon raised only one heap of high water, because, as the
moon only appears to revolve around the earth once a day, or, more
accurately, once in that same average period of twenty-four hours and
fifty-one minutes, it would be impossible for us to have high tides
succeeding each other as they do in periods a little longer than
twelve hours, if only one heap were carried round the earth.

The first question then is, as to how these two opposite heaps of
water are placed in respect to the position of the moon. The most
obvious explanation would seem to be, that the moon should pull the
waters up into a heap directly underneath it, and that therefore there
should be high water underneath the moon. As to the other side, the
presence of a high tide there was, on this theory, to be accounted for
by the fact that the moon pulled the earth away from the waters on the
more remote side, just as it pulled the waters away from the more
remote earth