Each of these bodies excites, by its attraction upon the waters of the
sea, two gigantic waves, which flow in the same direction round the
world as the attracting bodies themselves apparently do. The two waves
of the moon, on account of her greater nearness, are about 3½ times as
large as those excited by the sun. One of these waves has its crest on
the quarter of the earth’s surface which is turned towards the moon;
the other is at the opposite side. Both these quarters possess the flow
of the tide, while the regions which lie between have the ebb. Although
in the open sea the height of the tide amounts to only about three
feet, and only in certain narrow channels, where the moving water is
squeezed together, rises to thirty feet, the might of the phenomenon
is nevertheless manifest from the calculation of Bessel, according to
which a quarter of the earth covered by the sea possesses during the
flow of the tide about 25,000 cubic miles of water more than during the
ebb; and that, therefore, such a mass of water must in 6¼ hours flow
from one quarter of the earth to the other.--_Professor Helmholtz._