The waters of the ocean are retained in their bed by the attraction of
gravitation. This power is great in proportion to the mass; and as the
earth is of much greater mass than the particles of water on its
surface, it attracts them and keeps them in their assigned places. But
the sun and moon also possess this power of attraction, and
notwithstanding their distance, attract and draw them up to a certain
elevation. The vast mass of the waters being drawn up by the moon into a
mountain or curve of water forms what is called the “great primary or
tidal wave.”
[Illustration: =VAST OCEAN CAVERN AT CAPRI, WIDELY KNOWN AS THE “BLUE
GROTTO”=
This remarkable cavern, on the shore of the island of Capri, at the
entrance of the Bay of Naples, is entered from the sea, and is one
hundred and eighteen feet long and forty feet high, with a breadth of
ninety-eight feet at its widest part. It derives its name from the
wonderful blue reflection of the sun’s rays through the water, which
gives the interior its marvelous beauty and majesty. The cavern has been
created by the ceaseless action of the tide.]
EBB-TIDE AND FLOOD-TIDE. This drawing up of the waters of mid-ocean
causes a recession from the shores, thus giving rise to ebb-tide, or low
water. But when the temporary attraction ceases the waters flow back to
their natural level, returning to shore and forming flood-tide, or high
water. This culmination or rising of the waters in the great tidal wave
takes place twice in twenty-four hours and fifty minutes. The combined
influence of the sun and moon at new and full moon augments the size of
this wave, and causes the “spring-tides” at those periods.
HEIGHT OF TIDES. High water at the various points along the coast is
dependent on the return of this great wave, though some variations are
caused by local peculiarities; and the height of the tide also varies
greatly in different parts of the earth.
On the eastern coast of North America, the average rise of the tide is
from nine to twelve feet. At the entrance to the Bay of Fundy, however,
it rises eighteen feet, while at the head of that bay it reaches sixty,
and in the highest spring tides, even seventy feet. At Bristol, in
England, the spring tides rise to forty feet; and at St. Malo, on the
south coast of the English Channel, they reach fifty feet.