Читать книгу The Circle of Knowledge: A Classified, Simplified, Visualized Book of Answers - Various - Страница 114

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.”

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.