Tides

tidz, waves due to the attraction of the sun and moon. The liquid portions of the earth's surface are more free to move than the solid rocky portions, and rise in waves toward the sun and moon. The same attraction draws the solid earth away from the opposite liquid surface. The waters on the far side are free to linger behind under the influence of inertia. The moon causes two tides. The sun causes two. As the earth turns on its axis these four waves follow each other around the world ceaselessly. At the time of the new and the full moon, the tides of the moon coincide with those of the sun and are called spring tides. At half moon the tides are six hours apart, and are the lowest of the month. These are called neap tides. Rising water is a flood tide. Falling water is an ebb. A neap tide is about three-sevenths as high as a spring tide. Ships aim to take advantage of flood tide to enter or leave a harbor. As Shakespeare puts it:

Although the sun outweighs the moon enormously, it is so far distant that its attraction is only about two-fifths as great as that of the moon. The tides caused by the moon are over twice as high as those of the sun. As a matter of fact the tidal waves lag several hours behind the sun and moon and are prevented by the continents from running a free course. The tides of London are so retarded by land masses that they are over two days behind time when they reach the mouth of the Thames. The tides of each harbor are charted from actual observation, and are published for the guidance of pilots and navigators. Ordinarily a tide rises and subsides quietly.

If a tidal wave enters a wide bay through a narrow channel or over a bar, it expires quietly with little rise or fall of water level. The tides of the Mediterranean and Baltic are not high. When a tidal wave from deep water enters a broad, deep harbor or deep mouth of a river and spends its energy up a narrow channel at the far end, the momentum of the water forces a tremendous mass of water inland. The tides in the Bay of Fundy rise to a height of thirty feet at St. John, New Brunswick, and sixty feet at the head of Chignecto Bay. The tide comes into the upper bay, the Basin of Minas, in a huge roaring front, with the speed of a race horse. Hogs feeding on the sands seize clams in their mouths and race for dear life up the rocky cliffs to a place of safety.

In large rivers the tidal wave does not die out until it has ascended a considerable distance, in the case of the La Plata and the Amazon to a height of 100 feet above sea level. The bore, as the tidal wave in a river is called, is a dangerous feature in the navigation of many rivers. Ships are obliged to seek safety at anchorage until the crest goes roaring by. The bore in one of the rivers in eastern Asia is twenty feet high and advances ten miles an hour. Tides are not confined to the ocean, but they are difficult to detect in the Great Lakes. Chicago is said to have tides of about one and three-fourths inches.

The French have made considerable progress in the construction of tide-mills--in harnessing the racing tides to turn the wheels of mills. The tides of geologic ages are thought to have run thirty-six times as high as the tides of the present day--due to the more rapid rotation of the earth.