Tides are regular motions or successive rising and falling of the waters of oceans and seas. They are caused chiefly by the attraction of the moon, which draws up the waters as seen in cut No. 1. Thus, as the moon revolves round the earth, its attracting or drawing power passes with it over the surface of the ocean, and the elevation of the waters, following that attraction, is drawn along until the shore or coast of a country stops its progress; this causes, all along the coast, a rising of the water, and it is then called high tide. The tides occur twice in twenty-four hours.

The attraction of the moon in producing tides is affected in some degree by the sun. When the sun and moon attract in unison, the tides are very high; and when the sun tends to counteract the action of the moon the tides are not so high. Cut No. 2.

The highest tide is called spring tide, and the lowest, neap tide. These tides follow each other in regular succession, the daily tides gradually decreasing during fifteen days, from the highest to the lowest; and during the next fifteen days, increasing from the lowest to the highest.

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