What causes the tides?
The tides are produced by the gravitational action of the sun and moon upon the rotating earth. Theoretically all the stars, planet and satellites in the universe produce separate and distinct tides, but their influence is imperceptible. The moon, which in A Midsummer-Night's Dream Shakespeare calls the "governess of floods," is nearly two and a quarter times more potent as a tide-producing agency th': the sun, because the tide-generating force of a body depends not only on its mass but also on its distance from the earth. Although the effect may not always be appreciable, the tide-producing forces exert
proportion to their size and even on pools and ponds The water in a goldfish bowl or in a has tides. It is not true, as is .[page 80]
each other at the first and third quarters of the moon the high water level of the tide stands at its lowest point. This is called n tide. But twice each lunar month—at new and full moons—the moon and sun pull in the same line and the lunar and solar tides occur at the same time, producing the highest tide, known as spring tide, a term that has no reference to the seasons. When it is low tide on the coast of California it is also low tide on the Pacific coast of northern Japan; but, owing to the location, size and configuration of the Atlantic, there is several hours' difference between low tide on th eastern coasts of the United States and low tide on the western coasts of Spain and France. The United States Coast and Geodetic Survey has developed a "tide-predicting machine," an instrument that indicates the rise and fall of tides at given places for a year in advance. This remarkable mechanism makes precise mathematical calculate and is said to do the work of a hundred mathematicians.