The tides are caused by the gravitational action of the sun and moon upon the rotating earth. Theoretically all the stars, planets and satellites in the universe produce separate and distinct tides, but their influence, of course, cannot be detected. The moon is nearly two and a quarter times more potent as a tide-producing agent than 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 their effect may not always be appreciable, the tide-producing forces exert themselves on all bodies of water in proportion to their size and depth, even on ponds and pools. It is not true, as sometimes stated, that there are no tides in such bodies as the Mediterranean Sea and the Great Lakes. The same forces produce tides in these bodies, but their area is relatively small and the tides are so slight that they are generally masked by differences in mean level resulting from winds, changes in atmospheric pressure, and the discharge of rivers.

At Chicago the tide in Lake Michigan rises only about two inches. The subject of tides is exceedingly complex because of the motions of the earth and moon and because the actual operation of the tide-producing forces is modified by numerous local factors, particularly the location and configuration of the oceans. The pull of the moon tends to separate the solid part of the earth from the more mobile waters, which therefore pile up. When the moon is over a body of water lt pulls the fluid upward and sets a tidal wave in motion. But the earth and the moon are swinging around a common center of gravity and pulling against each other, the moon actually causing the earth to deviate slightly from what would otherwise be its normal path. Therefore, while the moon is piling up the water on its own side of the earth it appears to push it out on the opposite side.

In reality the more mobile water merely takes a slightly greater orbit than the solid part of the earth does when the entire earth is held back by the moon. The more or less free water on the opposite side of the earth bulges away from the moon much as the passengers in the rear seat of an automobile are thrown outward when the machine suddenly turns a sharp corner. Thus two vast tidal waves originate at the same time on opposite sides of the earth and sweep through the oceans. The crests of these waves are 180 degree apart, but their troughs vary in width, depending on the position of the moon. These tidal waves continually sweep through the seas as the earth rotates and produce two high and two low tides in every period Of 24 hours and about 51 minutes. During a great part of the time the sun and the moon pull in different directions, or at angles to each other, but twice each lunar month-at new and full moons-they pull in the same line and then the highest tides occur. 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 the eastern coasts of the United States and low tide on the western coasts of Spain and France.