HOW THE TIDES ARE PRODUCED BY THE SUN AND MOON.
Each of these bodies excites, by its attraction upon the waters of the sea, two gigantic waves, which flow in the same direction round the world as the attracting bodies themselves apparently do. The two waves of the moon, on account of her greater nearness, are about 3½ times as large as those excited by the sun. One of these waves has its crest on the quarter of the earth’s surface which is turned towards the moon; the other is at the opposite side. Both these quarters possess the flow of the tide, while the regions which lie between have the ebb. Although in the open sea the height of the tide amounts to only about three feet, and only in certain narrow channels, where the moving water is squeezed together, rises to thirty feet, the might of the phenomenon is nevertheless manifest from the calculation of Bessel, according to which a quarter of the earth covered by the sea possesses during the flow of the tide about 25,000 cubic miles of water more than during the ebb; and that, therefore, such a mass of water must in 6¼ hours flow from one quarter of the earth to the other.—Professor Helmholtz.