The tides of the ocean haye been a subject of wonder and a puzzle to those who have not investigated their cause. It is related of the soldiers of Alexander the Great, who were natiyes of the Mediterranean shores, that when they reached the confines of the Indian Ocean and saw its waters rolling up to a great hight, and then flowing back, twice every day, that they became alarmed, and attributed the phenomena to a special interposition of the deities of the country which they had invaded. Various remarkable theories have been advanced regarding the causes of the tides. Kepler, the celebrated German astronomer, believed that the earth was a real living animal, that the tides were due to its respirations, and that men and beasts were like insects feeding on its back. The tides have also been attributed to the water. of the ocean running in and out through some great hole at each pole communicating by a subterranean passage through the earth ; and the Maelstrom—now a myth—that whirl. 100I on the coast of Norway, has been sug-gested as an opening into this supposed ender-ground canal. We would not be yery much urprised if some speculative being would yet discover that the very cold water found by Lieut. Berryman, as described in our last number, was melted ice that had come from he Arctic regions through this inner passage, and had oozed out near Bermuda. If the tides were due to the breathings of a great monster, of course, we can suppose that it just respired twice every twenty-four hours, but this never could account for the variations of the tides. The hole through the earth also cannot account for these variations, as this theory must involve the necessity of a regular ocean current running round and round, not ebbing and Sowing. Many persons—according to letters sometimes received by us—still seem to be much perplexed with the phenomena of the tides. They cannot understand why they are higher at one time than another, nor why they rise to the hight of sixty feet in the Bay of Fundy; forty feet in the ports of Bristol, England, and St. Malo, France, and only rise to a few feet in hight in New York and other places, while they are scarcely perceptible in the Baltic and other seas. Descartes was the first philosopher who advanced the theory that the tides were due to the influence of the moon, but Newton was the first who worked out the problem, and discovered the true cause. Descartes believed that the moon acted on the waters of the ocean by pressure; Newton demonstrated that it acted upon the ocean by attraction : that instead of pressing the waters, it rolled them up directly under it, and also at its antipodes at the samo time, thus producing the two tides every day. The tides are caused by the attraction of both the moon and the sun. If this earth had no moon, the attraction of the sun would produce two tides every day, but their ebb and flow would take place at the same hours regularly, not varying as they do now ; these tides would also be much smaller than those of the moon. Although the mass of the sun is far greater than that of the moon, and although attractionis in proportion to the mass, yet it is also inversely as the square of the distance. As the sun, therefore, is four hundred times further distant than the moon, th0 attraction of the waters of the sea towards the sun is found to be about three times less than that of the moon ; the tides producedby the sun would therefore be three times less than those of the moon. There are really two ocean tides, the lunar and solar, but the latter is absorbed by the former, which is wholly observable in respect to time, the solar only as it influences the hight of the tidal wave. That caused by the moon is three times greater than that of the sun, and it follows the moon's motion around the earth, rising and falling eyery twelve hours, and each succeeding tide later by three quarters of an hour than the preceding one, exactly in accordance with the positions of the moon, or as it is commonly called, " its rising and setting." Sometime$ there are very low and at other times very high tides—that is, their hight is not uniform. Thisis caused by the positions of the sun and moon relative to the earth. Thus, as at the time of the new moon, the sun and the moon being in the same parts of the heavens—the tides produced in the ocean are then the highest, because they are equal to the sum of the two tides—lunar and solar. This should also take place at the time of the fun moon when our satellite is opposite the sun, the attractive force being equal and opposite, in producing the tidal wave. This is found to be true. The tides are greater at new and full moon than at the first and last quarters, as during the latter periods the attraction of the sun, not acting in unison with that of the moon, tends to lower the tides. Reasoning from this data, it 'will naturally be inferred that when the Bun and moon are equally distant from the two poles of the globe, such as at the times of the equinoxes ;n March and September, the tides would be greatest. This is aho found to be the case, thus demonstrating beyond all donbt that the flux and reflux of the sea are due to the attraction of the sun and moon upon the waters of the ocean. The difference in the hight of tho tides at various places is due to the peculiar formation of sea coasts. They are very high in the Bay of Fundy, because an immense quantity of water is piled into a wide-mouthed narrow spac', in the same manner that a certain quantity of water will rise higher in a narrow than in a wide channel. Some have advanced the popular belief against the lnnar influence causing tides, namely, that the Mediterranean is a tideless sea. This is not strictly true. The ocean tides, owing to tbe narrow passage into the Mediterranean at Gibraltar, scarcely affect this sea, but for all this there are regular tides observable at some places. At Vienna they sometimes rise to two feet, and in the Faro of Messina to twenty inches.
This article was originally published with the title "The Tides" in Scientific American 13, 20, 157 (January 1858)