The Book of the Ocean

by Ernest Ingersoll

LOW TIDE, ST. JOHN’S HARBOR, N. B.

The tide has been an inevitable circumstance of the existence on the earth of the ocean, or any other great body of water, ever since its origin, yet it was not until Sir Isaac Newton made us comprehend the law of gravitation that its mystery was explained. We now know with certainty—if you want the mathematical formulæ and so forth, consult some good modern encyclopædia under the word tide—that this periodical rising and falling of the sea is due to the attraction of the sun and moon,—to the last three times as much as to the first, because it is so much nearer. This attraction is exerted toward the globe as a whole; and its visible effect upon the movable water is to lift it bodily on that side nearest the moon, and at the same time to pull away the earth from the water on the opposite side, which amounts to the same thing; and thus high tides are simultaneously produced at these antipodes, which accounts for the two a day. At the same time, however, the intermediate spaces have low tides caused by an attraction there toward the center of the earth. “There are thus always simultaneously and directly under the moon two high waters opposite each other, and two low waters at equal distances between them. Owing to the rotation of the earth, this permanent system of swells and troughs travels from east to west over every part of the ocean and of its coast, and explains the regular succession of rising and falling waters, at equal intervals of time, which we call the tides.”

THE EARTHQUAKE WAVE PASSING OVER
THE LIGHTHOUSE ON POINT ANJER.

But the sun also exerts a similar but lesser influence, producing four daily solar tides, which most of the time are lost to view in the greater lunar tides. When, however, the moon gets into line with the earth and the sun, so that both the heavenly bodies pull together like a tandem team, as happens twice a month,—at new moon and full moon,—their combined action causes unusually high water, which is the sum of the lunar and solar tides, and is called the spring tide. High water is then highest, and low water lowest. On the other hand, in the midst of these fortnightly intervals, when the moon is at its first or third quarter, the sun is a full quarter of the heavens (90°) away from the moon. Its influence, therefore, acts at right angles to or practically against that of the moon, and the solar tides go to swell the low waters and diminish the high waters, forming what sailors call neap tides,—preserving an old English word meaning low.

Now remember that the globe is not standing still, even while we make these explanations, but is revolving at a tremendous speed, so that the water under the moon lifted by lunar attraction is changing place every instant at the rate of over one thousand miles an hour, and you have the conception of a low wave on each side of the earth, reaching north and south, highest and swiftest on the equator and diminishing toward the poles. These are the true tidal waves. Were the globe covered with an unbroken mantle of water, such waves, each about twenty inches (or twenty-nine inches at springtide) high on the average at the equator, would follow one another round and round the earth at the rate of one complete circuit in every twenty-four hours. That must have been the case in the primeval ocean before any continents existed; and something of it still exists in the belt of unobstructed water surrounding the Antarctic continent of ice. It would then be flood tide or ebb tide at the same hour along the whole length of any one meridian. But in the present condition of the globe, where the oceans are separated by continents and broken by islands, the progress of the tidal waves is obstructed, deflected, and wholly stopped in a great variety of ways and places, so that the hours, amount, and behavior of the tides are exceedingly varied in different regions, and are often very puzzling, forming one of the most difficult matters with which the practical navigator has to deal. Interference of tidal currents forms the Maelstrom, off the coast of Norway, whose revolution is reversed twice daily, the classic Scylla and Charybdis, in the Straits of Messina, so much dreaded by the navigators of old, and many other whirlpools of less celebrity. The tidal wave sweeping northward across the Atlantic has time to round the northern end of Scotland and flood the German Ocean with southward swelling currents before the rising water pouring into the southern end of the English Channel has time to push its way through that narrow and shallow passage; hence the two floods meet in the Straits of Dover, which accounts for the miserable chop-sea so sadly prevalent in that unfortunate bit of water.

The natural height of the tide seems to be from two to five feet, as shown in the midst of the broad Pacific. “But when dashing against the land, and forced into deep gulfs and estuaries,” to quote Professor Simon Newcomb, “the accumulating tide-waters sometimes reach a very great height. On the eastern coast of North America, which is directly in the path of the great Atlantic wave, the tide rises on an average from 9 to 12 feet. In the Bay of Fundy, which opens its bosom to receive the full wave, the tide, which at the entrance is 18 feet, rushes with great fury into that long and narrow channel, and swells to the enormous height of 60 feet, and even to 70 feet in the highest spring tides. In the Bristol Channel, on the coast of England, the spring tides rise to 40 feet, and swell to 50 in the English Channel at St. Malo on the coast of France.”

To this cause is also due in some degree those great oceanic currents which form another striking fact in the history of the sea; but they are mainly due to temperature, wind, and the rotation of the earth.