Description of the Phenomenon.—Devastations of Storm-Floods on Flat Coasts.—What did the Ancients know of the Tides?—Their Fundamental Causes revealed by Kepler and Newton.—Development of their Theory by La Place, Euler, and Whewell.—Vortices caused by the Tides.—The Maelstrom.—Charybdis.—The Barre at the mouth of the Seine.—The Euripus.
Living on the sea-coast would undoubtedly be deprived of one of its greatest attractions, without the phenomenon of the tides, which, although of daily recurrence, never loses the charm of novelty, and gives constant occupation to the fancy by the life, movement, and perpetual change it brings along with it. How wonderful to see the sandy plain on which, but a few hours ago, we enjoyed a delightful walk, transformed into a vast sheet of water through which large vessels plough their way! How agreeable to trace the margin of the rising flood, and listen to its murmurs! Those of the rustling grove or waving cornfield are not more melodious. And then the variety of interesting objects which the reflux of the tide leaves behind it on the beach—the elegantly formed shell, the feathery sertularia, the delicate fucoid, and so many other strange or beautiful marine productions, that may well challenge the attention of the most listless lounger.
But the spectacle of the tides is not merely pleasing to the eye, or attractive to the imagination; it serves also to rouse the spirit of scientific inquiry. It is indeed hardly possible to witness their regular succession without feeling curious to know by what causes they are produced, and when we learn that they are governed by the attraction of distant celestial bodies, and that their mysteries have been so completely solved by man, that he is able to calculate their movements for months and years to come, then indeed the pleasure and admiration we feel at their aspect must increase, for we cannot walk upon the beach without being constantly reminded that all the shining worlds that stud the heavens are linked together by one Almighty power, and that our spirit, which has been made capable of unveiling and comprehending so many of the secrets of creation, must surely possess something of a divine nature!
On all maritime coasts, except such as belong to mediterranean seas not communicating freely with the ocean, the waters are observed to be constantly changing their level. They regularly rise during about six hours, remain stationary for a few minutes, and then again descend during an equal period of time, when after having fallen to the lowest ebb, they are shortly after seen to rise again, and so on in regular and endless succession. In this manner twelve hours twenty-four minutes elapse on an average from one flood to another, so that the sea twice rises and falls in the course of a day, or rather twice during the time from one passage of the moon through the meridian to the next, a period equivalent on an average to 1-35/1000 day, or nearly twenty-five hours. Thus the tides retard from one day to another; least at new and full moon, when our more active satellite accomplishes her apparent diurnal motion round the earth in twenty-four hours, thirty-seven minutes; and most at half-moon, when, sailing more leisurely through the skies, she takes full twenty-five hours and twenty-seven minutes to perform her daily journey.
As the retarding of the tides regularly corresponds with the retarding of the moon, they always return at the same hour after the lapse of fourteen days, so that at the end of each of her monthly revolutions, the moon always finds them in the same position. The knowledge of this fact is extremely useful to navigators, as it is easy to calculate the time of any tide in a port by knowing when it is high-water on the days of new and full moon.
The height of the tides in the same place is as unequal and changing as the period of their intervals, and is equally dependent on the phases of the moon, increasing with her growth, and diminishing with her decrease. New and full moon always cause a higher rising of the flood (spring-tide), followed by a deeper ebb, while at half-moon the change of level is much less considerable (neap-tide). Thus in Plymouth, for instance, the neap-tides are only twelve feet high, while the ordinary spring-tides rise to more than twenty feet.
The highest tides take place during the equinoxes; and eclipses of the sun and moon are also invariably accompanied by considerable floods, a circumstance which cannot fail to add to the terror of the ignorant and superstitious when a mysterious obscurity suddenly veils the great luminaries of the sky. It has also been remarked that the tides are stronger or weaker, according as the moon is at a greater or smaller distance from the earth.
Thus as the height of the floods is always regulated by the relative position of the sun and moon, and the movements of these heavenly bodies can be calculated a long time beforehand, our nautical calendars are able to tell us the days when the highest spring-tides may be expected.
This however can only be foretold to a certain extent, as the tidal height not only depends upon the attraction of the heavenly bodies, but also upon the casual influences of the wind, which defies all calculation, and of the pressure of the air. Thus Mr. Walker observed on the coasts of Cornwall and Devonshire that when the barometer falls an inch, the level of the sea rises sixteen inches higher than would otherwise have been the case.
When a strong and continuous wind blows in an opposite direction to the tide-wave, and at the same time the barometer is high, the curious spectators will therefore be deceived in their expectations, however promising the position of the attracting luminaries may be; while an ordinary spring-tide, favoured by a low state of the barometer and chased by a violent storm against the coast, may attain more than double the usual height. When all favourable circumstances combine, an event which fortunately but rarely occurs, those dreadful storm-tides take place, as menacing to the flat coasts of the Netherlands as an eruption of Etna to the towns and hamlets scattered along its base, for here also a vast elementary power is let loose which bids defiance to human weakness. It is then that the rebel sea affords a spectacle of appalling magnificence. The whole surface seethes and boils in endless confusion. Gigantic waves rear their monstrous heads like mighty Titans, and hurl their whole colossal power against the dunes and dykes, as if, impelled by a wild lust of conquest, they were burning to devour the rich alluvial plains which once belonged to their domain. Far inland, the terrified peasant hears the roar of the tumultuous waters, and well may he tremble when the mountain-waves come thundering against the artificial barriers, that separate his fields from the raging floods, for the annals of his country relate many sad examples of their fury, and tell him that numerous villages and extensive meads, once flourishing and fertile, now lie buried fathom-deep under the waters of the sea.