Читать книгу The Sea and Its Living Wonders - G. Hartwig - Страница 36

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THE TIDES.

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.

Thus, on the first of November, 1170, the storm-flood, bursting through the dykes, submerged all the land between the Texel, Medenblik, and Stavoren, formed the island of Wieringen, and enlarged the openings by which the Zuiderzee communicated with the ocean. The inundations of 1232 and 1242 caused, each of them, the death of more than 100,000 persons, and that of 1287 swept away more than 80,000 victims in Friesland alone. The irruption of 1395 considerably widened the channels between the Flie and the Texel, and allowed large vessels to sail as far as Amsterdam and Enkhuizen, which had not been the case before. Whilst reading these accounts, we are led to compare the inhabitants of the Dutch lowlands with those of the fertile fields and vineyards that clothe the sides of Vesuvius: both exposed to sudden and irretrievable ruin from the rage of two different elements, and yet both contented and careless of the future; the first behind the dykes that have often given way to the ocean, the latter on the very brink of a menacing volcano.

The tides which sometimes cause such dreadful devastations on the shores of the North Sea are, as is well known, inconsiderable, or even hardly perceptible in the Mediterranean, and thus many years passed ere the Greeks and Romans first witnessed the grand phenomenon. The Phœnicians, the merchant princes of antiquity, who at a very early period of history visited the isolated Britons,—

"Penitus toto divisos orbe Britannos,"—

and sailed far away into the Indian Ocean, were of course well acquainted with it; but it first became known to the Greeks through the voyage of Colæus, a mariner of Samos, who, according to Herodotus, was driven by a storm through the Straits of Hercules into the wide Atlantic 600 years before Christ. About seventy years after this involuntary discovery, the Phoceans of Massilia, or Marseilles, first ventured to follow on the track of Colæus for the purpose of trading with Tartessus, the present Cadiz; and from that time remained in constant commercial intercourse with that ancient Phœnician colony.

With what eager attention may their countrymen have listened to the wondrous tale of the alternate rising and sinking of the ocean! Such must have been the astonishment of our forefathers when the first Arctic voyagers told them of the floating icebergs, and of the perpetually circling sun of the high northern summer.

Thus the tides became known to the Massilians about five centuries before Christ, but in those times of limited international intercourse, knowledge travelled but slowly from place to place; so that it was not before the conquests of Alexander, which first opened the Red Sea and the Persian Gulf to Grecian trade, that the great marine phenomenon began to attract the general attention of philosophers and naturalists.

The flux and reflux of the sea is evidently so closely connected with the movements and changes of the moon, that the intimate relations between both could not possibly escape the penetrating sagacity of the Greeks. Thus we read in Plutarch, that Pytheas of Marseilles, the great traveller who sailed to the north as far as the Ultima Thule, and lived in the times of Alexander the Great, ascribed to the moon an influence over the tides. Aristotle expressed the same opinion, and Cæsar says positively (Commentaries, De Bel. Gal. book iv. 29,) that the full-moon causes the tides of the ocean to swell to their utmost height. Strabo distinguishes a three-fold periodicity of the tides according to the daily, monthly, and annual position of the moon, and Pliny expresses himself still more to the point, by saying that the waters move as if obeying the thirsty orb which causes them to follow its course.

This vague notion of obedience or servitude was first raised by Kepler to the clear and well defined idea of an attractive power. According to this great and self-taught genius, all bodies strive to unite in proportion to their masses. "The earth and moon would mutually approach and meet together at a point, so much nearer to the earth as her mass is superior to that of the moon, if their motion did not prevent it. The moon attracts the ocean, and thus tides arise in the larger seas. If the earth ceased to attract the waters, they would rise and flow up to the moon."

The general notion of a mutual attraction, however, did no more than point out the way for the solution of the problem, and it was reserved to our great Newton to accomplish the prophecy of his great predecessor, "that the discovery of the true laws of gravitation would be accomplished in a future generation, when it should please the Almighty Creator of nature to reveal her mysteries to man."

Newton was the first who proved that the tide-generating power of a celestial body arises from the difference of the attraction it exerts on the centre and the surface of the earth. Thus it was at once made clear how the water not only rises on the surface facing the moon, but also on the opposite side of the earth, as in the latter case the moon acts more strongly on the mass of the earth than on the waters which cover the hemisphere most distant from her. The evident consequence is that the earth sinks (so to say), on the surface turned from the moon, whereby a deepening of the waters, or, in other words, a rising of the tide, is occasioned.

It now also became clear how the moon, whose attractive power upon the earth is 160 times smaller than that of the sun, is yet able to occasion a stronger tide, since, from her proximity to the earth, she attracts the surface more forcibly than the centre with the thirtieth part of her power, while the distant sun occasions a difference of attraction on these two points equal only to one twelve-thousandth part of her attractive force.

Now also a full explanation was first given why the highest tides take place at new and full moon: that is, when the moon stands between the sun and the earth; or the latter between the sun and the moon; as then the two celestial bodies unite their powers; while at half-moon the solar tide corresponding with the lunar ebb, or the lunar tide with the solar ebb, counteract each other.

But even Newton explained the true theory of the tides only in its more prominent and general features, and the labours of other mathematicians, such as MacLaurin, Bernoulli, Euler, La Place, and Whewell, were required for its further development, so as fully to explain all the particulars of the sublime phenomenon.

The reproach has often been made to science, that she banishes poetry from nature, and disenchants the forest and the field; but this surely is not the case in the present instance, for what poetical fiction can fill the soul with a grander image than that of the eternal restlessly-progressing tide-wave, which, following the triumphant march of the sun and moon, began as soon as the primeval ocean was formed, and shall last uninterruptedly as long as our solar system exists!

Were the whole earth covered with one sea of equal depth, the tides would regularly move onwards from east to west, and everywhere attain the same height under the same latitude. But the direction and the force of the tide-wave are modified by many obstacles on its way, such as coast-lines and groups of islands, and it has to traverse seas of very unequal depth and form. Flat coasts impede its current by friction, while it rolls faster along deep mural coasts. From all these causes the strength of the tides is very unequal in different places.

They are generally low on the wide and open ocean. Thus the highest tides at Otaheiti do not exceed eleven inches, three feet at St. Helena, one foot and a half at Porto Rico.

But when considerable obstructions oppose the progress of the tide-waves, such as vast promontories, long and narrow channels, or bays of diminishing width, and mouths of rivers directly facing its swell, it rises to a very great height. Thus, at the bottom of Fundy Bay, which stretches its long arm between Nova Scotia and New Brunswick, the spring-tides rise to sixty, seventy, or even one hundred feet, while at its entrance they do not exceed nine feet, and their swell is so rapid as frequently to sweep away cattle feeding on the shore.

The Bristol Channel and the bay of St. Malo in Brittany, are also renowned for their high tides. Near Chepstow, the flux is said sometimes to reach the surprising height of seventy feet, and at St. Malo the floods frequently rise to forty and fifty feet. When the water is low, this small sea-port town appears surrounded on all sides by fantastically shaped cliffs covered with sea-weeds and barnacles. Pools of salt water interspersed here and there among the hollowed stones, or on the even ground between them, and harbouring many curious varieties of marine animals, are the only visible signs of the vicinity of the ocean, whose hoarse murmurs are heard resounding from afar. But an astonishing change takes place a few hours after, when the town, surrounded by the sea, would be a complete island, but for a long, narrow causeway called "the Sillon," which connects it with the mainland. On the side fronting the open sea, the tide breaks with tremendous rage against the strong buttresses that have been raised to oppose its fury, rises foamingly to a height of thirty or forty feet, and threatens the tardy wanderer as he loiters on the narrow causeway. The cliffs that erewhile were seen to surround the town are now hidden under the waters, some few excepted, that raise their rugged heads like minute islands above the circumambient floods. The opposite side of the causeway is also washed by the sea: but here its motions are less tumultuous, for after having broken against numberless rocks and made a vast circuit, it scarce retains a vestige of its primitive strength. On this side lies the vast, but deserted harbour of St. Malo, completely dry at ebb-tide; a wide sea during the flood.

Two eminent French authors, Chateaubriand and Lamennais, were born at St. Malo, and there can be no doubt that the imposing spectacle I have briefly described must have greatly contributed to the widening of their intellectual horizon. Daily witnesses from their early childhood of one of the grandest phenomena of nature in all its wild sublimity, the boundless and the infinite soon grew familiar to their mind, enriching it with splendid imagery and bold conceptions.

Although the sun and the moon exert some attraction upon the smaller and inclosed seas, yet the development of a powerful flood-wave necessarily requires that the moon should act upon a sufficiently wide and deep expanse of ocean. Even the Atlantic is not broad enough for this purpose, as its equatorial width measures no more than one eighth of the earth's circumference: and the Pacific itself, notwithstanding its vast area, is so studded with islands and shallows, that it presents a much more obstructed basin for the action of the tide-wave than might be expected, from its apparent dimensions and equatorial position.

Thus it is in the Southern Ocean, where the greatest uninterrupted surface of deep water is exposed to the influence of the moon, that we must look for the "chief cradle of the tides." From this starting point they flow on all sides to the northward, progressing like any other wave that arises on a small scale in a pond from a gust of wind, the throwing of a stone, or any other cause capable of producing an undulating movement on the surface of the waters.

The tide-wave, which ultimately reaches our shores, arrives at the Cape of Good Hope thirteen hours after it has left Van Diemen's Land, and thence rolls onward in fourteen or fifteen hours to the coasts of Spain, France, and Ireland. It penetrates into the North Sea by two different ways. One of its ramifications turns round Scotland and thence flows onwards to the south, taking nineteen or twenty hours for the passage from Galway to the mouth of the Thames. A tide-wave, for instance, which appears at five in the afternoon on the west coast of Ireland, arrives at eight near the Shetland Islands, reaches Aberdeen at midnight, Hull at five in the morning, and Margate at noon.

The other ramification of the same tide-wave, taking the shorter route through the Channel, had meanwhile preceded it by twelve hours, having reached Brest about five o'clock of the afternoon (at the same time that the northern branch appeared at Galway), Cherbourg at seven, Brighton at nine, Calais at eleven, and the mouth of the Thames at midnight.

Thus, in this southern corner of the North Sea, two tide-waves unite that belong to two successive floods; the Scotch branch having started twelve hours sooner from the great Southern Ocean than the Channel branch, which thus results from the next following tide. The meeting of the two branches naturally gives rise to a more considerable rising of the waters, so that this circumstance, by allowing large ships to sail up the Thames, may be considered as one of the fundamental causes of the grandeur of London.

In other parts of the North Sea, where the two tide-waves appear at different times, the contrary takes place, for the ebb of the one coinciding with the rising of the other, they naturally weaken or even neutralise each other. This occasions the low tides on the coast of Jutland, in Denmark, where they are scarcely higher than in the Mediterranean, and explains the otherwise startling fact of there being a space in the North Sea where no periodical rise and fall of the waters whatsoever takes place.

Thus we see that the relations of the tides in the North Sea, with regard to height and time, are of a somewhat complicated nature, which could only be explained after the numerous observations (amounting to more than 40,000) made by order of the British Government in all parts of the world, under the direction of Professor Whewell, had proved that all the floods of the seas chiefly proceed from the great tide-wave of the Southern Ocean, which, by its numerous ramifications in narrow seas or through groups of islands and by the unequal rapidity of its progress, according to the depth or shallowness of the waters it traverses, occasions all the seeming anomalies which were quite inexplicable by the simple Newtonian theory.

As every twelve hours a new tidal-wave originates in the Southern Ocean which regularly follows in the same track as its predecessor, the tides everywhere succeed each other in regular and equal periods, and can thus everywhere be calculated beforehand.

In narrow straits or in the intricate channels which wind through clusters of islands, different tidal-waves meeting from opposite directions give rise to more or less dangerous whirlpools. One of the most famous of these vortices, though inconsiderable in itself, is the renowned Charybdis, which gave so much trouble to Ulysses on his passing through the strait which separates Sicily from Italy, but is at present an object of fear scarcely even to the poor fisherman's boat.

A much grander whirlpool, owing its celebrity, not to the fictions of poetry, but to the magnificent scale on which it has been constructed by nature, is the renowned Maelstrom, situated on the Norwegian coast in 68° N. lat., and near the island of Moskoe, from whence it also takes the name of Moskoestrom. It is four geographical miles in diameter, and in tempestuous weather its roar, like that of Niagara, is said to be heard several miles off. John Ramus gives us a terrible description of its fury, and mentions that in the year 1645 it raged with such noise and impetuosity, that on the island of Moskoe, the very stones of the houses fell to the ground. He tells us also that whales frequently come too near the stream, and, notwithstanding their giant strength, are overpowered by its violence, but, unfortunately adds, that it is impossible to describe their howlings and bellowings in their fruitless struggles to disengage themselves—impossible, no doubt, as whales happen to have no voice at all!

According to more modern travellers, such as the celebrated geologist Leopold von Buch, the Maelstrom is far from being so terrible as depicted by Ramus and other friends of the marvellous; so that, except during storms and spring-tides, large ships may constantly cross it without danger. The Norwegian fishermen are even said frequently to assemble on the field of the Maelstrom on account of the great abundance of fishes congregating in those troubled waters, and fearlessly to pursue their avocations, while the whirlpool moves their boats in a circular direction.

Sir Robert Sibbald describes a very remarkable marine whirlpool among the Orkney islands, which would prove dangerous to strangers, though it is of no consequence to the people who are used to it. It is not fixed to any particular place, but arises in various parts of the limits of the sea among these islands. Wherever it appears, it is very furious, and boats would inevitably be drawn in and perish with it, but the people who navigate them are prepared for it and always carry a bundle of straw or some such matter in the boat with them. This they fling into the vortex which immediately swallows it up, and, seemingly pleased with this propitiatory offering, subsides into smoothness, but soon after re-appears in another place.

A remarkable and sudden rising of the spring-tide takes place at the mouth of several rivers, for instance, the Indus (where the surprising phenomenon nearly caused the destruction of the fleet of Alexander the Great), the Hooghly, the Dordogne, &c. In the Seine it is observed on a scale of great magnitude. While the tide gradually rises near Havre and Harfleur, a giant wave is suddenly seen to surge near Quillebœuf, spanning the whole width of the river (from 30,000 to 36,000 feet). After this mighty billow has struck against the quay of Quillebœuf, it enters a more narrow bed and flows stream-upwards with the rapidity of a race horse, overflowing the banks on both sides, and not seldom causing considerable loss of property by its unexpected appearance. The astonishment it causes is increased when it takes place during serene weather, and without any signs of wind or storm. A deafening noise announces and accompanies this sudden swelling of the waters, which owes its first origin to the silent action of gravitation, and is the result of the diminishing velocity of the tide-wave over a shallow bottom.

While the tide-wave advances over the deep and open seas with an astonishing rapidity, its progress up the channel of a river is comparatively very slow, partly on account of the reason just mentioned, and partly from its meeting a current flowing in an opposite direction.

Thus, the tide takes no less than twelve hours for its progress from the mouth of the Thames to London, about the time it requires to travel all the way from Van Diemen's Land to the Cape of Good Hope. Consequently, when it is high-water at the mouth of the Thames at three o'clock in the afternoon, for instance, we have not high-water at London Bridge before three o'clock in the following morning, when it is again high water at the Nore. But, in the mean time, there has been low water at the Nore and high water about half-way to London, and while the high water is proceeding to London, it is ebbing at the intermediate places, and is low water there when it is high water at London and at the Nore. If the tide extended as far beyond London as London is from the Nore, we should have three high waters with two low waters interposed. The most remarkable instance of this kind is afforded by the gigantic river of the Amazons, as it appears by the observations of Condamine and others, that, between Para, at the mouth of the colossal stream, and the conflux of the Madera and Marañon, there are no less than seven simultaneous high waters with six low waters between them. Thus, four days after the tide-wave was first raised in the Southern Ocean, its last undulations expire deep in the bosom of the South American wilds.

The Mediterranean is generally supposed to be tideless, but this opinion is erroneous; and in the Adriatic, the flux of the sea is far from being inconsiderable, for, at Venice, the difference between high and low water is sometimes no less than six or even nine feet. Mr. W. Trevelyan, during a summer residence in the old port of Antium, on the Roman coast, found from a series of accurate observations, that the tides regularly succeed each other and attain a height of fourteen inches. In the eastern Mediterranean new measurements have proved that they are still more considerable, while in the western part of that inclosed sea they are almost imperceptible.

The differences of level caused by the Mediterranean tides, are indeed too inconsiderable to attract the general notice of the inhabitants on the coast, but in the famed Euripus, the narrow channel which separates the island of Eubœa or Negropont from continental Greece, the tide produces the striking phenomenon of very irregular fluctuations of the waters, from one end of the channel to the other.

This phenomenon was of course completely inexplicable to the ancient philosophers, and Aristotle is even said to have drowned himself in the Euripus in a fit of despair, since, with all his prodigious sagacity, he could not possibly solve the mystery. For us, who know that peculiar formations of the sea-bed and coasts are capable of considerably augmenting the force of the floods, and that tidal waves rushing into a narrow channel in opposite directions, and at different times, must necessarily produce irregular fluctuations of the waters, the phenomenon of the Euripus has ceased to be a mystery.