CHAPTER IX.

TIDES AND OCEANIC HIGHWAYS.

HERE are various movements to which the waters of the ocean are subject, which are of great importance to navigation, and of high interest to the physical enquirer. They are chiefly the effect of external causes of disturbance, either atmospheric or astronomical, which operate with mighty though changeful energy upon the yielding fluid. Waves, tides, and currents are three distinct forms under which its principal agitations appear. As water is susceptible of impression from the slightest force, the equilibrium at the surface is disturbed by the aerial currents in contact with it, and upon the particles of the fluid being displaced, the adjoining particles immediately

rush in to restore the balance. The cause continuing to act, the effect follows, in accordance with its duration and potency. In this manner waves are formed, from the power of the atmosphere in a state of motion, displacing the surface waters, and their own tendency to preserve an equilibrium. As the force of the wind varies, its impression upon the ocean varies proportionately. A gentle breeze wrinkles the surface; a brisk gale produces undulations which rock to and fro the largest vessels upon their bosom; a storm creates waves of enormous volume and appalling violence. These agitations of the sea may be compared to the waving of a forest, or of a field of grain, where commotion is displayed, apart from any continuous onward movement. They have far less resemblance to the motion of a river current. In a field of corn, under the action of the zephyr or a high wind, we see waves formed by the bending tops of the corn, apparently chasing one another across the field, yet without any advancing motion of the parts that form them. In like manner, the motion of waves of water is not necessarily accompanied by a current in the same direction, and though a continuous high wind produces this effect, the progress of the water is at a very slow rate, and has no correspondence with the impression which its outward aspect makes upon the mind of the spectator. The proof of this may be easily obtained, by throwing any light substance into the sea, a little beyond the breakers, or into a piece of standing water, the surface of which is ruffled. Such a floating body will rise and fall with the motion of the waves, but make little perceptible advance towards the shore.

Waves vary in their height, form, velocity, and extent. These diversities depend upon the depth of the sea, the size of its basin, and the force of the wind. A wave-summit produced by a breeze from the land, maintains constantly the same height while the impulse is the same; but the heights increase according to the distance from the shore. In open seas, where the wind blows upon the water in a parallel manner, through a considerable tract, the waves are generally in the shape of straight and long furrows; but in more confined situations, they appear in short straight lines, or in arcs of circles, according to

the configuration of the coasts, and their contiguity. When the wind blows fresh, the motion of the waves not being sufficiently quick, their thin and light tops are impelled forward and broken, falling upon their own slopes in a torrent of white foam, particles of which, in the form of spray, are carried to a vast distance by the gale. It is no uncommon circumstance during a violent gale, for persons far inland to be sensible of a saline impregnation in the atmosphere, the spray of the waves, which have been torn by the blast. In severe tempests, enormous volumes of water are accumulated in ridges, which literally consist of wave on wave; for owing to the permanence of the wind, its action will raise a second upon the first, and a third upon the second, in the same manner as it raised the first upon the flat surface of the water. From a number of experiments, and the experience of divers, it appears that the disturbance of the sea by the action of the winds, extends but a comparatively small way below the surface. The observations of the Committee appointed by the British Association in 1836 show that, with a depth of water equal to 12 feet, waves nine inches high and four or five feet long did not sensibly affect the water at the bottom; and probably at the depth of 200 feet the sea is undisturbed in the roughest weather. After the subsidence of the wind which has put the surface in motion, waves continue to roll for some hours, upon the principle of the pendulum swinging for some time after it has received an impulse. Hence in a completely calm state of the atmosphere, the ocean exhibits a great undulatory movement, called the swell, which seldom entirely dies away, before the action of the disturbing cause is renewed. The swell proceeds from the combined influence of winds and currents, and upon the mighty oscillation being checked in its career by sand banks, or a rocky coast, a roaring and violent surge is produced. Such is the surf at Madras, caused by the swell of the ocean across the Bay of Bengal, a sweep of nearly five hundred miles, coming into contact with the shore, where it exhibits the "wild waves' play," whose voice is heard far over the level plains of the Carnatic. It frequently occurs, that while the swell is advancing in one direction, the wind is blowing from an opposite quarter, or the wind suddenly chops about, or the surface of the ocean receives impulses in various directions from different breezes; and in any of these cases, a series of compound waves is produced, and the aspect of the deep becomes complex in the extreme.

The Sound at Plymouth was formerly exposed to a heavy swell occasioned by the south-west gales, incommoding and endangering the shipping of that great naval arsenal; but this has been remedied by the construction of the Breakwater, one of the most stupendous undertakings ever accomplished by the genius and power of man.

"The billows sleep

Within the shelter of a wondrous pile

Of man's best workmanship—that new-made isle,

That marble isle-brought piecemeal from the shore,

To break the weltering waves, and check their savage roar."

The Breakwater is a barrier or dyke, nearly a mile in length, raised above the surface of the water, and stretching across the Sound so as to leave entrances at both ends. It was formed by an immense quantity of large stones, quarried from some limestone hills, which were shipped in vessels specially constructed for the purpose, and precipitated into the sea at the spot where it was proposed to keep the waves at bay. About fifty vessels were employed in this service, by which were deposited in the year 1812 about 16,000 tons of stone; in 1813, 71,000; in 1814, 240,000; in 1815, 264,000; in 1816, 300,000; the whole quantity amounting to nearly 40,000,000 cubic feet. Baron Dupin inspected this extraordinary work while in progress, and enthusiastically records the impression made upon his mind by the "order, regularity, and activity which reign throughout all the operations; the embarking and disembarking of the materials; the working and

placing of the enormous blocks which form the upper part of the breakwater; the difficulties conquered by the dexterity and ingenuity of the workmen; the transport of the blocks, and, above all, their extraction from the quarries. When we visit the workshops of the artificers and the operations of the quarry-men," he continues, "it is admirable to observe man, so weak and so feeble, manage at his will the enormous masses he has detached from their beds, in order to precipitate them into the ocean, to form other hills. The roads formed in the air for the transport of the useless earth and broken fragments; the lines of cranes and their combined labour; the movements of the carriages; the arrival, the loading, and the departure of the vessels-present to the eye of an admirer of great works and of the mechanical arts, one of the most pleasing and imposing spectacles it is possible to contemplate." The artificial barrier has answered the purpose for which it was constructed, as admirably as if a natural rampart of rock occupied its site; and Plymouth Sound is now a safe and convenient roadstead for the largest menof-war.

The effect attributed to "a soft answer," the moderation of wrath, has frequently been illustrated by a reference to the action of oil upon waves. From the time of Plutarch and Pliny, who relate, that the mariners of their day were accustomed to still waves in a storm by pouring oil into the sea, it has passed current in popular speech, that this effect by such means may be produced, and though treated with discredit in modern times, experiment proves that there is some truth in the statement. Among the facts reported in favour of it, the following occurs in a letter to Count Bentinck from M. Tengragel, dated Batavia, Jan. 5th, 1770:-" Near the islands Paul and Amsterdam we met with a storm, which had nothing particular in it worthy of being communicated to you, except that the captain found himself obliged, for greater safety, in wearing the ship, to pour oil into the sea, to prevent the waves breaking over her; which had an excellent effect, and succeeded in preserving us. As he poured out but a little at a time, the East India Company owes perhaps its ship to only six demi-aumes of olive oil. I was present upon deck when this was done, and I should not have mentioned this circumstance to you, but that we have found people here so prejudiced against the experiment, as to make it necessary for the officers on board, and myself, to give a certificate of the truth on this head, of which we made no difficulty." It was the practice of the fishermen of Lisbon, when about to return into the river, if they saw before them too great a surf upon the bar, which they apprehended might fill their boats in passing, to empty a bottle of oil into the sea, to suppress the breakers. Previous to the time of Franklin, no man of science made experiments upon the subject; but his attention was called to it by a circumstance which he thus narrates:- "In 1757, being at sea in a fleet of ninety-six sail bound for Louisbourg, I observed the wakes of two of the ships to be remarkably smooth, while all the others were ruffled by the wind, which blew fresh. Being puzzled with the differing appearance, I at last pointed it out to our captain, and asked him the meaning of it. The cooks,' said he, have, I suppose, been just emptying their greasy water through the scuppers, which has greased the sides of those ships a little;' and this answer he gave me with an air of some little contempt, as to a person ignorant of what everybody else knew. In my own mind I at first slighted his solution, though I was not able to think of another." The issue of one of Franklin's experiments upon a pond on Clapham Common, is detailed in a volume of the Philosophical Transactions. After dropping a little oil into the water, he states: "I saw it spread itself with surprising swiftness upon the surface, but the effect of smoothing the waves was not produced; for I had applied it first upon the leeward side of the pond, where the waves were largest, and the wind drove my oil back upon the shore. I then went to the windward side, where they began to form; and there the oil, though not more than a tea-spoonful, pro

duced an instant calm over a space several yards square, which spread amazingly, and extended itself gradually till it reached the lee-side, making all that quarter of the pond, perhaps half an acre, as smooth as a looking-glass." Franklin again experimented at the entrance of Portsmouth harbour, opposite to Haslar hospital, in company with Sir Joseph Banks, Dr. Blagden, and Dr. Solander, where the waves, though not destroyed, were reduced to calm and gently swelling undulations. It seems evident, therefore, that the mollifying effect attributed to the action of oil upon disturbed waters is not without some foundation. Though the course of large waves is not arrested by it, for these have acquired a power of oscillation independent of the force of the wind, yet it will smooth their surface, and perhaps prevent their formation altogether under the influence of but a gentle breeze. "I imagine," says Franklin, accounting for the effect, "that the wind blowing over water covered with a film of oil cannot easily catch upon it, so as to raise the first wrinkles, but slides over it, and leaves it smooth as it finds it."

The second great movement which the waters of the ocean exhibit is the Tides. Here we have periodical fluctuations of its level, the causes of which are astronomical, and arise from the attractive influence of the sun and moon, the latter being the more potent agent of the two. The sea rises, or flows, as it is called, by degrees, about six hours; it remains stationary about a quarter of an hour; it then retires, or ebbs, during another six hours, to flow again after a brief repose. Thus every day, or the period elapsing between successive returns of the moon to the meridian of a place, which is 24 hours 50 minutes, the sea ebbs and flows twice, much less, indeed, towards the poles than within the tropics, where the waters lie under the direct influence of the lunar attraction. The connection between the periodical flux and reflux of the sea, and the positions of the moon, is too obvious to have escaped the attention of mankind in early ages, whose geographical situation brought oceanic phenomena under their notice, for the highest tides occur at the period of new and full moon, and the lowest when her phase is a semicircle in the heavens. Accordingly, the philosophers of antiquity remark upon the tides varying with the moon; and the elder Pliny, in a very striking passage in his Natural History, attributes them to lunar action, and proceeds to give a very accurate description of their leading circumstances. Keppler clearly indicated the principle of gravitation, and referred the tides to the attraction of the moon; an explanation which Galileo regretted, who ascribed them to the rotation of the earth combined with its revolution about the sun. Dr. Wallis, in 1666, in letters to Mr. Boyle, attributed the alternate rise and depression of the ocean to the consideration, that the common centre of gravity of the moon and the earth describes an orbit about the sun, while they revolve about this common centre. In answer to an objection which was made to him in the form of a query, how two bodies which have no tie can have one common centre of gravity? Wallis states: "It is harder to show how they have, than that they have it. As to the present case, how the earth and moon are connected, I will not undertake to show, nor is it necessary to my purpose; but that there is somewhat that does connect them, as much as what connects the loadstone and the iron which it draws, is past doubt to those who allow them to be carried about the sun, as one aggregate or body, whose parts keep a respective position to one another, like as Jupiter with his four satellites, and Saturn with his one. Some tie there is that makes those satellites attend their lords and move in a body, though we do not see that tie, nor hear the words of command." This language evinces great sagacity, but it is the language of surmise merely. To Newton the glory belongs of demonstrating the existence of a principle which had previously been a matter of philosophical speculation, of explaining its laws, and showing how the tides are produced by the influence of gravitation, the grand agent of the movements of the universe, and the

conservator of its harmony. The theory of the tides is exceedingly simple in its principles, but the most complex of all physical problems in its details.

If we suppose a zone of water to surround the globe in every part, and to be subject to no external influence, it is obvious, that the waters would uniformly maintain the same level, and be arranged upon its surface at an invariable depth, assuming the form of the circle E. It is easy, however, to perceive what effect, according to the law of gravity; the presence of a body like the moon will produce upon the zone of water, a fluid so susceptible of mobility, yielding to the slightest impressions. As a solid body draws toward it any other body, by a force which varies with its distance from the point attracted, then if m be the moon, the particles of water at a will feel more powerfully the effect of

m

E

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her attraction than those on either side, or than the particles of land at the bottom of the water, b, which are at a greater distance, and the result will be a bulging out of the watery zone, as in the diagram, immediately next the lunar globe. But while high water is thus produced at a, b, it is also produced at the same time in the opposite hemisphere at c, d, owing to the varying distance of c and d from m, while at e and ƒ no such diversity of attraction can be experienced, and the effect is, the reduction of the watery zone to the form of an ellipsoid, instead of that of a sphere. There is some difficulty to many minds in conceiving of the moon's influence raising the waters directly under her, and producing at the same time the like effect upon the waters on the opposite side of the earth. But a little consideration will at once remove it; for, obviously, the earth's centre, E, will be more powerfully drawn towards the moon than the waters at c d, and recede from them, producing the same effect as though they receded, or rose up from the centre of the earth. Now, as the waters cannot rise in one place without falling in another, they are depressed at e and ƒ when elevated at a and d; and as the moon is a month in making her circuit round the earth, there would be two elevations and depressions of the waters in the month in each place, if the earth remained stationary upon its axis. But by the diurnal rotation, the moon passes every day the superior and the inferior meridian of every part of the globe, producing daily two seasons of high and low water. The tidal swell of the ocean, or high water, at any given point, is thus occasioned by the point in question being in the direct line of the lunar attraction, or the moon being upon the meridian of its position; and the ebb of the tide, or low water at the point, is caused by the moon being upon a circle which cuts the meridian at right angles. But the facts of the case do not exactly tally with what we should expect from the theory, for the times of high and low water are not coincident with the fulfilment of the conditions stated. Thus, if the moon passes the meridian of Ushant, on the north-west coast of France, at twelve o'clock in the day, high water will not take place until about three hours afterwards, or three o'clock in the afternoon. The explanation of this is, that the impulse received by the waters of the sea in being brought by the rotation of the earth under the influence of the lunar attraction, continues to act for some time after the rotation has carried them away from it, so that there is still a tendency to ascend, as the effect of the impulse received, though the immediate action of the planet is not so strong as it was.

Besides the action of the moon in elevating the waters of the ocean, there is that of the sun to be considered. The solar attraction is not so influential a cause in the production of the tides as the lunar, because, though a much larger body, the sun is at far greater distance from the earth. Still the sun acts upon the ocean in precisely the same manner, though in a feebler degree, and his influence is sufficiently energetic to produce

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