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parts of the mainland, where the shores are low. This is particularly observable along the south coast of Lincolnshire and in the Bay of Lancaster. The latter estuary becomes

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at low water an expanse of sand, across which there is a road, the channel of the river Lune presenting the chief obstacle to the passage dry-shod. In other narrow seas, however, there is scarcely any perceptible tide at all, owing to the conditions under which they communicate with the general sweep of the deep. The distribution and configuration of the land, together with the influence of the winds, greatly involve the problem of the tides, and render it one of the most difficult in the whole range of physics. We shall merely mention a few of its leading features.

The diurnal rotation of the earth being from west to east, the apparent course of the moon is from east to west, and consequently in an ocean of considerable extent in that direction, a tidal wave is formed following the lunar course. The only great belt of water which answers to this condition is the Pacific Ocean, for the general direction of the Atlantic is from north to south, its breadth from east to west being comparatively small. It is the southern part of the Pacific, including the Indian Ocean, that exhibits the greatest extent of surface in the direction of the moon's path; and accordingly a very regular tide-wave is there produced, the general course of which is from east to west, but running towards the tropics, the region of the direct line of the lunar attraction. From the mouth of the Red Sea to the Cape of Good Hope the whole east coast of Africa is reached about the same time by the summit of a single tide-wave, causing the hours of high water at its different stations to be coincident. It is otherwise with the tides of the Atlantic, along the coasts of which the hours of high water are successively later as we travel northward, that being the general direction pursued by its tidal waves, which the

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annexed diagram will explain. Suppose A B a belt of water, and CD another narrower belt opening into it, the other lines being ridges of waves passing along AB. It is obvious that when a wave has arrived at c, part of it will run along the belt C D to find its

level, and there will be a succession of waves in CD numerically the same as in A B, but pursuing a different direction. The relation between the Atlantic and the Pacific

and Great Southern Oceans, is analagous to that of the upper to the lower belt; and upon the tide-waves of the Pacific reaching the mouth of the Atlantie, they move along its basin, with some local exceptions, in the line of its direction, from south to north, across from the African to the American coast. The subjoined plate will convey a generally accurate idea of the direction of the tide-waves of the south Pacific and Atlantic Oceans. The tide-waves of the Atlantic thus advancing northwards cause the time of high

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water to be successively later at the different ports on the west coasts of Africa and Europe, and the east coast of North and South America, except from Rio Janeiro to the Falkland Islands, where the advance of the wavesummits is from east to west. The tide-wave, which is at the Cape of Good Hope at a certain hour, is at the British Isles about fifteen hours afterwards, where, interrupted in its progress, it divides into three branches : one branch flows eastward up the English Channel, passing through the Straits of Dover, and is off the mouth of the Thames at the Nore in about eight hours from the time it entered the Channel; a second branch advances through St. George's Channel into the

Irish Sea; while the third and principal branch of the same wave proceeds along the west coasts of Ireland and Scotland, rounds the northern extremity of the latter, flows slowly down the North Sea, and meets the first branch at the mouth of the Thames, having taken about twenty hours to compass the distance. The annexed sketch represents the course of the latter around Great Britain. Supposing the moon to have passed the meridian of Brest, at the north-western extremity of the French coast, at twelve o'clock in the day, it is high water there soon after three o'clock in the afternoon, the ridge of the tide-wave stretching out into the Atlantic in a north-west direction from Ushant, falling a little to the south of Cape Clear, in Ireland. By six o'clock the wave has gained the north coast of Ireland, the ridge maintaining the same direction; and three hours afterwards it has reached the Orkneys, the ridge bearing due north. By twelve o'clock the wave has entered the German Ocean, the ridge-line extending eastward from the Scottish coast to the south point of Norway: and in about eleven hours afterwards it has flowed down the eastern extent of England to the entrance of the Thames. It has been observed, that while in open seas, as around the islands of the Pacific, and St. Helena in the South Atlantic, the tides have only an elevation of one or two feet, in many narrow channels, like the English and St. George's, they rise to a far greater height, owing to the confined space into which the water is crowded. The Bristol Channel opens widely to the south-west, where it receives the tide-wave of the Atlantic, but it is very contracted at its upper end, and the water is heaped up in consequence, much above the level to which it otherwise would rise, attaining to an elevation of forty

and fifty feet. At St. Maloes, on the north coast of France, the tide attains the height of fifty feet, and even sixty in the Bay of Fundy in the United States, where its rise is at the same time so rapid, that cattle feeding on the shores have been surrounded and swept off by it. On the contrary, in narrow seas situated like the Mediterranean and the Baltic, there is scarcely any tide whatever; while in Hudson's and Baffin's Bays and the Red Sea, the influence of the tidal current is strongly felt. A slight inspection of their geographical position will explain the reason of this. The mouths of the latter oceanic estuaries open in the direction of its advancing tide-waves, while the entrances of the Mediterranean and the Baltic are at acute angles with reference to them, and being turned from the main direction of the Atlantic tide, but a small portion of its waters passes through them, not sufficient to produce any marked alteration in the level of those seas. In addition to this, their dimensions are too limited to allow of the moon's action being unequally exerted upon them, were they in the direct line of her attraction, so that the equilibrium of the surface is not greatly disturbed. The highest tidal rise in the Mediterranean occurs to the eastward of Sicily, where a wave is raised which flows up the Adriatic, elevating the waters of that close sea nearly four feet at new and full moon, and half that height at neap tides, alternately covering and laying bare the bottom of the Venetian lagoons. At Antium, Mr. Trevalyan found, by a series of observations, regular tides in the summer of 1836, rising there to fourteen inches; and a tide was noticed by M. D'Angos, at Toulon, on the coast of France, where the sea rose a foot about three hours and a half after the moon passed the meridian. In the east of the Mediterranean also the tides are felt, and slightly so in the Grecian Archipelago, where a gentle rise of the waters in the port of Egina and the gulf of Corinth has been observed. But the general level of the Mediterranean fluctuates only a few inches.

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Port of Egina.

Hence the soldiers of Alexander were alarmed on beholding the high tide at the mouth of the Indus, and the troops of Cæsar were filled with consternation on witnessing a similar spectacle upon our own coast, their previous knowledge of oceanic phenomena having been confined to the seas of Italy and Greece.

Winds have a powerful influence upon the tidal currents, especially in narrow seas and river channels, keeping them back when blowing from an opposite quarter, and accelerating their flow when pursuing the same direction; so that the tide will rise above its usual level, or fall below it, according as a strong wind co-operates with it or not. An experiment made by Smeaton shows, that in a canal four miles long, the level of the

water at one end was four inches higher than at the other, owing to the action of the wind upon it. Major Rennell states, that a piece of water, ten miles broad, and generally only three feet deep, has, by a strong wind, had its waters driven to one side, and so sustained as to become six feet deep, while the windward side was laid dry. In rivers also, when they are swollen by the rains, the increased force of their current materially affects the advance of the tide; and when their waters are low the tidal wave will proceed along their channel to a greater distance, and hold up in them during a longer interval than under opposite circumstances. The astronomer Flamstead, who had frequent occasion to go to Greenwich by water, turned his attention to the subject of the tide in the Thames; and he remarks: "When, by reason of great droughts in summer, or extreme frosts in winter, the springs are low, and the fresh waters less than usual, the tides may hold up longer than the times noted in the table; also when strong northwesterly or northerly winds blow, which bring in an extraordinary flood from the northern seas, and keep it up longer than other times; so, on the contrary, when the winds blow hard on the opposite points of the compass, or when we have much rain and great freshes, the tides hold not out so long as the times shown in the table, the freshes overpowering and checking them sooner; yet have I never found that the difference between the calculated and observed high waters have much exceeded half an hour-most commonly they are scarce half so much." There are some singular circumstances connected with the tides of rivers which have thus been noticed by a recent writer:-"They are not of equal duration, as is the case in most parts of the sea; but the ebb tides frequently last twice as long as the flowing tides. At Rotterdam the tide flows for about four hours and five minutes, but the ebb lasts seven hours and fifty-five minutes. The Meerwede at Dortrecht flows against the current of the river for three hours and fifty-one minutes, and with it eight hours and nine minutes. This difference is easily explained when the force of the river current is taken into account. The same circumstance explains the difference in the velocity of the ebbing and flowing tide. Between the North Sea and Hamburgh the flowing tide takes five minutes to run up a mile, but the ebb tide performs the same distance in less than four minutes. But it is difficult to explain the wellestablished fact that the tides advance much farther into a river than might be expected. When the tide at the mouth of a river rises four feet, we might suppose that it would advance only to such a point in the river where the surface is four feet above the sea; but it has been ascertained that it advances farther. It seems that the volume of water which is carried up by the tide is pushed onwards by the mass behind it, and carried to a greater distance than the inclination of the river bed would seem to allow. It has also been observed, that during the flowing of the tide the surface of the water in the river presents a somewhat convex form, the water along the banks being a little lower than in the middle of the river, and that during the ebb the contrary takes place. The flowing tide raises the water from below, and thus sooner affects the main body of the river, where it has more room to operate than the water near the margin. In accordance with this explanation, it is observed that the flowing tide is perceptible in the middle, while it is still ebbing along the banks, and that vessels which are at anchor near the banks are turned round before the water on the surface of the river near the banks begins to flow upward."

The change produced in the aspect of rivers by the advance of the tide is of the most striking description, and confers important advantages upon the towns seated along their banks, rendering them essentially maritime, though at a considerable distance from the sea. The Avon at Bristol supplies a remarkable example of the alteration, and of the commercial benefits resulting from it. Its natural character at St. Vincent's rocks is that of a shallow brawling stream, scarcely navigable by the smallest craft; but upon the flow

of the tide it receives an accession of near forty feet to its depth of water, which enables the largest West Indiamen and steamers to communicate directly with the city. The change brings not only a supply of water adequate for navigation, but an alternate current every twelve hours, which is just as useful as having a fair wind up and down the river, the regular occurrence of which being certain may immediately be turned to account by previous preparation. The same phenomenon is exhibited on the Solway Firth, the sands of which are so dry at low water that travellers on horseback can cross them, while the tide returns so rapidly as to render this a somewhat hazardous

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experiment. Besides the advantage to navigation, to a great metropolis like London, the tidal flux and reflux of the Thames is of immense consequence to physical health. The river may be said to be thoroughly washed out twice a day, by which the drainage of a million and a half of people is carried off to the sea, and fresh water and air returned back-a process which has been aptly compared to a system of lungs, furnished to the city for the purpose of securing a healthy vital action.

A third movement to which the ocean is subject is known by the name of currents, which involve not merely the surface stratum of the sea, but probably extend to the bottom, where they prevail, and constitute great oceanic highways. The effect of currents was perceived long before any thing was known of their direction and velocity; and Columbus was strengthened in his belief, that land might be reached across the Atlantic westward, by substances which had been drifted from that quarter. A pilot in the service of the King of Portugal, Martin Vicenti, assured him, that after sailing four hundred and fifty leagues to the west of Cape St. Vincent he had taken a piece of carved wood from the sea, evidently not laboured with an iron instrument, which must have floated from some unknown land in a westerly direction. Columbus was also informed by his brother-in-law, Pedro Correo, that he had seen a similar piece of wood off Porto Santo, a small island to the north-east of Madeira, which seemed to have come from the same region; and it was commonly reported that reeds of an immense size had floated to those islands from the west, which the great discoverer fancied were identical with

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