The most extensive and best determined system of currents is that which has its source in the Indian Ocean, under the influence of the trade winds; and which, after doubling the Cape of Good Hope, inclines to the northward, along the western coast of Africa; then crosses the Atlantic near the Equator, and is lost in the Caribbean Sea; yet seems to be again revived in the current which issues from the Gulph of Mexico, by the Straits of Bahama, and flows rapidly in a north-easterly direction, by the bank of Newfoundland, towards the Azores.
Rennell informs us, that the Lagullas current, so called from the cape and bank of that name, is formed by the junction of two streams flowing from the Indian Ocean, the one from the channel of Mozambique, down the south-east coast of Africa, the other from the ocean at large.—The collective stream is from ninety to one hundred miles in breadth, and runs at the rate of from two and a half to more than four miles per hour. It is at length turned westward by the Lagullas bank, which rises from a sea of great depth, to within one hundred fathoms of the surface. It must therefore be inferred, says Rennell, that the current here is more than one hundred fathoms deep, otherwise the main body of it would pass across the bank, instead of being deflected eastward, so as to flow round the Cape of Good Hope. From this Cape it flows northward, along the western coast of Africa, taking the name of the South Atlantic current. It then enters the Bight or Bay of Benin, and is turned westward, partly by the form of the coast there, and partly, perhaps, by the Guinea current, which runs from the north into the same great bay. From the centre of this bay proceeds the Equatorial current, holding a westerly direction towards the Atlantic, which it traverses from the coast of Guinea to that of Brazil, flowing afterwards by the shores of Guiana to the West Indies. The breadth of this current varies from one hundred and sixty to four hundred and fifty geographical miles, and its velocity is from twenty five to seventy nine miles per day, the mean rate being about thirty miles. The length of its whole course is about four thousand miles. As it skirts the coast of Guiana, it is increased by the influx of the waters of the Amazon and Orinoco, and by their junction acquires accelerated velocity. After passing the island of Trinadad, it expands, and is almost lost in the Caribbean Sea; but there appears to be a general movement of that sea towards the Mexican Gulph, which discharges the most powerful of all currents through the Straits of Florida, where the waters run in the northern part with a velocity of five miles an hour, having a breadth of from thirty five to fifty miles. [25]
The temperature of the Gulph of Mexico is 86° in summer, or 6° higher than that of the ocean in the same parallel (25° N. lat.) and a large proportion of this warmth is retained, even where the stream reaches the 43° N. lat. After issuing from the Straits of Florida, the current runs in a northerly direction to Cape Hatteras, in North Carolina, about 35° N. lat. where it is more than seventy miles broad, and still moves at the same rate of seventy five miles per day. In about 40° N. lat. it is turned more towards the Atlantic by the extensive banks of Nantucket and St. George, which are from two hundred to three hundred feet beneath the surface of the sea; a clear proof that the current exceeds that depth. On arriving near the Azores, the stream widens and overflows, as it were forming a large expanse of warm water in the centre of the north Atlantic, over a space of two or three hundred miles from north to south, and having a temperature of from 8° to 10° Fahr. above the surrounding ocean. The whole area covered by the gulph water is estimated by Rennell at two thousand miles in length, and at a mean, three hundred and fifty miles in breadth, an area more extensive than that of the Mediterranean. The warm water has been sometimes known to reach the Bay of Biscay, still retaining five degrees of temperature above that of the adjoining ocean; and a branch of the gulf current drifts fruits, plants, and wood, the produce of America and the West Indies, to the shores of Ireland and the Hebrides. [26]
From the above statements, observes Mr. Lyell, we may understand why Rennell has characterised some of the principal currents as oceanic rivers, which he describes as being from fifty to two hundred and fifty miles in breadth, and having a rapidity exceeding that of the largest navigable rivers of the continent, and so deep as to be sometimes obstructed and occasionally turned aside by banks, the tops of which do not rise within forty, fifty, or even one hundred fathoms of the surface of the sea.
The ordinary velocity of the principal currents of the ocean is from one to three miles per hour; but when the boundary lands converge, large bodies of water are driven gradually into a narrow space, and then, wanting lateral room, are compelled to raise their level. Whenever this occurs, their velocity is much increased. The current which runs through the Race of Alderney, between the island of that name and the main land, has a velocity of about eight English miles an hour. The late Captain Hewett found that in the Pentland Firth the stream, in ordinary spring tides, runs ten miles and a half an hour, and about thirteen miles during violent storms. The greatest velocity of the tidal current through the “Shoots or New Passage,” in the Bristol Channel, is fourteen miles an hour; and Captain King observed, in his recent survey of the Straits of Magellan, that the tide ran at the same rate through the “First Narrows,” and about eight geographical miles an hour in other parts of those straits.
The course of currents on the British shores is ascertained to be as winding as that of ordinary rivers. Sometimes they run between banks of sand, which consist of matter thrown down at certain points where the velocity of the stream has been retarded, but it very frequently occurs, that as in a river one bank is made of low alluvial gravel, while the other is composed of some hardy and lofty rocks constantly undermined, so the current in its bends strikes here and there upon a coast which then forms one bank, whilst a shoal under water forms the other. If the coast be formed of solid materials, it yields slowly; so also if it be of great height, for in that case a large quantity of matter must be removed before the sea can penetrate to any distance.
Currents depend, like tides, on no temporary or accidental circumstances, but on the laws which preside over the motions of the heavenly bodies. The height to which tides rise, and the violence and velocity of the currents, depend in a great measure on the actual configuration of the land, the contour of a long line of continental or insular coast, the depth and breadth of channels, the peculiar form at the bottom of the seas—in a word, on a combination of circumstances which are made to vary continually by many igneous and aqueous causes, and among the rest, by the tides and currents themselves. Although these agents of decay and reproduction are local in reference to periods of short duration, such as those which history embraces, they are nevertheless universal, if we extend our views to a sufficient lapse of ages. [28]
Currents, observes Goldsmith, act their part in a smaller sphere, being generally greatest where the motions of the sea are least, namely, nearest the shores, and with the tides, produce the most rapid changes; their motion agitates the substances of which their bed is composed, and at the bottom of the sea, the greatest wonders are performed, for while the sea has been known to recede from some lands, so it has been found to encroach upon others, and probably these depredations on one part of the shore may account for the dereliction of another, for the current which rested upon some certain bank, having got an egress in some other place, it no longer presses upon its former bed, but pours all its stream into the new entrance, so that every inundation of the sea may be attended with some correspondent dereliction of another shore, where the sea meets no obstacles, it spreads with a gentle intumescence, till all the power is destroyed by wanting depth to aid the motion, but when the progress is checked in the midst by the prominence of rocks or the abrupt elevation of land, it dashes with all its force its depth against the obstacle, and forms, by its repeated violence, that abruptness of the shore which confines its impetuosity. Where the sea is extremely deep, and very much vexed with tempests, it is no small obstacle that can confine its rage; and for this reason, we see the boldest shores projected against the deepest waters, all less impediments having long before been surmounted and washed away. In places where the force of the sea is less violent, or its tides less rapid, the shores are generally seen to descend with a more gradual declivity. Upon these shores the sea seldom beats with any great violence, as a large wave has not depth sufficient to float it onwards, so that here only are to be seen gentle surges making towards the land, and lessening as they approach. As the sea, in the former description, is generally seen to present prospects of tumult and uproar, here it more usually exhibits a repose and tranquil beauty. Its waters which, when surveyed from the precipice, afforded a muddy greenish hue, arising from their depth and position to the eye, [29] when regarded from a shelving shore, were the colour of the sky, and seem rising to meet it. The deafening noise of the deep sea is here converted into gentle murmurs; instead of the waters dashing against the face of the rock, it advances and recedes, still going forward but with just force enough to push its weeds and shells, by insensible approaches, to the shore.
CHAPTER III.
THE GERMAN OCEAN—ITS GEOGRAPHICAL POSITION—ITS TIDES—DISASTROUS EFFECTS IN COMBINATION WITH GALES OF WIND FROM THE NORTH-WEST ON DIFFERENT PARTS OF THE COAST UNDER CONSIDERATION—EXAMPLES.