Physical Geography Of The Sea 1855/15
CHAPTER XV. — THE DRIFT OF THE SEA.
Object of Plate IX., § 528. — The Eastern Edge of the Gulf Stream sometimes visible, 529. — The Polar Drift about Cape Horn, 533. — How the Polar Waters drift into the South Atlantic, and force the Equatorial aside, 535. — How this is accomplished, 537. — A Harbor in a Bend of the Gulf Stream for Icebergs, 539. — Why Icebergs are not found in the North Pacific, 540. — The Womb of the Sea, 541. — Drift of warm Waters out of the Indian Ocean, 543. — A Suggestion from Lieutenant Jansen, of the Dutch Navy, 544. — A Current of warm Water sixteen hundred Miles wide, 545. — The Pulse of the Sea, 546. — How the Gulf Stream beats Time, 547. — The Circulation of the Sea likened to that of the Blood, 548. — THE FISH: Number of Vessels engaged in the Fisheries of the Sea, 551. — The Sperm Whale delights in warm Water, 552. — The Torrid Zone impassable to the Right Whale, 553.
526. THERE is a movement of the waters of the ocean which, though it be a translation, yet does not amount to what is known to the mariner as current, for our nautical instruments and the art of navigation have not been brought to that state of perfection which will enable navigators generally to detect as currents the flow to which I allude as drift.
527. If we imagine an object to be set adrift in the ocean at the equator, and if we suppose that it be of such a nature that it would obey only the influence of sea water, and not of the winds, this object, I imagine, would, in the course of time, find its way to the icy barriers about the poles, and again back among the tepid waters of the tropics. Such an object would illustrate the drift of the sea, and by its course would indicate the route which the surface waters of the sea follow in their general channels of circulation to and fro between the equator and the poles.
528. The object of Plate IX., therefore, is to illustrate, as far as the present state of my researches enables me to do, the circulation of the ocean, as influenced by heat and cold, and to indicate the routes by which the overheated waters of the torrid zone escape to cooler regions, on one hand, and on the other, the great channel ways through which the same waters, after having been deprived of this heat in the extra-tropical or polar regions, return again toward the equator; it being assumed that the drift or flow is from the poles when the temperature of the surface water is below, and from the equatorial regions when it is above that due the latitude. Therefore, in a mere diagram, as this plate is, the numerous eddies and local currents which are found at sea are disregarded.
529. Of all the currents in the sea, the Gulf Stream is the best defined; its limits, especially those of the left bank, are always well marked, and, as a rule, those of the right bank, as high as the parallel of the thirty-fifth degree of latitude, are quite distinct, being often visible to the eye. The Gulf Stream shifts its channel (§ 53), but nevertheless its banks are often very distinct. As I write these remarks, the abstract log of the ship Herculean (William M. Chamberlain), from Callao to Hampton Roads, in May, 1854, is received. On the eleventh of that month, being in latitude 33º 39’ north, longitude 74º 56’ west (about one hundred and thirty miles east of Cape Fear), he remarks: “Moderate breezes, smooth sea, and fine weather. At ten o’clock fifty minutes, entered into the southern (right) edge of the Stream, and in eight minutes the water rose six degrees; the edge of the stream was visible, as far as the eye could see, by the great rippling and large quantities of Gulf weed — more’weed’ than I ever saw before, and I have been many times along this route in the last twenty years.”
530. In this diagram, therefore, I have thought it useless to attempt a delineation of any of those currents, as the Rennell Current of the North Atlantic, the “connecting current” of the South, “Mentor’s Counter Drift,” Rossel’s Drift of the South Pacific, &c., which run now this way, now that, and which are frequently not felt by navigators at all.
531. In overhauling the log-books for data for this chart, I have followed vessels with the water thermometer to and fro across the seas, and taken the registrations of it exclusively for my guide, without regard to the reported set of the currents. When, in any latitude, the temperature of the water has appeared too high or too low for that latitude, the inference has been that such water was warmed or cooled, as the case may be, in other latitudes, and that it has been conveyed to the place where found through the great channels of circulation in the ocean. If too warm, it is supposed (§ 528) that it had its temperature raised in warmer latitudes, and therefore the channel in which it is found leads from the equatorial regions.
532. On the other hand, if the water be too cool for the latitude, then the inference is that it has lost its heat in colder climates, and therefore is found in channels which lead from the polar regions. The arrow-heads point to the direction in which the waters are supposed to flow. Their rate, according to the best information that I have obtained, is, at a mean, only about four knots a dayrather less than more.
533. Accordingly, therefore, as the immense volume of water in the Antarctic regions is cooled down, it commences to flow north. As indicated by the arrow-heads, it strikes against Cape Horn, and is divided by the continent, one portion going along the west coast as Humboldt’s Current (§ 267); the other, entering the South Atlantic, flows up into the Gulf of Guinea, on the coast of Africa. Now, as the waters of this polar flow approach the torrid zone, they grow warmer and warmer, and finally themselves become tropical in their temperature. They do not then, it may be supposed, stop their flow; on the contrary, they keep moving, for the very cause which brought them from the extra-tropical regions now operates to send them back. This cause is to be found in the difference of the specific gravity at the two places. If, for instance, these waters, when they commence their flow from the hyperborean regions, were at 30º, their specific gravity will correspond to that of sea water at 30º. But when they arrive in the Gulf of Guinea or the Bay of Panama, having risen by the way to 80º, or perhaps 85º, their specific gravity becomes such as is due sea water of this temperature; and, since fluids differing in specific gravity can no more balance each other on the same level than can unequal weights in opposite scales, this hot water must now return to restore that equilibrium which it has destroyed in the sea by rising from 30º to 80º or 85º.
534. Hence it will be perceived that these masses of water which are marked as cold are not always cold. They gradually pass into warm; for in traveling from the poles to the equator they partake of the temperature of the latitudes through which they flow, and grow warm.
535. Plate IX., therefore, is only introduced to give general ideas; nevertheless, it is very instructive. See how the influx of cold water into the South Atlantic appears to divide the warm water, and squeeze it out at the sides, along the coasts of South Africa and Brazil. So, too, in the North Indian Ocean, the cold water again compelling the warm to escape along the land at the sides, as well as occasionally in the middle.
536. In the North Atlantic and North Pacific, on the contrary, the warm water appears to divide the cold, and to squeeze it out along the land at the sides. The impression made by the cold current from Baffin’s Bay upon the Gulf Stream is strikingly beautiful.
537. Why is it that these polar and equatorial waters should appear now to divide and now to be divided? The Gulf Stream has revealed to us a fact in which the answer is involved. We learn from that stream that cold and warm sea waters are, in a measure (§ 53), like oil and vinegar; that is, there is among the particles of sea water at a high temperature, and among the particles of sea water at a low temperature, a peculiar molecular arrangement that is antagonistic to the free mixing up of cold and hot together. At any rate, that salt waters of different temperatures do not readily intermingle at sea is obvious.
538. Does not this same repugnance exist, at least in degree, between these bodies of cold and warm water of the plate? And if so, does not the phenomenon we are considering resolve itself into a question of masses? The volume of warm water in the North Atlantic is greater than the volume of cold water that meets and opposes it; consequently, the warm thrusts the cold aside, dividing and compelling it to go round. The same thing is repeated in the North Pacific, whereas the converse obtains in the South Atlantic. Here the great polar flow, after having been divided by the American Continent, enters the Atlantic, and filling up nearly the whole of the immense space between South America and Africa, seems to press the warm waters of the tropics aside, compelling them to drift along the coast on either hand.
539. Another feature of the sea expressed by this plate is a sort of reflection or recast of the shore-line in the temperature of the water. This feature is most striking in the North Pacific and Indian Ocean. The remarkable intrusion of the cool into the volume of warm waters to the southward of the Aleutian Islands, is not unlike that which the cool waters from Davis’s Straits make in the Atlantic upon the Gulf Stream. As I write, I receive from Captain N. B. Grant the abstract log of the American ship Lady Arbella, bound from Hamburg to New York, in May, 1854. In sailing through this “horse-shoe,” or bend in the Gulf Stream (§ 522), he passed, from daylight to noon, twenty-four large “bergs,” besides several small ones, “the whole ocean, as far as the eye could reach, being literally covered with them.” “I should,” he continues, “judge the average height of them above the surface of the sea to be about sixty feet; some five or six of them were at least twice that height, and, with their frozen peaks jutting up in the most fantastic shapes, presented a truly sublime spectacle.”
540. This “horse-shoe” of cold in the warm water of the North Pacific, though extending 5 degrees farther toward the south, can not be the harbor for such icebergs. The cradle of those of the Atlantic was perhaps in the Frozen Ocean, for they may have come thence through Baffin’s Bay. But in the Pacific there is no nursery for them. The water in Bering’s Strait is too shallow to let them pass from that ocean into the Pacific, and the climates of Russian America do not favor the formation of large bergs. But, though we do not find in the North Pacific the physical conditions which generate icebergs like those of the Atlantic, we find them as abundant with fogs. The line of separation between the warm and cold water assures us of these conditions.
541. What beautiful, grand, and benign ideas do we not see expressed in that immense body of warm waters which are gathered together in the middle of the Pacific and Indian Oceans! * It is the womb of the sea. In it, coral islands innumerable have been fashioned, and pearls formed in “great heaps;” there, multitudes of living things, countless in numbers and infinite in variety, are hourly conceived. With space enough to hold the four continents and to spare, its tepid waters teem with nascent organisms. They sometimes swarm so thickly there that they change the color of the sea, making it crimson, brown, black, or white, according to their own hues. These patches of colored water sometimes extend, especially in the Indian Ocean, as far as the eye can reach. The question, “What produces them?” is one that has elicited much discussion in sea-faring circles.
The Brussels Conference [1853] deemed them an object worthy of attention, and recommended special observations with regard to them. The discolorations of which I speak are no doubt caused by organisms of the sea, but whether wholly animal or wholly vegetable, or whether sometimes the one and sometimes the other, has not been satisfactorily ascertained. I have had specimens of the coloring matter sent to me from the pink-stained patches of the sea. They were animalculae well defined. Quantities of slimy, red coloring matter are, at certain seasons of the year, washed up along the shores of the Red Sea, which Dr. Ehrenberg, after an examination under the microscope, pronounces to be a very delicate kind of sea weed: from this matter that sea derives its name. So also the Yellow Sea. Along the coasts of China, yellowish colored spots are said not to be uncommon. I know of no examination of this coloring matter, however. In the Pacific Ocean I have often observed these discolorations of the sea. Red patches of water are most frequently met with, but I have also observed white or milky appearances, which at night I have known greatly to alarm navigators, they taking them for shoals.
542. These teeming waters bear off through their several channels the surplus heat of the tropics, and disperse it among the icebergs of the Antarctic. See the immense equatorial flow to the east of New Holland. It is bound for the icy barriers of that unknown sea, there to temper climates, grow cool, and return again, refreshing man and beast by the way, either as the Humboldt Current, or the ice-bearing current which enters the Atlantic around Cape Horn, and changes into warm again as it enters the Gulf of Guinea. It was owing to this great southern flow from the coral regions that Captain Ross was enabled to penetrate so much farther south than Captain Wilkes, on his voyage to the lations of identity with other provinces. The Red Sea and Persian Gulf are its offsets.” — From Professor Forbes’s Paper on the “Distribution of Marine Life.” Plate 31st, Johnston’s Physical Atlas, 2d ed.: Wm. Blackwood & Sons, Edinburgh and London, 1854.
Antarctic, and it is upon these waters that that sea is to be penetrated, if ever. The North Pacific, except in the narrow passage between Asia and America, is closed to the escape of these warm waters into the Arctic Ocean. The only outlet for them is to the south. They go down toward the Antarctic regions to dispense their heat and get cool; and the cold of the Antarctic, therefore, it may be inferred, is not so bitter as is the extreme cold of the Frozen Ocean of the north.
543. The warm flow to the south from the middle of the Indian Ocean is remarkable. Masters who return their abstract logs to me mention sea weed, which I’suppose to be brought down by this current, as far as 45º south. There it is generally, but not always, about 5 degrees warmer than the ocean along the same parallel on either side.
544. But the most unexpected discovery of all is that of the warm flow along the west coast of South Africa, its junction with the Lagullas current, called, higher up, the Mozambique, and then their starting off as one stream to the southward. The prevalent opinion used to be that the Lagullas current, which has its genesis in the Red Sea (§ 55), doubled the Cape of Good Hope, and then joined the great equatorial current of the Atlantic to feed the Gulf Stream. But my excellent friend Lieutenant Marin Jansen, of the Dutch Navy, suggested to me a few months ago that this was probably not the case. This induced a special investigation, and I found as he suggested, and as is represented on Plate IX. Captain N. B. Grant, in the admirably well-kept abstract log of his voyage from New York to Australia, found this current remarkably developed. He was astonished at the temperature of its waters, and did not know how to account for such a body of warm water in such a place. Being in longitude 14º east and latitude 39º south, he thus writes in his abstract log: “That there is a current setting to the eastward across the South Atlantic and Indian Oceans is, I believe, admitted by all navigators. The prevailing westerly winds seem to offer a sufficient reason for the existence of such a current, and the almost constant southwest swell would naturally give it a northerly direction. But why the water should be warmer here (38º 40’ south) than between the parallels of 35º and 37º south is a problem that, in my mind, admits not of so easy solution, especially if my suspicions are true in regard to the northerly set. I shall look with much interest for a description of the ’currents’ in this part of the ocean.”
545. In latitude 38º south, longitude 6º east, he found the water at 56º. His course thence was a little to the south of east, to the meridian of 41º east, at its intersection with the parallel of 42º south. Here his water thermometer stood at 50º, but between these two places it ranged at 60º and upward, being as high on the parallel of 39º as 73º. Here, therefore, was a stream — a mighty “river in the ocean” — one thousand six hundred miles across from east to west, having water in the middle of it 23º higher than at the sides. This is truly a Gulf Stream contrast. What an immense escape of heat from the Indian Ocean, and what an influx of warm water into the frozen regions of the south! This stream is not always as broad nor as warm as Captain Grant found it. At its mean stage it conforms more nearly to the limits assigned it in the diagram (Plate IX.).
546. We have, in the volume of heated water reported by Captain Grant, who is a close and accurate observer, an illustration of the sort of spasmodic efforts — the heaves and throes — which the sea, in the performance of its ceaseless task, has sometimes to make. By some means, the equilibrium of its waters, at the time of Captain Grant’s passage, December — the southern summer1852, appears to have been disturbed to an unusual extent; hence this mighty rush of overheated waters from the great inter-tropical caldron of the two oceans down toward the south. Instances of commotion in the sea at uncertain intervals — the making, as it were, of efforts by fits and starts to keep up to time in the performance of its manifold offices — are not unfrequent, nor are they inaptly likened to spasms. The sudden disruption of the ice which arctic voyagers tell of, the immense bergs which occasionally appear in groups near certain latitudes, the variable character of all the currents of the sea—now fast, now slow, now running this way, then that — may be taken as so many signs of the tremendous throes which occur in the bosom of the ocean. Sometimes the sea recedes from the shore, as if to gather strength for a great rush against its barriers, as it did when it fled back to join with the earthquake and overwhelm Callao in 1746, and again Lisbon nine years afterward. The tide-rips in mid ocean, the waves dashing against the shore, the ebb and flow of the tides, may be regarded, in some sense, as the throbbings of the great sea pulse.
547. The motions of the Gulf Stream (§ 53), beating time for the ocean and telling the seasons for the whales, also suggest the idea of a pulse in the sea, which may assist us in explaining some of its phenomena. At one beat there is a rush of warm water from the equator toward the poles, at the next beat a flow from the poles toward the equator. This sort of pulsation is heard also in the howlings of the storm and the whistling of the wind; the needle trembles unceasingly to it, and tells us of magnetic storms of great violence, which at times extend over large portions of the earth’s surface; and when we come to consult the records of those exquisitely sensitive anemometers, which the science and ingenuity of the age have placed at the service of philosophers, we find there that the pulse of the atmosphere is never still: in what appears to us the most perfect calm, the recording pens are moving to the pulses of the air.
548. Now if we may be permitted to apply to the Gulf Stream and to the warm flows of water from the Indian Ocean an idea suggested by the functions of the human heart in the circulation of the blood, we perceive how these pulsations of the great seaheart may perhaps assist in giving circulation to its waters through the immense system of aqueous veins and arteries that run between the equatorial and polar regions. The waters of the Gulf Stream, moving together in a body (§ 1) through such an extent of ocean, and being almost impenetrable to the cold waters on either side which are, indeed, the banks of this mighty river — may be compared to a wedge-shaped cushion placed between a wall of waters on the right and a wall of waters on the left. If now we imagine the equilibrium of the sea to be disturbed by the heating or cooling of its waters to the right or the left of this stream, or the freezing or thawing of them in any part, or if we imagine the disturbance to take place by the action of any of those agencies which give rise to the motions which we have called the pulsations of the sea, we may conceive how it might be possible for them to force the wall of waters on the left to press this cushion down toward the south, and then again for the wall on the right to press it back again to the north, as (§ 54) we have seen that it is. Now the Gulf Stream, with its head in the Straits of Florida, and its tail in the midst of the ocean (§ 492), is wedge-shaped; its waters cling together (§ 28), and are pushed to and fro, squeezed, if you please — by a pressure (§ 53), now from the right, then from the left, so as to work the whole wedge along between the cold liquid walls which contain it. May not the velocity of this stream, therefore, be in some sort the result of this working and twisting, this peristaltic force in the sea?
549. In carrying out the views suggested by the idea of pulsations in the sea, and their effects in giving dynamical force to the circulation of its waters, attention may be called to the two lobes of polar waters that stretch up fr6m the south into the Indian Ocean, and which are separated by a feeble flow of tropical waters. Icebergs are sometimes met with in these polar waters as high up as the parallel of the fortieth degree of latitude. Now, considering that this tropical flow in mid ocean is not constant that many navigators cross the path assigned to it in the plate without finding their thermometer to indicate any increase of heat in the sea; and considering, therefore, that any unusual flow of polar waters, any sudden and extensive disruption of the ice there, sufficient to cause a rush of waters thence, would have the effect of closing for the time this mid-ocean flow of tropical waters, we are entitled to infer that there ie a sort of conflict, at times, going on in this ocean between its polar and equatorial flows of water. For instance, a rush of waters takes place from the poles toward the equator. The two lobes close, cut off the equatorial flow between them, and crowd the Indian Ocean with polar waters. They press out the overheated waters; hence the great equatorial flow encountered by Captain Grant. Thus this opening between the cold-water lobes appears to hold to the chambers of the Indian Ocean, with their heated waters, the relations which the valves and the ventricles of the human heart hold to the circulation of the blood. The closing of these lobes at certain times prevents regurgitation of the warm waters, and compels them to pass through their appointed channels
.
550. From this point of view, how many new beauties do not now begin to present themselves in the machinery of the ocean! its great heart not only beating time to the seasons, but palpitating also to the winds and the rains, to the cloud and the sunshine, to-day and night (§ 507). Few persons have ever taken the trouble to compute how much the fall of a single inch of rain over an extensive region in the sea, or how much the change even of two or three degrees of temperature over a few thousand square miles of its surface, tends to disturb its equilibrium, and consequently to cause an aqueous palpitation that is felt from the equator to the poles Let us illustrate by an example: The surface of the Atlantic Ocean covers an area of about twenty-five millions of square miles. Now, let us take one fifth of this area, and suppose a fall of rain one inch deep to take place over it. This rain would weigh three hundred and sixty thousand millions of tons; and the salt which, as water, it held in solution in the sea, and which, when that water was taken up as vapor, was left behind to disturb equilibrium, weighed sixteen millions more of tons, or nearly twice as much as all the ships in the world could carry at a cargo each. It might fall in an hour, or it might fall in a day; but, occupy what time it might in falling, this rain is calculated to exert so much force — which is inconceivably great — in disturbing the equilibrium of the ocean. If all the water discharged by the Mississippi River during the year were taken up in one mighty measure, and cast into the ocean at one effort, it would not make a greater disturbance in the equilibrium of the sea than would the fall of rain supposed. Now this is for but one fifth of the Atlantic, and the area of the Atlantic is about one fifth of the seaarea of the world; and the estimated fall of rain was but one inch, whereas the average for the year is (§ 144) sixty inches, but we will assume it for the sea to be no more than thirty inches. In the aggregate, and on an average, then, such a disturbance in the equilibrium of the whole ocean as is here supposed occurs seven hundred and fifty times a year, or at the rate of once in twelve hours. Moreover, when it is recollected that these rains take place now here, now there; that the vapor of which they were formed was taken up at still other places, we shall be enabled to appreciate the better the force and the effect of these pulsations in the sea.
551. Between the hottest hour of the day and the coldest hour of the night there is frequently a change of four degrees in the temperature of the sea.*
- Vide Admiral Smyth’s Memoir of the Mediterranean, p. 125.
Let us, therefore, to appreciate the throbbings of the sea-heart, which take place in consequence of the diurnal changes in its temperature, call in the sunshine, the cloud without rain, with day and night, and their heating and radiating processes. And to make the case as strong as to be true to nature we may, let us again select one fifth of the Atlantic Ocean for the scene of operation. The day over it is clear, and the sun pours down his rays with their greatest intensity, and raises the temperature two degrees. At night the clouds interpose, and prevent radiation from this fifth, whereas the remaining four fifths, which are supposed to have been screened by clouds, so as to cut off the heat from the sun during the day, are now looking up to the stars in a cloudless sky, and serve to lower the temperature of the surface waters, by radiation, two degrees. Here, then, is a difference of four degrees, which we will suppose extends only ten feet below the surface. The total and absolute change made in such a mass of sea water by altering its temperature four degrees is equivalent to a change in its volume of three hundred and ninety thousand millions of cubic feet.
552. Do not the clouds, night and day, now present themselves to us in a new light? They are cogs, and rachets, and wheels in that grand and exquisite machinery which governs the sea, and which, amid all the jarrings of the elements, preserves in harmony the exquisite adaptations of the ocean.
553. It seems to be a physical law, that cold-water fish are more edible than those of warm water. Bearing this fact in mind as we study Plate IX., we see at a glance the places which are most favored with good fish-markets. Both shores of North America, the east coast of China, with the west coasts of Europe and South America, are all washed by cold waters, and therefore we may infer that their markets abound with the most excellent fish. The fisheries of Newfoundland and New England, over which nations have wrangled for centuries, are in the cold water from Davis’s Strait. The fisheries of Japan and Eastern China, which almost, if not quite, rival these, are situated also in the cold water. Neither India, nor the east coasts of Africa and South America, where the warm waters are, are celebrated for their fish.
554. THREE THOUSAND AMERICAN VESSELS, it is said, are engaged in the fisheries. If to these we add the Dutch, French, and English, we shall have a grand total, perhaps, of not less than six or eight thousand, of all sizes and flags, engaged in this one pursuit. Of all the industrial pursuits of the sea, however, the whale fishery is the most valuable. Wherefore, in treating of the physical geography of the sea, a map for the whales would be useful.
555. The sperm whale likes warm-water. The right whale delights in cold water. An immense number of log-books of whalers have been discussed at the National Observatory, with the view of detecting the parts of the ocean in which the whales are to be found at the different seasons of the year. Charts showing the result have been published; they form a part of the series of Maury’s Wind and Current Charts.
556. In the course of these investigations, the discovery was made that the torrid zone is to the right whale as a sea of fire, through which he can not pass; that the right whale of the northern hemisphere and that of the southern are two different animals; and that the sperm whale has never been known to double the Cape of Good Hope — he doubles Cape Horn.
557. With these remarks, and the explanations given on Plate IX., the parts of the ocean to which the right whale most resorts, and the parts in which the sperm are found, may be seen at a glance.