Physical Geography of the Sea and its Meteorology/Chapter 18

by Matthew Fontaine Maury

CHAPTER XVIII.

§ 740-772. TIDE-RIPS AND THE SEA DRIFT.

740. The glories of the sea, and the destiny of the nautilus.—We never tire of the sea; like the atmosphere, it is a laboratory; in which wonders by processes the most exquisite are continually going on. Its flora and its fauna, its weaves and its tides, its currents and its salts, all in themselves afford profitable subjects of study and charming themes for thought. But as interesting as they are individually, and as marvellous too, they are not half so marvellous, nor nearly so wonderful as the offices which, with their aid, the sea performs in the physical economy of our planet. In this aspect the sea, with its insects, its salts, and its vapours, is a machine of the most beautiful construction. Its powers are vast, multitudinous, and varied. It is so stable and true in its work that nothing can throw it out of gearing, and yet its compensations are so delicate that the task of preserving them is assigned to the tiniest of its inhabitants, and to agents apparently the most subtle and fickle. They preserve its harmonies and make its adjustments, in beauty and sublimity of effect, to vie with the glories of the heavens. Take the tiny little nautilus, one of the oldest families in the sea, for example. Where, inquires M. Lucien Dubois, do they go in such fleets with their purple sails so nicely trimmed to the breeze? Who pilots them, and what master hand holds the helm? What compass, and of whose workmanship, is that which guides these delicate and graceful little argonauts from sea to sea? Arriving off the "Stormy Capes," the flotilla is separated, one division holding its way for the Pacific, the other hauling up for the Atlantic, each bound on its high and secret mission. They build, equip, and repair as they go; the fleet is imperishable, but individual life in it is ephemeral. They die, these tiny "men o' war," one after another, but the same watchful Providence that cared for them while living, now provides for their burial being dead. The inanimate shell, drawn to distant seas by under currents, descends like autumnal leaves from depth to depth by an insensible fall. In future times the seaman's sounding-rod may reach the bottom on which it has fallen, and thus reveal to man the secret paths of the sea,—or when the geological clock next strikes the hour, the same little shell may, by some throe of nature, be brought up to the surface, and spread out in its marl bed, to fertilize and make fruitful unknown lands.

741. Drift described.—There is a movement of the waters of the ocean which, though it be a translation, yet it 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. If an object be set afloat in the ocean, as at the equator, it would, in the course of time, even though it should not be caught up by any of the known currents, 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.

742. Plate IX.—The object of Plate IX., therefore, is to illustrate, as far as the present state of my researches enable me to do, the circulation of the ocean as influenced by heat and cold, and to indicate, on one hand, the routes by which the overheated waters of the torrid zone escape to cooler regions, and to point out, 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 towards 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. 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 (§ 124), but nevertheless its banks are often very distinct. Ships, in crossing! the edges of it, can sometimes know it by the colour of the water; at other times they find, as they pass along, the temperature of the water to change 8° or 10° in the course of as many minutes; as an example of this, I quote from the abstract log of the "Herculean," in which Captain William M. Chamberlain, being in latitude 33° 39' north, longitude 74° 56' west (about one hundred and thirty miles east of Cape Fear), 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; tho 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." In this diagram, therefore, I have thought it useless to attempt a delineation of any of those currents, as the Kennell Current of the North Atlantic, the "connecting current" of the South, "Mentor's Counter Drift," "Rossel's Drift of the South Pacific," etc., which run now this way, now that, and which are frequently not felt by navigators at all. 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 the 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 oceanic circulation. If too warm, it is supposed that it had its temperature raised in warmer latitudes, and therefore the channel in which it is found leads from the equatorial regions. 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 day—rather less than more. 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 (§ 398); 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 30°, their specific gravity will correspond to that of sea water at 30°. But when they arrive in the Gulf of Guinea or the Bay of Panama, having risen by the way to 80°, or perhaps 85°, their specific gravity becomes such as is due to 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 the opposite scales of a true balance, this hot water must now return to restore that equilibrium which it has destroyed in the sea by rising from 30° to 80° or 85°. 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 travelling from the poles to the equator they partake of the temperature of the latitudes through which they flow, and grow warm. 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. 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.

743. The great bend in the Gulf Stream.—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 Oceans. The remarkable intrusion of the cool into the volume of warm waters to the southward of the Aleutian Islands is not unlike that (§ 731) which the cool waters from Davis' Straits make in the Atlantic upon the Gulf Stream. In sailing through this " horse-shoe," or bend in the Gulf Stream (§ 731), Captain N. B. Grant, of the American ship "Lady Arbella," bound from Hamburgh to New York, in May, 1854, 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 continued, "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."

744. The horse-shoe in the Japan current,—The "horse-shoe" of cold in the warm water of the North Pacific, though extending 5 degrees farther towards the south, cannot be the harbour 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 Behring's Strait is too shallow to let them pass from that ocean into the Pacific, and the climates of Russian America do not favour 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.

745. The animalculæ of the sea.—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, the tepid waters of this part of the ocean teem with nascent organisms.^[1] They sometimes swarm so thickly there that they change the colour of the sea, making it crimson, brown, black, or white, according to their own hues. These patches of coloured 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 seafaring circles. The Brussels Conference deemed them an object worthy of attention, and recommended special observations with regard to them.

746. Coloured patches.—Capt. W. E. Kingman, of the American clipper ship the " Shooting' Star," reports in his abstract log a remarkable white patch, which he encountered in lat. 8° 46′ S., long. 105° 30′ E., and which, in a letter to me, he thus describes: "Thursday, July 27, 1854. At 7h. 45m. p.m., my attention was called to notice the colour of the water, which was rapidly growing white. Knowing that we were in a much frequented part of the ocean, and having never heard of such an appearance being observed before in this vicinity, I could not account for it. I immediately hove the ship to and cast the lead; had no bottom at 60 fathoms. I then kept on our course, tried the water by thermometer, and found it to be 78½°, the same as at 8 a.m. We filled a tub, containing some sixty gallons, with the water, and found that it was filled with small luminous particles, which, when stirred, presented a most remarkable appearance. The whole tub seemed to be active with worms and insects, and looked like a grand display of rockets and serpents seen at a great distance in a dark night; some of the serpents appeared to be six inches in length, and very luminous. We caught, and could feel them in our hands, and they would emit light until brought within a few feet of a lamp, when, upon looking to see what we had, behold, nothing was visible; but, by the aid of a sextant's magnifier, we could plainly see a jelly-like substance without colour. At last a specimen was obtained of about two inches in length, and plainly visible to the naked eye; it was about the size of a large hair, and tapered at the ends. By bringing one end within about one-fourth of an inch of a lighted lamp, the flame was attracted towards it, and burned with a red light; the substance crisped in burning something like a hair, or appeared of a red heat before being consumed. In a glass of the water there were several small round substances (say 1/16th of an inch in diameter), which had the power of expanding to more than twice their ordinary size, and then contracting again; when expanded, the outer rim appeared like a circular saw, only that the teeth pointed towards the centre. This patch of white water was about 23 miles in length, north and south, divided near its centre by an irregular strip of dark water half a mile wide; its east and west extent I can say nothing about. I have seen what is called white water in about all the known oceans and seas in the world, but nothing that would compare with this in extent or whiteness. Although we were going at the rate of nine knots, the ship made no noise either at the bow or stern. The whole appearance of the ocean was like a plain covered with snow. There was scarce a cloud in the heavens, yet the sky, for about ten degrees above the horizon, appeared as black as if a storm was raging; the stars of the first magnitude shone with a feeble light, and the 'Milky Way' of the heavens was almost entirely eclipsed by that through which we were sailing. The scene was one of awful grandeur; the sea having turned to phosphorus, and the heavens being hung in blackness, and the stars going out, seemed to indicate that all nature was preparing for that last grand conflagration which we are taught to believe is to annihilate this material world. After passing through the patch, we noticed that the sky, for four or five degrees above the horizon, was considerably illuminated, something like a faint aurora borealis. We soon passed out of sight of the whole concern and had a fine night, without any conflagration (except of midnight oil in trying to find out what was in the water). I send you this because I believe you request your corps of 'one thousand assistants' to furnish you with all such items, and I trust it will be acceptable. But as to its furnishing you with much, if any, information relative to the insects or animals that inhabit the mighty deep, time will only tell; I cannot think it will."

747. Whence the Red Sea derives its name.—These discolourations 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 colouring matter sent to me from the pink-stained patches of the sea. They were animalculæ well defined. The tints which have given to the Red Sea its name may perhaps be in some measure due to agencies similar to those which, in the salt-makers' ponds, give a reddish cast (§ 71) to the brine just before it reaches that point of concentration when crystallization is to commence. Some microscopists maintain that this tinge is imparted by the shells and other remains of infusoria which have perished in the glowing saltness of the water. The Red Sea may be regarded, in a certain light, as the scene of natural salt-works on a grand scale. The process is by solar evaporation. No rains interfere, for that sea (§ 376) is in a riverless district, and the evaporation goes on unceasingly, day and night, the year round. The shores are lined with incrustations of salt, and the same causes which tinge with red (§71) the brine in the vats of the salt-makers probably impart a like hue to the arms and ponds along the shore of this sea. Quantities, also, of slimy, red colouring 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 derived its name. So also the Yellow Sea. Along the coasts of China, yellowish-coloured spots are said not to be uncommon. I know of no examination of this colouring matter, however. In the Pacific Ocean I have often observed these discolourations 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 by their being taken for shoals.

748. The escape of warm waters from the Pacific.—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 Australia, and which I have called the Polynesian Drift. 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 Antarctic. 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 towards 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.

749. Ditto from the Indian Ocean.—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 45° south. There, it is generally, but not always, about 5 degrees warmer than the ocean along the same parallel on either side.

750. A wide current.—But the most unexpected discovery of all is that of the warm flow along the west coast of South Africa, its junction with the Lagulhas 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 Lagulhas current, which has its genesis in the Red Sea (§ 390), 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 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 14° east, and latitude 39° 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 south-west swell would naturally give it a northerly direction. But why the water should be warmer here (38° 40′ south) than between the parallels of 35° and 37° 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." In latitude 38° south, longitude 6° east, he found the water at 56°. His course thence was a little to the south of east, to the meridian of 41° east, at its intersection with the parallel of 42° south. Here his water thermometer stood at 50°, but between these two places it ranged at 60° and upward, being as high on the parallel of 39° as 73°. 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 23° 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.).

751. Commotions in the sea.—Instances of commotions in the sea at uncertain intervals are not unfrequent. There are some remarkable disturbances of the sort which I have not been able wholly to account for. Near the equator, and especially on this side of it in the Atlantic, mention is made, in the "abstract log," by almost every observer that passes that way, of "tide-rips," which are a commotion in the water not unlike that produced by a conflict of tides or of other powerful currents. These "tide-rips" sometimes move along with a roaring noise, like rifts over rocks in rivers, and the inexperienced navigator always expects to find his vessel drifted by them a long way out of her course; but when he comes to cast up his reckoning the next day at noon, he remarks with surprise that no current has been felt.

752. Humbolt's description of tide-rips.—Tide-rips present their most imposing aspect in the equatorial regions. Humboldt met some in 34° N., and thus describes them: "When the sea is perfectly calm, there appear on its surface narrow belts, like small rivulets, and in which the water runs with a noise very perceptible to the ear of an experienced pilot. On the 15th of June, in about 34° 36′ N., we found ourselves in the midst of a great number of these belts of currents; we were able to determine their direction by the compass. Some were flowing to the N.E.; others E.N.E., although the general motion of the ocean, indicated by a comparison of the log and the longitude by meter, continued towards the S.E." It is very common to see a mass of motionless water crossed by ridges of water which run in different directions. This phenomena may be observed every day on the surface of our lakes; but it is more rare to find partial movements impressed by local causes on small portions of water in the midst of an oceanic river occupying an immense space, and moving in a constant direction, although with an inconsiderable velocity. In this conflict of currents, as in the oscillation of waves, our imagination is struck with these movements, which seem to penetrate each other, and by which the ocean is incessantly agitated.

753. Horsburgh's.—Horsburgh, in his East India Directory, thus remarks on them, when speaking of the north-east monsoon about Java: "In the entrance of the Malacca Straits, near the Nicobar and Acheen Islands, and between them and Junkseylon, there are often very strong ripplings, particularly in the south-west monsoon; these are alarming to persons unacquainted, for the broken water makes a great noise when the ship is passing through the ripplings in the night. In most places ripplings are thought to be produced by strong currents, but here they are frequently seen when there is no perceptible current. Although, there is no perceptible current experienced so as to produce an error in the course and distance sailed, yet the surface of the water is impelled forward by some undiscovered cause. The ripplings are seen in calm weather approaching from a distance, and in the night their noise is heard a considerable time before, they come near. They beat against the sides of a ship with great violence, and pass on, the spray sometimes coming on deck;. and a small boat could not always resist the turbulence of these remarkable ripplings."

754. Tide-rips in the Atlantic.—Captain Higgins, of the " Maria," when bound from New York to Brazil, thus describes, in his. abstract log, one of these "tide-rips," as seen by him, 10th October, 1855, in N. lat. 14°, W. long. 34°: "At 3 p.m. saw a tide-rip; in the centre, temp, air 80°, water 81°. From the time it was seen to windward, about three to five miles, until it had passed to leeward out of sight, it was not five minutes. I should judge it travelled at not less than sixty miles per hour, or as fast as the bores of India. Although we have passed through several during the night, we do not find they have set the ship to the westward any; it may be that they are so soon passed that they have no influence on the ship, but they certainly beat very hard against the ship's sides, and jarred her all over. They are felt even when below, and will wake one out of sleep."

755. Mock vigias.—Captain Wakeman, of the "Adelaide," in January, 1856, lat. 11° 21′ N., long. 33° 33’ W., encountered "tide-rips " which broke and foamed with such violence that he took them for breakers or a shoal. They sometimes are most alarming. Approaching through the stillness of the night with a roaring noise, and in the shape of tremendous rollers combing and foaming, they seem to threaten to overwhelm vessel and crew; but, breaking over the deck, they pass by, and in a few moments the sea is as smooth and as unruffled as before. Many of the "vigias" which disfigure our charts have no other foundation than the foam of a tide-rip. Captain Arquit's log of the "Comet" gives an account of many tide-rips which he encountered also in the north-east trade-wind region of the Atlantic. Thus, November 15, 1855, lat. 7° 34’ N., long. 40° 30’ W.: "Many tide-rips, which we had a good opportunity of observing when becalmed. They came up in ridges as long as the eye could reach, from all parts of the compass, but mostly from the E. I examined the ridges very closely, but could not see any fine drift-matter of any kind, as you can on the ridges of currents in many parts of the ocean. We have had no currents unless they have been from different directions, and one counteracting the other. November 16th, lat. 6° 07' N.: Light winds and pleasant. There has been no time since noon to midnight but there have been tide-rips either in sight or hearing, mostly tending N.E. and S.W. in long narrow ridges. From 8 p.m. to 9 p.m. the ocean appeared like a boiling caldron, which we sailed through for three miles. The bubbling made a loud noise, which we heard for a long time after we had sailed through it. The ship had a very singular motion, like striking her keel on a soft muddy bottom in a short rough sea-way— the same as I have felt in the harbour of Montevideo. The motion was noticed by all on board. We have had a current of fifteen miles going west. I have often noticed tide-rips in this part of the ocean before, particularly when bound home (for I have never been where I am now, bound out, before), and have mentioned them in my abstract log, but they were different from what we had last night. The ship would come to and fall off three points without any regard to the rudder."

756. Bores, eagres, and the earthquake wave of Lisbon.—But, besides tide-rips, bores, and eagres,^[2] there are the sudden disruptions in the ice which arctic voyagers tell of, the immense icebergs which occasionally appear in groups near certain latitudes, the variable character of all the currents of the sea—now fast, now slow (§ 401), now running this way, then that—all of which 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.

757. Rains at sea and their effect upon its equilibrium.—Few persons have ever taken the trouble to compute (§ 402) 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 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 vapour, 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. This rain might fall in an hour, or it might fall in a day; but, to occupy what time it might in falling, it 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 supposed rain-fall. Now this is for but one fifth of the Atlantic, and the area of the Atlantic is about one fifth of the sea area of the world; and the estimated fall of rain was but one inch, whereas the average for the year is (§ 757) 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 vapour of which they were formed was taken up at still other places, we shall be the better enabled to appreciate the force and effect of these irregular movements in the sea.

758. Ditto of cloud and sunshine.—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.^[3] Let us, therefore, the more thoroughly to appreciate those agitations of the sea 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, with truth 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 of the water 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 of 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 two degrees is equivalent to a change in its volume of three hundred and ninety thousand millions of cubic feet. And yet there be philosophers who maintain (§ 123) that evaporation and precipitation, changes of temperature and saltness, and the secretions of insects, are not to be reckoned among the current-producing agents of the sea. That the gentle trade-winds do it all!

759. Day and night.—Do not the clouds, night and day, now present themselves to us in a new light? They are cogs, and pinions, and wheels in that grand and exquisite machinery which governs the sea, and which, amid all the jarring of the elements, preserves the harmonies of the ocean.

760. Logs overhauled for kelp arid ice.—The log-books of not less than 1843 vessels cruising on the polar side of 35° S. have, by the officers of the Observatory, been overhauled for kelp and ice. Of these, 367 (or one in five) mentioned kelp or sea-weed east of Cape Horn; 142 mention "rock- weed and drift matter" between the previous meridian and 10° W., and chiefly between 35° and 40° S. " Long kelp " is also found by Australian traders after passing the Cape of Good Hope; 146 logs make mention of it between the meridians of 40° and 120° E. It most abounds along this line, however, between the meridians of 45° and 65° E., and the parallels of 42° and 48° S. These sargassos are sketched with a free hand on Plate IX.

761. A sargasso in the South Pacific.—Sea-weed is frequently mentioned also by the homeward-bound Australian traders on their way to Cape Horn: this collection has (§ 139) already been alluded to. It now appears that instead of three, as stated in former editions of this work, there are really five true sargassos, as shown on Plate IX.

762. Sea-weed about the Falkland Islands.—The weedy space, marked as such, about the Falkland Islands, is probably not a true sargasso. The sea-weed reported there probably comes from the Straits of Magellan, where immense masses of it grow. These straits are so encumbered with sea-weed that steamers find great difficulty in making their way through it. It so encumbers their paddles as to make frequent stoppages necessary.

763. The African sargasso.—The sargasso to the west of the Cape of Good Hope, though small, is perhaps the best defined of them all. Mention is generally made of it in the logs as "rock-weed" and "drift matter." Now when it is recollected that weeds have been found as frequently, nearly (§ 760), in this email space as they have been in the large space between the Cape and Australia, the reader will be able to form a more correct idea as to the relative abundance of weed in these seas of weed.

764. Icebergs.—By going far enough south, icebergs may be found on any meridian; but in searching for them, we can only look where commerce carries our colleagues of the sea. Out of the 1843 tracks traced on the polar side of 35° S., only 109 make mention of ice. Few of these went, except in doubling Cape Horn, beyond the parallel of 55° S., therefore we have not been able to track the ice back into the "chambers of the frost." We can only say that north of 50° antarctic icebergs most abound between the meridians of 15° W. and 55° E.

765. The largest drift farthest.—As a rule, the bergs which are the largest last longest, and approach nearest to the equator. Here, then, is the great line of antarctic drift; by studying it we may perhaps catch a glimmer of light from south polar shores. These icebergs, be it remembered, have drifted north through a belt of westerly winds. Their course, therefore, was probably not due north, but to the east of that rhomb.

766. The line of antarctic drift.—Tracing this line of drift, then, backward in a south-westerly direction, it should guide us to that part of the southern continent where the icebergs have their principal nursery. This would take us to the sources of the Humboldt current, and seem to indicate that these glaciers are launched in its waters; but, as their motion is slow, the winds bear the bergs to the east, while the general drift sets them to the north.

767. Necessity for, and advantages of an antarctic expedition.—Arrived at this point, fiords, deep bays, and capacious gulfs loom up before the imagination, reminding us to ask the question. Is there not embosomed in the antarctic continent a Mediterranean, the shores of which are favourable to the growth and the launching of icebergs of tremendous size? and is not the entrance to this sea near the meridian of Cape Horn, perhaps to the west of it? Circumstances like these beget longings, and we sigh for fresh antarctic explorations. Surely, when we consider the advantages which the improvements of the age, the lights of the day, would afford an exploring expedition there now; when we reflect upon the drawbacks and difficulties with which former expeditions thither had to contend; when we call to mind the facilities with which one might be conducted now: surely, I say, when we thus reflect, no one can doubt as to the value and importance of the discoveries which a properly equipped expedition would now be sure to make.

768. Commercial considerations.—In those regions there are doubtless elements of commercial wealth in the number of seals and abundance of whales, if in nothing else. 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 favoured 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' 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.

769. Value of the fisheries.—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, it was thought, would be useful: it has so proved itself.

770. Sperm whales.—The sperm whale is a warm-water "fish". 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 results have been published; they form a part of the series of Maury's Wind and Current Charts.

771. A sea of fire to them.—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 cannot 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.

772. Right whales.—With these remarks, and the explanation 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. The sargassos, or places of weed, are also represented on this plate.

1. "It is the realm 'of reef-building corals, and of the wondrously-beautiful assemblage of animals, vertebrate and invertebrate, that live among them or prey upon them. The brightest and most definite arrangements of colour are here displayed. It is the seat of maximum development of the majority of marine genera. It has but few relations 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' Physical Atlas, 2nd ed.; Wm. Blackwood and Sons, Edinburgh and London, 1854.
2.  The bores of India, of the Bay of Fundy, and the Amazon, are the most celebrated. They are a tremulous tidal wave, which at stated periods comes rolling in from the sea, threatening to overwhelm and ingulf everything that moves on the beach. This wave is described, especially in the Bay of Fundy, as being many feet high; and it is said oftentimes to overtake deer, swine, and other wild beasts that feed or lick on the beach, and to swallow them up before the swiftest of foot among them have time to escape. The swine, as they feed on mussels at low water, are said to snuff the "bore," either by sounder smell, and sometimes to dash off to the cliffs at great speed before it rolls on.

   The eagre is the bore of Tsien-Tang River. It is thus described by Dr. Macgowan, in a paper before the Royal Asiatic Society, 12 January, 1853, and as seen by him from the city of Hang-chow, in 1848:—

   "At the upper part of the bay, and about the mouth of the river, the eagre is scarcely observable; but, owing to the very gradual descent of the shore, and the rapidity of the great flood and ebb, the tidal phenomena even here present a remarkable appearance. Vessels, which a few moments before were afloat, are suddenly left high and dry on a strand nearly two miles in width, which the returning wave as quickly floods. It is not until the tide rushes beyond the mouth of the river that it becomes elevated to a lofty wave constituting the eagre, which attains its greatest magnitude opposite the city of Hang-chow. Generally there is nothing in its aspect, except on the third day of the second month, and on the eighteenth of the eighth, or at the spring-tide about the period of the vernal and autumnal equinoxes, its great intensity being at the latter season. Sometimes, however, during the prevalence of easterly winds, on the third day after the sun and moon are in conjunction, or in opposition, the eagre courses up the river with hardly less majesty than when paying its ordinary periodical visit. On one of these unusual occasions, when I was travelling in native costume, I had an opportunity of witnessing it, on December 14th, 1848, at about 2 p.m.

   "Between the river and the city walls, which are a mile distant, dense suburbs extend several miles along the banks. As the horn-of flood-tide approached, crowds gathered in the streets running at right angles with the Tsien-Tang, but at safe distances. My position was a terrace in front of the Tki-WAVE Temple, which afforded a good view of the entire scene. On a sudden, all traffic in the thronged mart was suspended, porters cleared the front street of every description of merchandize, boatmen ceased lading and unlading their vessels, and put out in the middle of the stream, so that a few moments sufficed to give a deserted appearance to the busiest part of one of the busiest cities of Asia. The centre of the river teemed with craft, from small boats to huge barges, including the gay 'flower-boats.' Loud shouting from the fleet announced the appearance of the flood, which seemed like a glittering white cable, stretched athwart the river at its mouth, as far down as the eye could reach. Its noise, compared by Chinese poets to that of thunder, speedily drowned that of the boatmen; and as it advanced with prodigious velocity—at  the rate, I should judge, of twenty-five miles an hour—it assumed the appearance of an alabaster wall, or, rather, of a cataract four or five miles across, and about thirty feet high, moving bodily onward. Soon it reached the advanced guard of the immense assemblage of vessels awaiting its approach. Knowing that the bore of the Hooghly, which scarcely deserves mention in connection with the one before me, invariably overturned boats which were not skilfully managed, I could not but feel apprehensive for the lives of the floating multitude. As the foaming wall of water dashed impetuously onward, they were silenced, all being intensely occupied in keeping their prows towards the wave which threatened to submerge everything afloat; but they all vaulted as it were to the summit with perfect safety. The spectacle was of great interest when the eagre had passed about one half way among the craft. On one side they were quietly reposing on the surface of the unruffled stream, while those on the nether portion were pitching and heaving in tumultuous confusion on the flood; others were scaling with the agility of salmon the formidable cascade. This grand and exciting scene was but of a moment's duration; it passed up the river in an instant, but from this point with gradually diminishing force, size, and velocity, until it ceased to be perceptible, which Chinese accounts represent to be eighty miles distant from the city. From ebb to flood tide the change was almost instantaneous; a slight flood continued after the passage of the wave, but it soon began to ebb. Having lost my memoranda, I am obliged to write from recollection. My impression is that the fall was about twenty feet; the Chinese say that the rise and fall is sometimes forty feet at Hang-chow, The maximum rise and fall at spring-tides is probably at the mouth of the river, or upper part of the bay, where the eagre is hardly discoverable. In the Bay of Fundy, where the tides rush in with amazing velocity, there is at one place a rise of seventy feet; but there the magnificent phenomenon in question does not appear to be known at all. It is not, therefore, where tides attain their greatest rapidity, or maximum rise and fall, that this wave is met with, but where a river and its estuary both present a peculiar configuration.

   "Dryden's definition of an eagre, appended in a note to the verse above quoted from the Threnodia Atigustalis, is, 'a tide swelling above another tide,' which he says he had himself observed in the Eiver Trent. Such, according to Chinese oral accounts, is the character of the Tsien-Tang tides—a wave of considerable height rushes suddenly in from the bay, which is soon followed by one much larger. Other accounts represent three successive waves riding in; hence the name of the temple mentioned, that of the Three Waves. Both here and on the Hooghly I observed but one wave; my attention, however, was not particularly directed to this feature of the eagre. The term should, perhaps, be more comprehensive, and express the instantaneous rise and advance of a tidal wave;' the Indian barbarism 'bore' should be discarded altogether.

   "A very short period elapsed between the passage of the eagre and the resumption of traffic. The vessels were soon attached to the shore again; women and children were occupied in gathering articles which the careless or unskilful had lost in the aquatic melee. The streets were drenched with spray, and a considerable volume of water splashed over the banks into the head of the grand canal, a few feet distant."—Vide Transactions of Chinese Branch of the Royal Asiatic Society.

3. Vide Admiral Smyth's Memoir of the Mediterranean, p. 125.