Physical Geography of the Sea and its Meteorology/Chapter 8

by Matthew Fontaine Maury

CHAPTER VIII.

 

§ 370-409.—CURRENTS OF THE SEA.

 

370. Obedient to order.—We here set out with the postulate that the sea, as well as the air, has its system of circulation, and that this system whatever it be, and wherever its channels lie, whether in the waters at or below the surface, is in obedience to law. The sea, by the circulation of its waters, doubtless has its offices to perform in the terrestrial economy; and when we see the currents in the ocean running hither and thither, we feel that they were not put in motion without a cause. On the contrary, we know they move in obedience to some law of Nature, be it recorded down in the depths below, never so far beyond the reach of human ken; and being a law of Nature, we know who gave it, and that neither chance nor accident had anything to do with its enactment. Nature grants us all that this postulate demands, repeating it to us in many forms of expression: she utters it in the blade of green grass which she causes to grow in climates and soils made kind and genial by warmth and moisture that some current of the sea or air has conveyed far away from under a tropical sun. She murmurs it out in the cooling current of the north; the whales of the sea tell of it (§ 158); and all its inhabitants proclaim it.

371. The fauna and flora of the sea.—The fauna and the flora of the sea are as much the creatures of climate (§ 104), and are as dependent for their well-being upon temperature, as are the fauna and the flora of the dry land. Were it not so we should find the fish and the algae, the marine insect and the coral, distributed equally and alike in all parts of the ocean. The arctic whale would delight in the torrid zone, and the habitat of the pearl oyster would be also under the iceberg, or in the frigid waters of polar seas.

372. Those of southern unlike those of northern seas.—Nevertheless, though the constituents of sea water be the same in kind, we must not infer that they are the same in degree throughout all parts of the ocean, for there is a peculiarity, perhaps of temperature, perhaps of transparency, which marks the inhabitants of trans-equatorial seas. MM. Peron and Le Sueur, who have turned their attention to the subject, assert that out of many thousand examples they did not find a single one in which the inhabitants of trans-equatorial were not distinguishable from, those of their species in cis-equatorial seas.

373. The capacity of water to convey heat.—Water, while its capacities for heat are scarcely exceeded by those of any other substance, is one of the most complete of non-conductors. Heat does not permeate water as it does iron, for instance, or other good conductors. Heat the top of an iron plate, and the bottom becomes warm; but heat the top of a sheet of water, as in a pool or basin, and that at the bottom remains cool. The heat passes through iron by conduction, but to get through water it requires to be conveyed by a motion, which in fluids we call currents. Therefore the study of the climates of the sea involves a knowledge of its currents, both cold and warm. They are the channels through which the waters circulate, and by means of which the harmonies of old ocean are preserved.

374. Currents of the sea to be considered in pairs.—Hence, in studying the system of oceanic circulation, we set out with the very simple assumption, viz., that from whatever part of the ocean a current is found to run, to the same part a current of equal volume is bound to return; for upon this principle is based the whole system of currents and counter-currents of the air as well as of the water. Hence, the advantage of considering them as the anatomist does the nerves of the human system—in pairs. Currents of water, like currents of air, meeting from various directions, create gyrations, which in some parts of the sea, as on the coast of Norway, assume the appearance of whirlpools, as though the water were drawn into a chasm below. The celebrated Maelstrom is caused by such a conflict of tidal or other streams. The late Admiral Beechey, R.N.,^[1] gave diagrams illustrative of many " rotatory streams in the English Channel, a number of which occur between the outer extremities of the channel tide and the stream of the oceanic or parent wave." "They are clearly to be accounted for," says he, " by the streams acting obliquely upon each other."

375. Marine currents do not, like those on land, run of necessity from higher to lower levels.—It is not necessary to associate with oceanic currents the idea that they must, of necessity, as on land, run from a higher to a lower level. So far from this being the case, some currents of the sea actually run up hill, while others run on a level. The Gulf Stream is of the first class (§ 83).

376. The Red Sea current.—The currents which run from the Atlantic into the Mediterranean, and from the Indian Ocean into the Red Sea, are the reverse of this. Here the bottom of the current is probably a water-level, and the top an inclined plane, running down hill. Take the Red Sea current as an illustration. That sea lies, for the most part, within a rainless and riverless district. It may be compared to a long and narrow trough. Being in a rainless district, the evaporation of it is immense; none of the water thus taken up is returned to it either by rivers or rains. It is about one thousand miles long; it lies nearly north and south, and extends from latitude 13° to the parallel of 30° north. From May to October, the water in the upper part of this sea is said to be two feet lower than it is near the mouth.^[2] This change or difference of level is ascribed to the effect of the wind, which, prevailing from the north at that season, is supposed to blow the water out. But from May to October is also the hot season; it is the season when evaporation is going on most rapidly: and when we consider how dry and how hot the winds are which blow upon this sea at this season of the year; that it is a narrow sea; that they blow across it and are not saturated, we may suppose the daily evaporation to be immense. The evaporation from this sea and the Persian Gulf is probably greater than it is from any other arms of the ocean. We know that the waste from canals by evaporation, in the summer-time, is an element which the engineer, when taking the capacity of his feeders into calculation, has to consider. With him it is an important element: how much more so must the waste by evaporation from this sea be when we consider the physical conditions under which it is placed! Its feeder, the Arabian Sea, is a thousand miles from its head; its shores are burning sands; the evaporation is ceaseless; it is a natural evaporating dish (§ 525) on a grand scale; none of the vapours which the scorching winds that blow over it carry away are returned to it again in the shape of rains. The Red Sea vapours are carried off and precipitated elsewhere. The depression in the level of its head waters in the summer-time, therefore, it appears, is owing to the effect of evaporation, as well as to that of the wind blowing the waters back. The evaporation in certain parts of the Indian Ocean is supposed to be (§ 103) from three fourths of an inch to an inch daily. Whatever it be, it is doubtless greater in the Red Sea. Let us assume it, then, in the summer-time to average only half an inch a day. Now, if we suppose the velocity of the current which runs into that sea to average, from mouth to head, twenty miles a day, it would take the water fifty days to reach the head of it. If it lose half an inch from its surface by evaporation daily, it would, by the time it reaches the Isthmus of Suez, have lost twenty-five inches from its surface. Thus the waters of the Red Sea ought to be lower at the Isthmus of Suez than they are at the Straits of Babelmandeb. Independently of the forcing out by the wind, the waters there ought to be lower from two other causes, viz., evaporation and temperature; for the temperature of that sea is necessarily lower at Suez, in latitude 30°, than it is at Babelmandeb, in latitude 13°. To make it quite clear that the surface of the Red Sea is not a sea level, but is an inclined plane, suppose the channel of the Red Sea to have a perfectly smooth and level floor, with no water in it, and a wave ten feet high to enter the Straits of Babelmandeb, and to flow up the channel, like the present surface current, at the rate of twenty miles a day for fifty days, losing daily, by evaporation, half an inch; it is easy to perceive that, at the end of the fiftieth day, this wave would not be so high by two feet (twenty-five inches) as it was the first day it commenced to flow. The top of that sea, therefore, may be regarded as an inclined plane, made so by evaporation.

377. Upper and under currents through straits explained.—But the salt water, which has lost so much of its freshness by evaporation, becomes salter, and therefore heavier. The lighter water at the Straits cannot balance the heavier water at the Isthmus, and the colder and salter, and therefore heavier water, must either run out as an under current, or it must deposit its surplus salt in the shape of crystals, and thus gradually make the bottom of the Red Sea a salt-bed, or it must abstract all the salt from the ocean to make the Red Sea brine—and we know that neither the one process nor the other is going on. Hence we infer that there is from the Red Sea an under and outer current, as there is from the Mediterranean through the Straits of Gibraltar, and that the surface waters near Suez are salter than those near the mouth of the Red Sea. And, to show why there should be an outer and under current from each of these two seas, let us suppose the case of a vat of oil, and a vat of wine connected by means of a narrow trough—the trough being taken to represent the straits connecting seas the waters of which differ as to specific gravity. Suppose the trough to have a flood-gate, which is closed until we are ready for the experiment. Now let the two vats be filled, one with wine the other with oil, up to the same level. The oil is introduced to represent the lighter water as it enters either of these seas from the ocean, and the wine the same water after it has lost some of its freshness by evaporation, and therefore has become salter and heavier. Now suppose the flood-gate to be raised, what would take place? Why, the oil would run in as an upper current, overflowing the wine, and the wine would run out as an under current.

378. The Mediterranean current.—The rivers which discharge their waters into the Mediterranean are not sufficient to supply the waste of evaporation, and it is by a process similar to this that the salt which is carried in from the ocean is returned to the ocean again: were it not so, the bed of that sea would be a mass of solid salt. The unstable equilibrium of the seas is a physical necessity. Were it to be lost, the consequences would be as disastrous as would be any derangement in the forces of gravitation. Without doubt, the equilibrium of the sea is preserved by a system of compensation as exquisitely adjusted as are those by which the "music of the spheres" is maintained. It is difficult to form an adequate conception of the immense quantities of solid matter which the current from the Atlantic, holding in solution, carries into the Mediterranean. In his abstract log for March 8th, 1855, Lieutenant William Grenville Temple, of the United States ship Levant, homeward bound, has described the indraught there: "Weather fine; made 1¼ pt. lee-way. At noon, stood in to Almiria Bay and anchored off the village of Koguetas. Found a great number of vessels waiting for a chance to get to the westward, and learned from them that at least a thousand sail are weather-bound between this and Gibraltar. Some of them have been so for six weeks, and have even got as far as Malaga, only to be swept back by the current. Indeed, no vessel had been able to get out into the Atlantic for three months past." Now suppose this current, which baffled and beat back this fleet for so many days, ran no faster than two knots the hour. Assuming its depth to be 400 feet only, and its width seven miles, and that it carried in with it the average proportion of solid matter—say one thirtieth—contained in sea water; and admitting these postulates into calculation as the basis of the computation, it appears that salts enough to make no less than 88 cubic miles of solid matter, of the density of water, were carried into the Mediterranean during these 90 days. Now, unless there were some escape for all this solid matter, which has been running into that sea, not for 90 days merely, but for ages, it is very clear that the Mediterranean would, ere this, have been a vat of very strong brine, or a bed of cubic crystals.

379. The Suez Canal.—We have in this fact, viz., the difficulty of egress from the Mediterranean, and the tedious character of the navigation, under canvas, within it, the true secret of the indifference which, in commercial circles in England and the Atlantic states of Europe, is manifested towards the projected Suez Canal. But to France and Spain on the Mediterranean, to the Italian States, to Greece, and Austria, it would be the greatest commercial boon of the age. The Mediterranean is a great gulf running from west to east, penetrating the old world almost to its very centre, and separating its most civilized from its most savage communities. Its southern shores are inhabited, for the most part by an anti-commercial and thriftless people. On the northern shores the climates of each nation are nearly duplicates of the climates of her neighbours to the east and the west; consequently, these nations all cultivate the same staples, and have wants that are similar: for a commerce among themselves, therefore, they lack the main elements, viz., difference of production, and the diversity of wants which are the consequence of variety of climates. To reach these, the Mediterranean people have had to encounter the tedious navigation and the sometimes difficult egress—just described—from their sea. Clearing the Straits of Gibraltar, their vessels do not even then find themselves in a position so favourable for reaching the markets of the world as they would be were they in Liverpool or off the Lizard. Such is the obstruction which the winds and the current from the Atlantic offer to the navigation there, that vessels bound to Lidia from the United States, England, or Holland, may often double the Cape of Good Hope before one sailing with a like destination from a Mediterranean port would find herself clear of the Straits of Gibraltar. It is therefore not surprising that none of the great commercial marts of the present day are found on the shores of this classic sea. The people who inhabit the hydrographic basin of the Mediterranean—which includes the finest parts of Europe—have, ever since the discovery of the passage around the Cape of Good Hope, been commercially pent up. A ship-canal across the Isthmus of Suez will let them out into the commercial world, and place them within a few days of all the climates, wants, supplies, and productions of India. It will add largely to their wealth and prosperity. As these are increased, trading intercourse is enhanced, and so by virtue of this canal they will become better customers for England and Holland, and all other trading nations whose ports are havens of the Atlantic. Occupying this stand-point in their system of commercial economy, the people of the United States await with a lively interest the completion of the Suez Canal.

380. Hydrometrical observations at sea wanted.—Of all parts of the ocean, the warmest water, the saltest and the heaviest too, is said to be found in the seas of the Indian Ocean. A good .series of observations there -with the hydrometer, at the different seasons of the year, is a desideratum. Taking, however, such as we have upon the density of the water in the Red Sea and the Mediterranean, and upon the under currents that run out from these seas, let us examine results.

381. Specific gravity of Red Sea water.—Several years ago, Mr. Morris, chief engineer of the Oriental Company's steam-ship Ajdaha, collected specimens of Red Sea water all the way from Suez to the Straits of Babelmandeb, which were afterwards examined by Dr. Giraud, who reported the following results:^[3]

			Latitude. Degrees	Longitude. Degrees.	Spec. Grav.	Saline Cont. 1000 parts.
No.	1.	Sea of Suez	..	..	1027	41.0
	2.	Gulf of Suez	27.49	33.44	1026	40.0
	3.	Red Sea	24.29	36.00	1024	39.2
	4.	Ditto	20.55	38.18	1026	40.5
	5.	Ditto	20.43	40.03	1024	39.8
	6.	Ditto	14.35	42.43	1024	39.9
	7.	Ditto	12.39	44.45	1023	39.2

These observations agree with the theoretical deductions just announced, and show that the surface waters at the head are heavier and Salter than the surface waters at the mouth of the Red Sea.

382. Evaporation from.—In the same paper, the temperature of the air between Suez and Aden often rises, it is said, to 90°, "and probably averages little less than 75° day and night all the year round. The surface of this sea varies in heat from 65° to 85°, and the difference between the wet and dry bulb thermometers often amounts to 25°—in the kamsin, or desert winds to from 30° to 40°; the average evaporation at Aden is about eight feet for the year." Now assuming," says Dr. Buist, "the evaporation of the Red Sea to be no greater than that of Aden, a sheet of water eight feet thick, equal in area to the whole expanse of that sea, will be carried off annually in vapour; or, assuming the Red Sea to be eight hundred feet in depth at an average—and this, most assuredly, is more than double the fact—the whole of it would be dried up, were no water to enter from the ocean, in one hundred years. The waters of the Red Sea, throughout, contain some four per cent, of salt by weight—or, as salt is a half heavier than water, some 2.7 per cent, in bulk—or, in round numbers, say three per cent. In the course of three thousand years, on the assumptions just made, the Red Sea ought to have been one mass of solid salt, if there were no current running out." Now we know the Red Sea is more than three thousand years old, and that it is not filled with salt; and the reason is, that as fast as the upper currents bring the salt in at the top, the under currents carry it out at the bottom.

383. The Mediterranean Currents.—With regard to an under current from the Mediterranean, we may begin by remarking that we know that there is a current always setting in at the surface from the Atlantic, and that this is a salt-water current, which carries an immense amount of salt into that sea. "We know, moreover, that that sea is not salting up; and therefore, independently of the postulate (§ 374) and of observations, we might infer the existence of an under current, through which this salt finds its way out into the broad ocean again.^[4]

384. The drift of the Phoenix.—With regard to this outer and under current, we have observations telling of its existence as long ago as 1712. "In the year 1712," says Dr. Hudson, in a paper communicated to the Philosophical Society in 1724, "Monsieur du L'Aigle, that fortunate and generous commander of the privateer called the Phoenix, of Marseilles, giving chase near Ceuta Point to a Dutch ship bound to Holland, came up vith her in the middle of the Gut between Tariffa and Tangier, and there gave her one broadside, which directly sunk her, all her men being saved by Monsieur du L'Aigle; and a few days after, the Dutch ship, with her cargo of brandy and oil, arose on the shore near Tangier, which is at least four leagues to the westward of the place where she sunk, and directly against the strength of the current, which has persuaded many men that there is a recurrency in the deep water in the middle of the Gut that sets outward to the grand ocean, which this accident very much demonstrates; and, possibly, a great part of the water which runs into the Straits returns that way, and along the two coasts before mentioned; otherwise this ship must, of course, have been driven towards Ceuta, and so upwards. The water in the Gut must be very deep; several of the commanders of our ships of war having attempted to sound it with the longest lines they could contrive, but could never find any bottom."

385. Saltness of the Mediterranean.—In 1828, Dr. Wollaston, in a paper before the Philosophical Society, stated that he found the specific gravity of a specimen of sea water, from a depth of six hundred and seventy fathoms, fifty miles within the Straits to have a "density exceeding that of distilled water by more than four times the usual excess, and accordingly leaves, upon evaporation, more than four times the usual quantity of saline residuum. Hence it is clear that an under current outward of such denser water, if of equal breadth and depth with the current inward near the surface, would carry out as much salt below as is brought in above, although it moved with less than one fourth part of the velocity, and would thus prevent a perpetual increase of saltness in the Mediterranean Sea beyond that existing in the Atlantic." The doctor obtained this specimen of sea water from Captain, now Admiral Smyth, of the English Navy, who had collected it for Dr. Marcet. Dr. Marcet died before receiving it, and it had remained in the admiral's hands some time before it came into those of Wollaston. It may, therefore, have lost something by evaporation; for it is difficult to conceive that all the river water, and three fourths of the sea water which runs into the Mediterranean, is evaporated from it, leaving a brine for the under current having four times as much salt as the water at the surface of the sea usually contains. Very recently, M. Coupvent des Bois is said to have shown, by actual observation, the existence of an outer and under current from the Mediterranean.

386. The escape of salt and heavy water by under currents.—However that may be, these facts, and the statements of the Secretary of the Geographical Society of Bombay (§ 382), seem to leave no room to doubt as to the existence of an under current both from the Red Sea and Mediterranean, and as to the cause of the surface current which flows into them. I think it a matter of demonstration. It is accounted for (§ 377) by the salts of the sea. Writers whose opinions are entitled to great respect differ with me as to the conclusiveness of this demonstration. Among those writers are Admiral Smyth, of the British Navy, and Sir Charles Lyell, who also differ with each other. In 1820, Dr. Marcet being then engaged in studying the chemical composition of sea water, the admiral, with his usual alacrity for doing "a kind turn," undertook to collect for the doctor specimens of Mediterranean water from various depths, especially in and about the Straits of Gibraltar. Among these was the one (§ 385) taken fifty miles within the Straits from the depth of six hundred and seventy fathoms (four thousand and twenty feet), which, being four times Salter than common sea water, left, as we have just seen, no doubt in the mind of Dr. Wollaston as to the existence of this under current of brine. But the indefatigable admiral, in the course of his celebrated survey of the Mediterranean, discovered that, while inside of the Straits the depth was upwards of nine hundred fathoms, yet in the Straits themselves the depth across the shoalest section is not more than one hundred and sixty^[5] fathoms. "Such being the case, we can now prove," exclaims Sir Charles Lyell, "that the vast amount of salt brought into the Mediterranean does not pass out again by the Straits for it appears by Captain Smyth's soundings, which Dr. Wollaston had not seen, that between the Capes of Trafalgar and Spartel, which are twenty-two miles apart, and where the Straits are shallowest, the deepest part, which is on the side of Cape Spartel, is only two hundred and twenty fathoms.^[6] It is therefore evident, that if water sinks in certain parts of the Mediterranean, in consequence of the increase of its specific gravity, to greater depths than two hundred and twenty fathoms, it can never flow out again into the Atlantic, since it must be stopped by the submarine barrier which crosses the shallowest part of the Straits of Gibraltar.^[7]"

387. Vertical circulation in the sea a physical necessity.—According to this reasoning, all the cavities, the hollows, and the valleys at the bottom of the sea, especially in the trade-wind region, where evaporation is so constant and great, ought to be salting up or filling up with brine. Is it probable that such a process is actually going on? No. According to this reasoning, the water at the bottom of the great American lakes ought to remain there for over, for the bottom of Erie is far below the barrier which separates this lake from the Falls of Niagara, and so is the bottom of every one of the lakes below the shallows in the straits or rivers that connect them as a chain. We may presume that the water at the bottom of every extensive and quiet sheet of water, whether salt or fresh, is at the bottom by reason of specific gravity; but that it does not remain there for ever we have abundant proof. If so the Niagara River would be fed by Lake Erie only from that layer of water which is above the level of the top of the rock at the Falls. Consequently, wherever the breadth of that river is no greater than it is at the Falls, we should have a current as rapid as it is at the moment of passing the top of the rock to make the leap. To see that such is not the way of Nature, we have but to look at any common mill-pond when the water is running over the dam. The current in the pond that feeds the overflow is scarcely perceptible, for "still water runs deep." Moreover, we know it is not such a skimming current as the geologist would make, which runs from one lake to another; for wherever above the Niagara Falls the water is deep, there we are sure to find the current sluggish, in comparison with the rate it assumes as it approaches the Falls; and it is sluggish in deep places, rapid in shallow ones, because it is fed from below. The common "wastes" in our canals teach us this fact.

388 The bars at the mouths of the Mississippi an illustration.—The reasoning of this celebrated geologist appears to be founded upon the assumption that when water, in consequence of its specific gravity, once sinks below the bottom of a current where it is shallowest, there is no force of traction, so to speak, in fluids, nor any other power, which can draw this heavy water up again. If such were the case, we could not have deep water immediately inside of the bars which obstruct the passage of the great rivers into the sea: the bar at the mouth of the Mississippi, with only fifteen feet of water on it, is estimated to travel out to sea at rates varing from twenty to one hundred yards a year. In the place where that bar was when it was one thousand yards nearer to New Orleans than it now is, whether it were fifteen years ago or a century ago, with only fifteen or sixteen feet of water on it, we have now four or five times that depth. As new bars were successively formed seaward from the old, what dug up the sediment which formed the old, and lifted it up from where specific gravity had placed it, and carried it out to sea over a barrier not more than a few feet from the surface? Indeed, Sir Charles himself makes this majestic stream to tear up its own bottom to depths far below the top of the bar at its mouth. He describes the Mississippi as a river having nearly a uniform breadth to the distance of two thousand miles from the sea.^[8] He makes it cut a bed for itself out of the soil, which is heavier than Admiral Smyth's deep sea water, to the depth of more than two hundred feet^[9] below the top of the bar which obstructs its entrance into the sea. Could not the same power which scoops out this solid matter for the Mississippi draw the brine up from the pool in the Mediterranean, and pass it out across the barrier in the Straits? The currents which run over the bars and shoals in our rivers are fed from the pools above with water which we know comes from depths far below the top of such bars. The breadth of the river where the bar is may be the same as its breadth where the deep pool is, yet the current in the pool may be so sluggish as scarcely to be perceptible, while it may dash over the bar or down the rapids with mill-tail velocity. Were the brine not drawn out again from the hollow places in the sea, if would be easy to prove that this indraught into the Mediterranean has taken, even during the period assigned by Sir Charles to the formation of the Delta of the Mississippi—one of the newest formations—salt enough to fill up the whole basin of the Mediterranean with solid matter. Admiral Smyth brought up bottom with his briny sample of deep sea water (six hundred and seventy fathoms), but no salt crystals.

389. Views of Admiral Smyth and Sir C. Lyell.—The gallant admiral—appearing to withhold his assent both from Dr. Wollaston in his conclusions as to this under current, and from the geologist in his inferences as to the effect of the barrier in the Straits—suggests the probability that, in sounding for the heavy specimen of sea water, he struck a brine spring. But the specimen, according to analysis, was of sea water, and it is not necessary to call in the supposition of a brine spring to account for this heavy specimen. If we admit the principle assumed by Sir Charles Lyell, that water from the great pools and basins of the sea can never ascend to cross the ridges which form these pools and basins, then the harmonies of the sea are gone, and we are forced to conclude they never existed. Every particle of water that sinks below a submarine ridge is ipso facto, by his reasoning, stricken from the channels of circulation, to become thenceforward for ever motionless matter. The consequence would be "cold obstruction" in the depths of the sea, and a system of circulation between different seas of the waters only that float above the shoalest reefs and barriers of each. If the water in the depths of the sea were to be confined there—doomed to everlasting repose,—then why was it made fluid, or why was the sea made any deeper than just to give room for its surface currents to skim along? If water once below the reefs and shallows must remain below them,—why were the depths of the ocean filled with fluid instead of solid matter? Doubtless, when the seas were measured and the mountains stood in the balance, the solid and fluid matters of the earth were adjusted in exact proportions to insure perfection in the terrestrial machinery. I do not believe in the existence of any such imperfect mechanism, or in any such failure of design as the imparting of useless properties to matter, such as fluidity to that which is doomed to be stationary, would imply. To my mind, the proofs—the theoretical proofs,—the proofs derived exclusively from reason and analogy—are as clear in favour of this under current from the Mediterranean as they were in favour of the existence of Leverrier's planet before it was seen through the telescope at Berlin. Now suppose, as Sir Charles Lyell maintains, that none of these vast quantities of salt which this surface current takes into the Mediterranean find their way out again. It would not be difficult to show, even to the satisfaction of that eminent geologist, that this indraught conveys salt away from the Atlantic faster than all the fresh-water streams empty fresh supplies of salt into the ocean. Now, besides this drain, vast quantities of salts are extracted from sea water for coral reefs, shell banks, and marl beds; and by such reasoning as this, which is perfectly sound and good, we establish the existence of this under current, or else we are forced to the very unphilosophical conclusion that the sea must be losing its salts, and becoming less and less briny.

390. The currents of the Indian Ocean.—By carefully examining the physical features of this sea (Plates VIII. and IX.) and studying its conditions, we are led to look for warm currents that have their genesis in this ocean, and that carry from it volumes of overheated water, probably exceeding in quantity many times that which is discharged by the Gulf Stream from its fountains (Plate VI.). The Atlantic Ocean is open at the north, but tropical countries bound the Indian Ocean in that direction. The waters of this ocean are hotter than those of the Caribbean Sea, and the evaporating force there (§ 300) is much greater. That it is greater we might, without observation, infer from the fact of a higher temperature and a greater amount of precipitation on the neighbouring shores (§ 298). These two facts, taken together, tend, it would seem, to show that large currents of warm water have their genesis in the Indian Ocean. One of them is the well-known Mozambique current, called at the Cape of Good Hope the Lagulhas current. Another of these warm currents from the Indian Ocean makes its escape through the Straits of Malacca, and, being joined by other warm streams from the Java and China Seas, flows out into the Pacific, like another Gulf Stream, between the Philippines and the shores of Asia. Thence it attempts the great circle route for the Aleutian Islands, tempering climates, and losing itself in the sea as its waters grow cool on its route towards the north-west coast of America.

391. The Black Stream of the Pacific contrasted with the Gulf Stream of the Atlantic.—Between the physical features of this, the "Black Stream" of the Pacific, and the Gulf Stream of the Atlantic there are several points of resemblance. Sumatra and Malacca correspond to Florida and Cuba; Borneo to the Bahamas, with the Old Providence Channel to the south, and the Florida Pass to the west. The coasts of China answer to those of the United States, the Philippines to the Bermudas, the Japan Islands to Newfoundland. As with the Gulf Stream, so also here with this China current, there is a counter current of cold water between it and the shore. The climates of the Asiatic coast correspond with those of America along the Atlantic, and those of Columbia, Washington, and Vancouver resemble those of Western Europe and the British Islands; the climate of California (State) resembles that of Spain; the sandy plains and rainless regions of Lower California reminding one of Africa, with its deserts between the same parallels, etc. Moreover, the North Pacific, like the North Atlantic, is enveloped, where these warm waters go, with mists and fogs, and streaked with lightning. The Aleutian Islands are almost as renowned for fogs and mists as are the Grand Banks of Newfoundland. A surface current flows north from Behring's Strait into the Arctic Sea: but in the Atlantic the current is from, not into the Arctic Sea: it flows south on the surface, north below; Behring's Strait being too shallow to admit of mighty under currents, or to permit the introduction from the polar basin of any large icebergs into the Pacific. Behring's Strait, in geographical position, answers to Davis' Strait in the Atlantic; and Alaska, with its Aleutian chain of islands, to Greenland. But instead of there being to the east of Alaska, as there is to the east of Greenland, an escape into the polar basin for these warm waters of the Pacific, a shore-line intervenes: being cooled here, and having their specific gravity changed, they are turned down through a sort of North Sea along the western coast of the continent toward Mexico. They appear here as a cold current. The effect of this body of cold water upon the littoral climate of California is very marked. Being cool, it gives freshness and strength to the sea breeze of that coast in summer-time, when the "cooling sea breeze" is most grateful. These contrasts show the principal points of resemblance and of contrast between the currents and aqueous circulation in the two oceans. The ice-bearing currents of the North Atlantic are not repeated as to volume in the North Pacific, for there is no nursery for icebergs like the frozen ocean and its Atlantean arms. The seas of Okotsk and Kamtschatka alone, and not the frozen seas of the Arctic, cradle the icebergs for the North Pacific.

392. The Lagulhas Current and the storms of the Cape.—The Lagulhas current, as the Mozambique is sometimes called, skirts the coast of Natal as our Gulf Stream does the coast of Georgia, where it gives rise to the most grand and terrible displays of thunder and lightning that are anywhere else to be witnessed. Missionaries thence report to me the occurrence there of thunder-storms in which for hours consecutively they have seen an uninterrupted blaze of lightning, and heard a continuous peal of thunder. Reaching the Lagulhas banks, the current spreads itself out there in the midst of cooler waters, and becomes the centre of one of the most remarkable storm-regions in the world. My friend and fellow-labourer, Lieut. Andrau of the Dutch Navy, has made the storms upon these banks a specialty for study. He has pointed out from the abstract logs at Utrecht the existence there of some curious and interesting atmospherical phenomena to which this body of warm water gives rise. The storms that it calls up come rushing from the westward;—sweeping along parallel with the coast of Africa, they curve along it. Though so near the land, they seldom reach it. They march into these warm waters with furious speed; reaching them with a low barometer, they pause and die out. That officer has conferred a boon upon the Indiamen of all flags, for he has taught them how to avoid these dreadful winter storms of the Cape.

393. The currents and drift of the Indian Ocean.—There is sometimes, if not always, another exit of warm water from the Indian Ocean. It seems to be an overflow of the great intertropical caldron of India;—seeking to escape thence, it works its way poleward more as a drift than as a current. It is to the Mozambique-current what the northern flow of warm waters in the Atlantic (§ 141) is to the Gulf Stream. This Indian overflow is very large. The best indication of it is afforded by the sperm whale curve (Plate IX.). This overflow finds its way south midway between Africa and Australia, and appears to lose itself in passing round a sort of Sargasso Sea, thinly strewed with patches of weed. Nor need we be surprised at such a vast flow of warm water as these three currents indicate from the Indian Ocean, when we recollect that this ocean (§ 392) is land-locked on the north, and that the temperature of its waters is frequently as high as 90° Fahr. There must, therefore, be immense volumes of water flowing into the Indian Ocean to supply the waste created by these warm currents.

394. The ice-hearing currents from the Antarctic regions.—On either side of this warm current that escapes from the intertropical parts of the Indian Ocean, but especially on the Australian side, an ice-bearing current (Plate IX.) is found wending its way from the Antarctic regions with supplies of cold water to modify climates and restore the aqueous equilibrium in that part of the world. There is a general drift up into the South Atlantic of ice-bearing waters from Antarctic seas. The icebergs brought thence, being often very large and high, are set to the eastward by the "brave west winds" of those regions. Hence the icebergs that are so often seen to the south of the Cape of Good Hope. They set off for the Atlantic, but are driven to the eastward by the west winds of these latitudes. The Gulf Stream seldom permits icebergs from Arctic waters to reach the parallel of 40° in the North Atlantic; but I have known the ice-bearing current which passes east of Cape Horn into the South Atlantic to convey its bergs as far as the parallel of 37° south latitude. This is the nearest approach of icebergs to the equator. These currents which run out from the intertropical basin of that immense sea—Indian Ocean—convey along immense volumes of water containing vast quantities of salt, and we know that sea water enough to convey back equal quantities of salt, and salt to keep up supplies for the outgoing currents, must flow into the intertropical regions of the same sea; therefore, if observations were silent upon the subject, reason would teach us to look for currents here that keep in motion immense volumes of water.

395. The currents of the Pacific—drift-wood.—The contrast has been drawn (§391) between the Japan or "Black Stream "of the North Pacific, and the Gulf Stream of the North Atlantic. The course of the former has never been satisfactorily traced out. There is (Plate IX.), along the coast of California and Mexico, a southwardly movement of waters, as there is along the west coast of Africa towards the Cape de Verd Islands. In the open space west of this southwardly set along the African coast there is the famous Sargasso Sea (Plate IX.), which is the general receptacle of the drift-wood and sea-weed of the Atlantic. So, in like manner, to the west from California of this other southwardly set, lies the pool into which the drift-wood and sea-weed of the North Pacific are generally gathered, but in small quantities. The shores of Johnston's Islands (17° N., 169° 30' W.), which are near the edge of this pool, are lined with drift-wood from the Columbia, and the red cedar of California. The immense trees that have been cast up on these guano islands were probably drifted down with the cool California current into the north-east trades, and by them wafted along to the west, thus showing that the currents of the North Pacific flow in a sort of circle, on the outer edge of which lie the Japanese and Aleutian Islands, and the north-west coast of America.

395. The Black Current of the Pacific, like the Gulf Stream, salter than the adjacent waters.—The natives of the Aleutian Islands, where no trees grow, depend upon the drift-wood cast ashore there for all the timber used in the construction of their boats, fishing-tackle, and household gear. Among this timber, the camphor-tree, and other woods of China and Japan, are said to be often recognised. In this fact we have additional evidence touching this China Stream, as to which (§ 395) but little, at best, is known. "The Japanese," says Lieutenant Bent,^[10] in a paper read before the American Geographical Society, January, 1856, "are well aware of its existence, and have given it the name of 'Kuro-Siwo,' or Black Stream, which is undoubtedly derived from the deep blue colour of its water, when compared with that of the adjacent ocean." From this we may infer (§71) that the blue waters of this China Stream also contain more salt than the neighbouring waters of the sea.

397. The cold current of Okotsk.—Inshore of, but counter to the "Black Stream," along the eastern shores of Asia, is found (§ 391) a streak or layer, or current of cold water answering to that between the Gulf Stream and the American coast. This current, like its fellow in the Atlantic, is not strong enough at all times sensibly to affect the course of navigation; but, like that in the Atlantic, it is the nursery (§ 158) of most valuable fisheries. The fisheries of Japan are nearly as extensive as those of Newfoundland, and the people of each country are indebted for their valuable supplies of excellent fish to the cold waters which the currents of the sea bring down to their shores.

398. Humboldt's Current.—The currents of the Pacific are but little understood. Among those about which most is thought to be known is the Humboldt Current of Peru, which the great and good man whose name it bears was the first to discover. It has been traced on Plate IX. according to the best information—defective at best—upon the subject. This current is felt as far as the equator, mitigating the rainless climate of Peru as it goes, and making it delightful. The Andes, with their snowcaps, on one side of the narrow Pacific slopes of this intertropical republic, and the current from the Antarctic regions on the other, make its climate one of the most remarkable in the world; for, though torrid as to latitude, it is such as to temperature that clothes are seldom felt as oppressive during any time of the year, especially after nightfall. 399. The "desolate" region.—Between Humboldt's Current and the great equatorial flow, there is an area marked as the "desolate region," Plate IX. It was observed that this part of the ocean was rarely visited by the whale, either sperm or right; why, it did not appear; but observations asserted the fact. Formerly, this part of the ocean was seldom whitened by the sails of a ship, or enlivened by the presence of man. Neither the industrial pursuits of the sea nor the highways of commerce called him into it. Now and then a roving cruiser or an enterprising whale-man passed that way; but to all else it was an unfrequented part of the ocean, and so remained until the gold-fields of Australia and the guano islands of Peru made it a thoroughfare. All vessels bound from Australia to South America now pass through it, and in the journals of some of them it is described as a region almost void of the signs of life in both sea and air. In the South Pacific Ocean especially, where there is such a wide expanse of water, sea-birds often exhibit a companionship with a vessel, and will follow and keep company with it through storm and calm for weeks together. Even those kinds, as the albatross and Cape pigeon, that delight in the stormy regions of Cape Horn and the inhospitable climates of the Antarctic regions, not unfrequently accompany vessels into the perpetual summer of the tropics. The sea-birds that join the ship as she clears Australia will, it is said, follow her to this region, and then disappear. Even the chirp of the stormy-petrel ceases to be heard here, and the sea itself is said to be singularly barren of life.

400. Polynesian drift.—In the intertropical regions of the Pacific, and among the heated waters of Polynesia, a warm current or drift of immense volume has its genesis. It rather drifts than floats to the south, laving as it goes, the eastern shore of Australia and both shores of New Zealand. These are the waters in which the little corallines delight to build their atolls and their reefs. The intertropical seas of the Pacific afford an immense surface for evaporation. No rivers empty there; the annual fall of rain there, except in the "Equatorial Doldrums," is small, and the evaporation is all that both the north-east and the south-east trade-winds can take up and carry off. I have marked on Plate IX. the direction of the supposed warm-water current which conducts these over-heated and briny waters from the tropics in mid-ocean to the extra-tropical regions where precipitation is in excess. Here, being cooled, and agitated, and mixed up with waters that are less salt, these over-heated and over-salted waters from the tropics are replenished and restored to their rounds in the wonderful system of oceanic navigation.

401. Equatorial currents.—There are also about the equator in this ocean some curious currents, which I have called the "Doldrum Currents" of the Pacific, but which I do not understand, and as to which observations are not sufficient yet to afford the proper explanation or description. There are many of them, some of which at times run with great force. On a voyage from the Society to the Sandwich Islands I encountered one running at the rate of ninety-six miles a day. These currents are generally found setting to the west. They are often, but not always, encountered in the equatorial Doldrums on the voyage between the Society and the Sandwich Islands. In Captain Pichon's abstract log of the French corvette "L'Eurydice," from Honolulu to Tahiti, in August, 1857, a "doldrum" current is recorded at 79 miles a day west by north. He encountered it between 1° N. and 4° S., where it was 300 miles broad. On the voyage to Honolulu in July of the same year, he experienced no such current; but in 6° N. he encountered one of 36 miles, setting south-east, or nearly in the opposite direction. This current does not appear to have been more than 60 miles broad. What else should we expect in this ocean but a system of currents and counter-currents apparently the most uncertain and complicated? The Pacific Ocean and the Indian Ocean may, in the view we are about to take, be considered as one sheet of water. This sheet of water covers an area quite equal in extent to one-half of that embraced by the whole surface of the earth; and, according to Professor Alexander Keith Johnston, who so states it in the new edition of his splendid Physical Atlas, the total annual fall of rain on the earth's surface is one hundred and eighty-six thousand two hundred and forty cubic imperial miles. Not less than three-fourths of the vapour which makes this rain comes from this waste of waters; but supposing that only half of this quantity, i.e., ninety-three thousand one hundred and twenty cubic miles of rain falls upon this sea, and that that much, at least, is taken up from it again as vapour, this would give two hundred and fifty-five cubic miles as the quantity of water which is daily lifted up and poured back again into this expanse. It is taken up at one place and rained down at another, and in this process, therefore, we have agencies for multitudes of partial and conflicting currents—all in their set and strength, apparently as uncertain as the winds.

402. The influence of rains and evaporation upon currents.—The better to appreciate the operation of such agencies in producing currents in the sea, nowhere, now there, first this way, and then that, let us, by way of illustration, imagine a district of two hundred and fifty-five square miles in extent to be set apart, in the midst of the Pacific Ocean, as the scene of operations for one day. We must now conceive a machine capable of pumping up, in the twenty-four hours, all the water to the depth of one mile in this district. The machine must not only pump up and bear off this immense quantity of water, but it must discharge it again into the sea on the same day, but at some other place. Now here is a force for creating currents that is equivalent in its results to the effect that would be produced by baling up, in twenty-four hours, two hundred and fifty-five cubic miles of water from one part of the Pacific Ocean, and emptying it out again upon another part. The currents that would be created by such an operation would overwhelm navigation and desolate the sea; and, happily for the human race, the great atmospherical machine which actually" does perform every day, on the average, all this lifting up, transporting and letting down of water upon the face of the grand ocean, does not confine itself to an area of two hundred and fifty-five square miles, but to an area three hundred thousand times as great; yet, nevertheless, the same quantity of water is kept in motion, and the currents, in the aggregate, transport as much water to restore the equilibrium as they would have to do were all the disturbance to take place upon our hypothetical area of one mile deep over the space of two hundred and fifty-five square miles. Now when we come to recollect that evaporation is lifting up, that the winds are transporting, and that the clouds are letting down every day actually such a body of water, we are reminded that it is done by little and little at a place, and by hairs' breadths at a time, not by parallelopipedons one mile thick, and that the evaporation is most rapid and the rains most copious, not always at the same place, but now here, now there. We thus see actually existing in nature a force perhaps quite sufficient to give rise to just such a system of currents as that which mariners find in the Pacific (§ 401)—currents which appear to rise in mid ocean, run at unequal rates, sometimes east, sometimes west, but which always lose themselves where they rise, viz., in mid ocean.

403. Under currents—Parker's deep-sea sounding.—Lieutenant J. C. Walsh, in the U. S. schooner "Taney," and Lieutenant-S. P. Lee, in the U. S. brig " Dolphin," both, while they were carrying on a system of observations in connection with the Wind and Current Charts, had their attention directed to the subject of submarine currents. They made some interesting experiments upon the subject. A block of wood was loaded to sinking, and, by means of a fishing-line or a bit of twine, let down to the depth of one hundred or five hundred fathoms, at the will of the experimenter. A small barrel as a float, just sufficient to keep the block from sinking farther, was then tied to the line, and the whole let go from the boat. To use their own expressions, "It was wonderful, indeed, to see this barrega move off, against wind, and sea, and surface current, at the rate of over one knot an hour, as was generally the case, and on one occasion as much as 1¾ knots. The men in the boat could not repress exclamations of surprise, for it really appeared as if some monster of the deep had hold of the weight below, and was walking off with it."^[11] Both officers and men were amazed at the sight. The experiments in deep-sea soundings, have also thrown much light upon the subject of under currents. There is reason to believe that they exist in all, or almost all parts of the deep sea, for never in any instance yet has the deep-sea line ceased to run out, even after the plummet had reached the bottom. If the line be held fast in the boat, it invariably parts, showing, when two or three miles of it are out, that the under currents are sweeping against the bight of it with what seamen call a swigging force, that no sounding twine has yet proved strong enough to withstand. Lieutenant J. P. Parker, of the United States frigate "Congress," attempted, in 1852, a deep-sea sounding off the coast of South America. He was engaged with the experiment eight or nine hours, during which time a line nearly ten miles long was payed out. Night coming on, he had to part the line (which he did simply by attempting to haul it in) and return on board. Examination proved that the ocean there, instead of being over ten miles in depth, was not over three, and that the line was swept out by the force of one or more under currents. But in what direction these currents were running is not known.

404. The compressibility of water—effect of in the oceanic circulation.'—Vertical circulation is as important in the sea as it is in the air (§ 231). In striving to understand the physical machinery of our planet and to comprehend its workings, we must, if we would learn, proceed upon the principle (§ 351) that at creation the waters were measured, the hills weighed, and the atmosphere meted out, and that each was endowed with its peculiar properties so proportioned and so adjusted as exactly to answer its purposes in the grand design. And, consequently, we are entitled to infer that fluidity instead of solidity was imparted to a certain quantity of matter which we call water, to enable it to perform the offices to be required of fluid matter, and which, in the terrestrial economy, solid matter was not adapted to perform. By this mode of reasoning we are taught to regard the fluidity of all the water in the sea as a physical necessity—and by this mode of reasoning we are required to reject as insufficient, any hypothesis touching the system of aqueous circulation on our planet which ignores, even in the profoundest depths of the ocean, an interchange of its particles between the bottom and the top. Were such interchange not to take place—were the water in the sea which once sinks below the level of its horizontal circulation doomed to remain there for ever, it would not be difficult to show that the sea would lose its balance and its counterpoises; that, not being able to preserve its status, the water at the bottom would have grown heavier and heavier, while that at the top would have become lighter and lighter, until the one became saturated with salt, the other entirely fresh. To prevent this state of things, we recognize the influences of the winds and tides, as well as the necessity of vertical movements in the sea. Whence, therefore, let us inquire, when a given quantity of water once finds its way to the bottom of the sea, whence—since it goes there by virtue of its own specific gravity, whence is power to be derived for bringing it up again? for sooner or later, according to this view, up it must come. We thus arrive precisely at one of those points (§ 287) at which hypothesis becomes absolutely necessary if we would make further progress. Here, therefore, let us pause to search among the physics of the sea for such a power and the foundation for hypothesis. Leslie has pointed out exactly such a power for the atmospheric ocean,—a power which, after the heaviest air has settled at the bottom of its subtile sea—after the lightest has come to rest at the top, and the whole arranged itself according to specific gravity—can haul that which is below to the top, and send that which is on the top down into the recesses and cavities below. Suppose the entire atmosphere to be, from the bottom to the top, nearly of the same temperature, and in a perfect state of the quiescent equilibrium, and that from some cause a certain volume of air above has its specific gravity so changed that it commences to descend. As it descends the pressure upon it increases—and air, being compressed, contracts and gives out heat. A like volume ascends to take its place, and in ascending it expands and grows cool. Thus the total mass, and the total pressure, and the total amount of caloric remain the same; but there is a transfer of heat from the top to the bottom, by which the equilibrium of the mass is destroyed, and a force established at the bottom of the atmospherical ocean which, with the assistance of an agent at the top to alter specific gravity, is capable of sending up the heavy air from the bottom, of drawing down the light from the top, and of turning, in course of time, the whole atmosphere upside down. All philosophers acknowledge the power of this omnipresent agent in the air, and that, by alternately assuming the latent and the sensible form, it, to say the least, assists to give to the atmosphere the dynamical force required for its system of vertical circulation as well as its horizontal. So with water and the salt sea where we do have an agent that is continually altering specific gravity at the surface. Notwithstanding the Florentine experiment upon water in the gold ball, it has since been abundantly proved that water is compressible—so much so, that at the depth of ninety-three miles its density would be doubled. Consequently, a given quantity of water—such, for instance, as a cubic foot measured at the surface—would not, if sunk to the depth of four miles, measure a cubic foot by seventy-two cubic inches. As a rule, the compressibility of water in the depths of the sea is one per cent, for every 1000 fathoms. Here, then, in the latent heat which is liberated in the processes of descent, have we not a power which is capable of sending up to the top water from the uttermost depths of the sea? Suppose that this cubic measure of water, by supplying vapour to the winds at the surface, to have its saltness so increased as to alter its specific gravity to sinking: Like the air, it is compressed, and contracts in its descent, giving out heat, raising the temperature, and changing the specific gravity of like quantities in the various thermal strata through which it has to pass. Thus heat is conveyed from the top to the bottom of the sea, there to be liberated and impart to its waters dynamical force for their upward movement. This is the power we paused to search for: whatever be its amount it is in the nature of a vera causa, and we must therefore recognize it, if not as the sole agent, nevertheless as one of the principal agents which nature employs in the system of vertical circulation that has been ordained for the waters of the sea.

405. Assisted by its salts.—Now, but for the salts of the sea this process could not go on so long as the laws of thermal dilatation remain as they are for sea water. Unlike fresh water, which expands as it is cooled below 39°. 5, sea water contracts until it has passed its freezing-point and attained the temperature of 25°.6.^[12] Were it not for its salts, sea water once near the surface within the tropics would, by reason of its warmth and thermal dilatation, remain near the surface. Vertical circulation would be confined to polar seas, and many of the living creatures that inhabit its waters would perish for the lack of currents to convey them their food.

406. The origin of currents.—If we except the tides, and the partial currents of the sea, such as those that may be created by the wind, we may lay it down as a rule (§ 103) that all the currents of the ocean owe their origin to difference of specific gravity between sea water at one place and sea water at another; for wherever there is such a difference, whether it be owing to difference of temperature or to difference of saltness, etc., it is a difference that disturbs equilibrium, and currents are the consequences. The heavier water goes towards the lighter, and the lighter whence the heavier comes; for two fluids differing in specific gravity (§ 106), and standing at the same level, can no more balance each other than unequal weights in opposite scales of a true balance. It is immaterial, as before stated, whether this difference of specific gravity be caused by temperature, by the matter held in solution, or by any other thing; the effect is the same, namely, a current. That the sea, in all parts, holds in solution the same kind of solid matter; that its waters in this place, where it never rains, are not Salter than the strongest brine; and that in another place, where the rain is incessant, they are not entirely without salt, may be taken as evidence in proof of a system of currents or of circulation in the sea, by which its waters are shaken up and kept mixed together as though they were in a phial.

407. Currents of the Atlantic.—The principal currents of the Atlantic have been described in the chapter on the Gulf Stream. Besides this, its eddies and its offsets are the equatorial current (Plate VI.), and the St. Roque or Brazil Current. Their fountain-head is the same: it is in the warm waters about the equator, between Africa and America. The former, receiving the Amazon and the Orinoco as tributaries by the way, flows into the Caribbean Sea, and becomes, with the waters (§ 103) in which the vapours of the trade-winds leave their salts, the feeder of the Gulf Stream. The Brazil current, coming from the same fountain, is supposed to be divided by Cape St. Roque, one branch going to the south under this name (Plate IX.), the other to the westward. This last has been a great bugbear to navigators, principally on account of the difficulties which a few dull vessels falling to leeward of St. Roque have found in beating up against it. It was said to have caused the loss of some English transports in the last century, which fell to leeward of the Cajoe on a voyage to the other hemisphere; and navigators, accordingly, were advised to shun it as a danger.

408. The St. Roque current.—This current has been an object of special investigation during my researches connected with the Wind and Current Charts, and the result has satisfied me that as a rule it is neither a dangerous nor a constant current, notwithstanding older writers. Horsburgh, in his East India Directory, cautions navigators against it; and Keith Johnston, in his great Physical Atlas, published in 1848, thus speaks of it: "This current greatly impedes the progress of those vessels which cross the equator west of 23° west longitude, impelling them beyond Cape St, Roque, when they are drawn towards the northern coast of Brazil, and cannot regain their course till after weeks or months of delay and exertion." So far from this being the case, my researches abundantly prove that vessels which cross the equator five hundred miles to the west of longitude 23° have no difficulty on account of this current in clearing that cape. I receive almost daily the abstract logs of vessels that cross the equator west of long. 30°, and in three days from that crossing they are generally clear of that cape. A few of them report the current in their favour; most of them experience no current at all; but, now and then, some do find a current setting to the northward and westward, and operating against them at the rate of 20 and occasionally of 50 miles a day. The intertropical regions of the Atlantic, like those of the other oceans (§ 401), abound with conflicting currents, which no researches yet have enabled the mariner to unravel so that he may at all times know where they are and tell how they run, in order that he may be certain of their help when favourable, or sure of avoiding them if adverse.

409. The Greenland current.—There are other currents, such as the Greenland Current, the cold current from Davis' Strait, the ice-bearing current from the antarctic regions, all setting into the Atlantic and the Gulf Stream, one branch of which finds its way into the Arctic Sea; the other (§89) finds its way back to the south partly as Rennell's current, all of which have been fully treated of in Chap. II., or are delineated on Plates VI. and IX. Judging by these, there would seem to be a larger flow of polar waters into the Atlantic than of other waters from it; and I cannot account for the preservation of the equilibrium of this ocean by any other hypothesis than that which calls in the aid of under currents. They, I have no doubt, like the water-ways, the mineral veins, the passages in the bowels of the earth, bear in their secret ways, an important part in the grand system of the terrestrial economy.

1. See an interesting paper by him on Tidal Streams of the North Sea and English Channel, p. 708 ; Phil. Transactions, Part ii., 1851.
2. Johnston's Physical Atlas.
3. Transact, of the Bombay Geograph. Sec. vol. ix., May, 1849, to August, 1850.
4. Dr. Smith appears to have been the first to conjecture this explanation, which he did in 1673 (vide Philosophical Transactions). This continual indraught into the Mediterranean appears to have been a vexed question among: the navigators and philosophers even of those times. Dr. Smith alludes to several hypotheses which had been invented to solve these phenomena, such as subterraneous vents, cavities, exhalation by the sun's beams, etc., and then offers his conjecture, which, in his own words, is, "that there is an under current, by which as great a quantity of water is carried out as comes flowing in. To confirm which, besides what I have said above about the difference of tides in the oflSng and at the shore in the Downs, which necessarily supposes an under current, I shall present you with an instance of the like nature in the Baltic Sound, as I received it from an able seaman who was at the making of the trial. He told me that, being there in one of the king's frigates, they went with their pinnace into the mid stream, and were carried violently by the current; that, soon after this, they sunk a bucket with a heavy cannon ball to a certain depth of water, which gave a check to the boat's motion; and, sinking it still lower and lower, the boat was driven ahead to the windward against the upper current: the current aloft, as he added, not being over four or five fathoms deep, and that the lower the bucket was let fall, they found the under current the stronger."
5. "The Mediterranean."
6. One hundred and sixty, Smyth.
7. Lyell's Principles of Geology, p. 334-5, ninth edition. London, 1853.
8. "From near its mouth at the Belize, a steam-boat may ascend for two thousand miles with scarcely any perceptible difference in the width of the river."—Lyell, p. 263.
9. "The Mississippi is continually shifting its course in the great alluvial plain, cutting frequently to the depth of one hundred, and even sometimes to the depth of two hundred and fifty feet"—Lyell, p. 273.
10. Lieutenant Bent was in the Japan Expedition with Commodore Perry, and used the opportunities thus afforded to study the phenomena of this stream.
11. Lieutenant Walsh.
12. See Prof. Hubbard's experiments, vol. i., Sailing Directions.