CHAPTER VI. — CURRENTS OF THE SEA.
Governed by Laws, § 232. — The Inhabitants of the Sea the Creatures of Climate, 233. — The Currents of the Sea an Index to its Climates, 235. — First Principles, 236. — Some Currents run up hill, 237. — Currents of the Red Sea, 238. — Top of that Sea an inclined Plane, 240. — How an under Current from it is generated, 245. — Specific Gravity of Sea Waters, 248. — Why the Red Sea is not salting up, 251. — Mediterranean Currents: How we know there is an under Current from this Sea, 252. — The sunken Wreck which drifted out, 253. —Both Currents caused by the Salts of the Sea, 254. — Currents Of The Indian Ocean: Why immense Volumes of warm Water flow from it, 255. — A Gulf Stream along the Coast of China, 256. — Points of Resemblance between it and the Gulf Stream of the Atlantic, 257. — A Current into Bering’s Strait, 258. — Geographical Features unfavorable to large Icebergs in the North Pacific, 260. — Necessity for cold to restore the Waste by the warm Currents, and Evaporation, 261. — Arguments in favor of return Currents, because Sea Water is salt, 262. — Currents Of The Pacific: Its Sargasso Sea, 264. — The Drift on the Aleutian Islands, 265. The cold China Current, 266. — Humboldt’s Current, 267. — Discovery of an immense Body of warm Water drifting South, 268. — Currents about the Equator, 270. — Under Currents: Experiments of Lieutenants Walsh and Lee, 271. — Proof of under Currents afforded by Deep Sea Soundings, 272. — Currents caused by Changes in Specific Gravity of Sea Water, 273. — Constituents of Sea Water every where the same; affords Evidence of a system of Oceanic Circulation, 274. — Currents Of The Atlantic: The great Equatorial Current: its Fountain-head, 275. — The Cape St. Roque Current proved to be not a constant Current, 276. — Difficulties of understanding all the Currents of the Seashore of the Atlantic can not be accounted for without the aid of under Currents, 277.
232. Let us, in this chapter, 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 physical laws. The sea, by the circulation of its waters, 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, reason assures us that they move in obedience to some law of Nature, be it recorded down in the depths below, never so far beyond the reach
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of human kind; and being a law of Nature, we know who gave it, and that neither chance nor accident had any thing 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 (§ 65), and all its inhabitants proclaim it.
233. The fauna and the flora of the sea are as much the creatures of climate (§ 66), 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 alga, the marine insect and the coral, distributed equally and alike in all parts of the ocean. The polar whale would delight in the torrid zone, and the habitat of the pearl oyster would be also under the iceberg, or in frigid waters colder than the melting ice.
234. Now 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.
235. 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.
236. 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 obliged 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.
237. It is not necessary to associate with oceanic currents the idea that they must of necessity, as on land, run from a higher to 12.5 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 (§ 10).
238. 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 from 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 13º to the parallel of 30º north
239. 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.* 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, we may suppose the daily evaporation to be immense; not less, certainly, than half an inch, and probably twice that amount. 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; and none of the vapors, which the scorching winds that blow over it carry away, are returned to it again in the shape of rains.
240. The Red Sea vapors are carried off and precipitated elsewhere. The depression in the level of its head waters in the summer time, therefore, it appears, is owing quite as much to the effect of evaporation as to that of the wind blowing the waters back.
- Johnston’s Physical Atlas.
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241. The evaporation in certain parts of the Indian Ocean (§ 33) is from three fourths of an inch to an inch daily. Suppose it for the Red Sea 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, lose twenty-five inches from its surface.
242. Thus the waters the Red Sea ought to be lower at the Isthmus of Suez than they are at the Straits of Bab-el-mandeb. Independently of the waters forced out by the wind, they 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 300, than it is at Bab-el-mandeb, in latitude 13º.
243. 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 Bab-el-mandeb, and to flow up the channel 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.
244. The top of that sea, therefore, may be regarded as an inclined plane, made so by evaporation.
245. 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 can not 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 or 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.
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246. 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 long trough, opening into a vat of oil, with a partition to keep the oil from running into the trough. Now suppose the trough to be filled up with wine on one side of the partition to the level of the oil on the other. 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 partition 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.
247. The rivers which discharge in 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 equilibrium of the seas is preserved, beyond a doubt, by a system of compensation as exquisitely adjusted as are those by which the “music of the spheres” is maintained.
248. The above about under currents is theory: Now let us see the results of actual observation upon the density of water in the Red Sea and the Mediterranean, and upon the under currents that run out from these seas.
Four or five years ago, Mr. Wm. M. 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 Bab-el-mandeb, which were afterward examined by Dr. Giraud, who reported the following results:
[CHART]
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249. 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.
250. In the same paper, the temperature of the air between Suez and Aden often rises, it is said, to 90º, “and probably averages little less than 75º day and night all the year round. The surface of the sea varies in heat from 65º to 85º, and the difference between the wet and dry bulb thermometers often amounts to 250-in the kamsin, or desert winds, to from 30º to 40º; 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 the sea, will be carried off annually in vapor; 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.”
251. 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.
252. 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, rmoreover, that sea is not salting up; and therefore, independently of the postulate (§ 236) and of observations (§ 253), we might infer the existence of an under current, through which this salt finds its way out into the broad ocean again.
Dr. Smith appears to have been the first to conjecture this explanation, which he did in 1683 (vide Philosophical Transactions).
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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 Ph(enix, of Marseilles, giving chase near Ceuta Point to a Dutch ship bound to Holland, came up with 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 toward Ceuta, and so upward. 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.” 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 dis 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, &c., 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 offing 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.”
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Tilled water by more than four times the usual excess, and accord ingly 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 perpet ual 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. who 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. However that may be, these facts, and the statements of the Secretary of the Geographical Society of Bombay (§ 250), 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 (§ 245) by the salts of the sea.
253. Writers whose opinions are entitled to great respect differ with me as to the proof of this demonstration. Among these 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 (§ 252) taken fifty miles within the Straits from the depth of six hundred and seventy
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fathoms (four thousand and twenty feet), which, being four times salter than common sea water, left, as we have just seen (§ 252), 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 upward of nine hundred fathoms, yet in the Straits themselves the depth across the shoalest section is not more than one hundred and sixty * 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.t 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.” t
254. 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 be salt, for the rivers and the rains, it is admitted, bring the salts from the land and empty them into the sea. It is also admitted that the great lakes would, from this cause, be salt, if they had no sea drainage. The Niagara River passes these river salts from the upper lakes into Ontario, and the St. Lawrence conveys them thence to the sea. Now the basins or bottoms of all these upper lakes are far below the top of the rock over which the Niagara pitches its flood. And, were the position assumed by this writer correct, viz., that if the water in *
“The Mediterranean.” t One hundred and sixty, Smyth. t Lyell’s Principles of Geology, p. 334-5, ninth edition. London, 1853.
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any of these lakes should, in consequence of its specific gravity, once sink below the level of the shoals in the rivers and straits which connect them, it never could flow out again, and consequently must remain there forever* — were this principle physically correct, would not the water at the bottom of the lakes gradually have received salt sufficient, during the countless ages that they have been sending it off to the sea, to make this everlastingly pent-up water briny, or at least quite different in its constituents from that of the surface? 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 forever 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. 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 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. Thus 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 varying from one hundred to twenty yards a year. *
- See paragraph (§ 253) from “Lyell’s Principles of Geology”
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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.*
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 below the top of the bar which obstructs its entrance into the sea. Could not the same power which scoops out this solid matter draw the brine up from the pool in the Mediterranean, and pass it out across the barrier in the Straits? The traction of locomotives on rail-roads and the force of that traction are well understood. Now have not currents in the deep sea power derived from some such force? Suppose this under current from the Mediterranean to extend one hundred and sixty fathoms down, so as to chafe the barrier across the Straits. Upon the bottom of this current, then, there is a pressure of more than fifty atmospheres. Have we not here a source of power that would be capable of drawing up, by almost an insensibly slow motion, water from almost any depth? At any rate, it appears that the effect of currents by traction, or friction, or whatever force, does extend far below the level of their beds in shallow places. Were it not so — were the brine not drawn out again — it 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 crystals*
“From near its mouth at the Balize, a steam-boat may ascend for two thousand miles with scarcely any perceptible difference in the width of the river.”—Lyell, p. 263.
“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. 134
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Admiral Smyth brought up bottom with his briny sample of deep sea water (six hundred and seventy fathoms), but no salt crystals. 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 how did a brine spring of sea water get under the sea but through the process of evaporation on the surface, or by parting with a portion of its fresh water in some other way? 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 thence forward forever 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. I do not believe in the existence of any such imperfect terrestrial mechanism, or in any such failures of design. To my mind, the proofs — the theoretical proofs — the proofs derived exclusively from reason and analogy — are as clear in favor of this under current from the Mediterranean as they were in favor 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 but again. It would not be difficult to show, even to the satisfaction of that eminent geologist, that this in draught conveys salt away from the Atlantic faster than all the fresh-water rivers empty fresh supplies of salt into the ocean. Now, besides this drain, vast quantities of salts are extracted from sea water for madrepores, 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 un-philosophical conclusion that the sea must be losing its salts, and becoming less and less briny.
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255. 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 (§ 146) 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 neighboring shores (§ 139). 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 Lagullas current.
256. Another of these currents 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. Then it attempts the great circle route (§ 69) for the Aleutian Islands, tempering climates, and losing itself in the sea on its route toward the northwest coast of America.
257. Between the physical features of this current and the Gulf Stream of the Atlantic there are several points of resemblance. Sumatra and Malacca correspond to Florida and Cuba; Borleo 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 countercurrent 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 are duplicates of those of Western Europe and the British Islands;
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the climate of California (State) resembling that of Spain; the sandy plains and rainless regions of Lower California reminding one of Africa, with its deserts between the same parallels, &c. 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 as renowned for fogs and mists as are the Grand Banks of Newfoundland.
258. A surface current flows north through Bering’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; Bering’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.
259. Bering’s Strait, in geographical position, answers to Davis’s 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, the Pacific shore-line intervenes, and turns them down through a sort of North Sea along the western coast of the continent toward Mexico.
260. These contrasts show the principal points of resemblance and of difference between the currents and aqueous circulation in the two oceans. The ice-bearing currents of the North Atlantic are not repeated as to degree in the North Pacific, for there is no nursery for icebergs like the frozen ocean and its arms. The seas of Okotsk and Kamtschatka alone, and not the frozen seas of the Arctic, cradle the icebergs for the North Pacific. There is, at times at least, another current of warm water from the Indian Ocean. It finds its way south midway between Africa and Australia. The whales (Plate IX.) give indications of it. Nor need we be surprised at such a vast flow of warm wate, as these three currents indicate from the Indian Ocean, when we recollect that this ocean (§ 255) is land-locked on the north, and that the temperature of its waters is frequently as high as 90º Fahrenheit.
261. There must, therefore, be immense volumes of water flowing into the Indian Ocean to supply the waste created by these warm currents, and the fifteen or twenty feet of water that observations ( 33) tell us are yearly carried off from this ocean by evaporation.
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On either side of this warm current that escapes from the intertropical parts of the Indian Ocean (§ 260), midway between Africa and Australia, 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. These cold currents sometimes get as far north with their icebergs as 40º south. The Gulf Stream seldom permits them to get so near the equator as that 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 37º south latitude. This is the nearest approach of icebergs to the equator.
262. These currents which run out from the inter-tropical basin of that immense sea — Indian Ocean — are active currents. They 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 or return to the inter-tropical 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.
263. THE CURRENTS OF THE PACIFIC. — The contrast has been drawn (§ 257) between the China or “Gulf Stream” of the North Pacific, and the Gulf Stream of the North Atlantic. The course of the China Stream has never been 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 toward the Cape Verde Islands.
264. 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.
265. 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 recognized. In this fact we have additional evidence touching this China Stream, as to which (§ 263) but little, at best, is known.
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266. THE COLD ASIATIC CURRENT.— Inshore of, but counter to the China current, along the eastern shores of Asia, is found (§ 257) 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 (§ 65) of most valuable fisheries. The fisheries of Japan are quite 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.
267. 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.
268. I have, I believe, discovered the existence of a warm current in the inter-tropical regions of the Pacific, midway between the American coast and the shore-lines of Australia.
269. This region affords an immense surface for evaporation. No rivers empty into it; the annual fall of rain, except in the “Equatorial Doldrums,” is small, and the evaporation is all that both the northeast and the southeast 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 overheated 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 overheated and over-salted waters from the tropics may be replenished and restored to their rounds in the wonderful system of oceanic circulation.
270. There are also about the equator in this ocean some curious currents 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,
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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. And 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 eightysix thousand, two hundred and forty cubic imperial miles. Not less than three fourths of the vapor 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 vapor, 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. The better to appreciate the operation of such agencies in producing currents in the sea, now here, 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 effects that would be produced by bailing 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
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operation would overwhelm navigation and desolate the sea; and, happily for the human race, the great atmospherical machine (§ 90), 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 do let down every day actually such a body of water, but that it is done by little and little at a place, and by hair’s breadths at a time, not by parallelopipedons one mile thick — that the evaporation is most rapid and the rains most copious, not always at the same place, but now here, now there, we shall see actually existing in nature a force sufficient to give rise to just such a system of currents as that which mariners find in the Pacific-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.
271. UNDER CURRENTS. — Lieutenant J. C. Walsh, in the United States schooner “Taney,” and Lieutenant S. P. Lee, in the United States 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 fishingline or a bit of twine, let down to the depth of one hundred or five hundred fathoms (six hundred or three thousand feet). A small 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 13 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.” * Both officers and men were amazed at the sight.
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272. The experiments in deep-sea soundings have also throwfi 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 paid 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.
273. It may, therefore, without doing any violence to the rules of philosophical investigation, be conjectured, that the equilibrium of all the seas is preserved, to a greater or less extent, by this system of currents and counter-currents at and below the surface. 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 (§ 34) 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, &c., it is a difference that disturbs equilibrium, and currents are the consequence. The heavier water goes toward the lighter, and the lighter whence the heavier comes; for two fluids differing in specific gravity (§ 36), and standing at the same
- Lieutenant Walsh.
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level, can not balance each other. 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.
274. 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. Moreover, we may lay it down as a law in the system. of oceanic circulation, that every current in the sea has its counter current; in other words, that the currents of the sea are, like the nerves of the human system, arranged in pairs; for wherever one current is found carrying off water from this or that part of the sea, to the same part must some other current convey an equal volume of water, or else the first would, in the course of time, cease for the want of water to supply it.
275. 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 Oronoco as tributaries by the way, flows into the Caribbean Sea, and becomes, with the waters (§ 35) in which the vapors 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 Cape on a voyage to the other hemisphere; and navigators, accordingly, were advised to shun it as a danger.
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276. 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 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 grand Physical Atlas, published in 1848, thus speaks of it: “This current greatly impedes the progress of those vessels which cross the equator west of 23º west longitude, impelling them beyond Cape St. Roque, when they are drawn toward the northern coast of Brazil, and can not 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 23º west 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 30º west, and in three days from that crossing they are generally clear of that cape. A few of them report the current in their favor; 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 twenty miles a day.
The inter-tropical regions of the Atlantic, like those of the other oceans (§ 270), 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 the navigator may be certain of their help when favorable, or sure of avoiding them if adverse.
277. I may here remark, that there seems to be a larger flow of polar waters into the Atlantic than of other waters from it, and I can not 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, bear an important part in the system of oceanic circulation. Admiral Sir Francis Beaufort, the venerable hydrographer of England, made, when in command of her Britannic majesty’s frigate Frederiksteen, in the Mediterranean, some interesting experiments upon under currents, which I should be glad to see repeated in other parts of the sea, especially between the tropics, in the Atlantic, Pacific, and Indian Oceans, and wherever the water is remarkably transparent.
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That officer says: “The counter currents, or those which return beneath the surface of the water, are also very remarkable; in some parts of the Archipelago they are at times so strong as to prevent the steering of the ship; and, in one instance, on sinking the lead, when the sea was calm and clear, with shreds of bunting of various colors attached to every yard of the line, they pointed in different directions all round the compass.” These shreds of bunting probably “tailed out” straight from the line, not in consequence of currents, but by reason of their specific gravity, and their tendency to lie straight out from, rather than to coil around, the line. At any rate, it is an interesting experiment, and I hope some of those noble-hearted mariners who are co-operating with me in collecting facts concerning the physical laws of the sea will avail themselves of calms to repeat the experiment. A submarine kite — that is, a contrivance upon the principle of a boy’s kite in the air — let down at various depths in the sea, would indicate both the direction and force of the under currents.