Physical Geography of the Sea and its Meteorology/Chapter 2

by Matthew Fontaine Maury

CHAPTER II.

§ 70-147.—THE GULF STREAM.

70. Its colour.—There is a river in the ocean: in the severest droughts it never fails, and in the mightiest floods it never overflows; its banks and its bottom are of cold water, while its current is of warm; it takes its rise in the Gulf of Mexico, and empties into Arctic seas; this mighty river is the Gulf Stream. There is in the world no other such majestic flow of waters. Its current is more rapid than the Mississippi or the Amazon, and its volume more than a thousand times greater. Its waters, as far out from the gulf as the Carolina coasts, are of indigo blue. They are so distinctly marked that their line of junction with the common sea-water may be traced by the eye. Often one-half of the vessel may be perceived floating in Gulf Stream water, while the other half is in common water of the sea—so sharp is the line, and such the want of affinity between those waters, and such, too, the reluctance, so to speak, on the part of those of the Gulf Stream to mingle with the littoral waters of the sea.

71. How caused.—At the salt-works of France, and along the shores of the Adriatic, where the "salines" are carried on by the process of solar evaporation, there is a series of vats or pools through which the water is passed as it comes from the sea, and is reduced to the briny state. The longer it is exposed to evaporation, the salter it grows, and the deeper is the hue of its blue, until crystallization is about to commence, when the now deep blue water puts on a reddish tint. Now the water of the Gulf Stream is salter (§ 102) than the littoral water of the sea through which it flows, and hence we can account for the deep indigo blue which all navigators observe in Gulf Stream water off the Carolina coasts. The salt-makers are in the habit of judging of the richness of sea-water in salt by its colour—the greener the hue the fresher the water. We have in this, perhaps, an explanation of the contrasts which the waters of the Gulf Stream present with those of the Atlantic, as well as of the light green of the North Sea and other Polar waters; also of the dark blue of intertropical seas, and especially of the Indian Ocean, which poets have described as the "black waters." Seamen who visit the Falls of Niagara never fail to remark upon the beautiful green of the water in the river below, and to contrast it with the dark blue of the sea in the trade-wind regions.

72. Speculations concerning the Gulf Stream.—What is the cause of the Gulf Stream has always puzzled philosophers. Many are the theories and numerous the speculations that have been advanced with regard to it. Modern investigations and examinations are beginning to throw some light upon the subject, though all is not yet entirely clear. But they seem to encourage the opinion that this stream, as well as all the constant currents of the sea, is due mainly to the constant difference produced by temperature and saltness in the specific gravity of water in certain parts of the ocean. Such difference of specific gravity is inconsistent with aqueous equilibrium, and to maintain this equilibrium these great currents are set in motion. The agents which derange equilibrium in the waters of the sea, by altering specific gravity, reach from the equator to the poles, and in their operations they are as ceaseless as heat and cold; consequently they call for a system of perpetual currents to undo their perpetual work.

73. Agencies concerned.—These agents, however, are not the sole cause of currents. The winds help to make currents by pressing upon the waves and drifting before them the water of the sea; so do the rains, by raising its level here and there; and so does the atmosphere, by pressing with more or less superincumbent force upon different parts of the ocean at the same moment, and as indicated by the changes of the barometric column. But when the winds and the rains cease, and the barometer is stationary, the currents that were the consequence cease. The currents thus created are therefore ephemeral. But the changes of temperature and of saltness, and the work of other agents which affect the specific gravity of sea-water and derange its equilibrium, are as ceaseless in their operations as the sun in his course, and in their effect they are as endless. Philosophy points to them as the chief cause of the Gulf Stream and of all the constant currents of the sea.

74. Early writers.—Early writers, however, maintained that the Mississippi River was the father of the Gulf Stream. Its floods, they said, produce it: for the velocity of this river in the sea (§ 70) might, it was held, be computed by the rate of the current of the river on the land.

75. Objection to the fresh-water theory.—Captain Livingston overturned this hypothesis by showing that the volume of water which the Mississippi River empties into the Gulf of Mexico is not equal to the three thousandth part of that which escapes from it through the Gulf Stream. Moreover, the water of the Gulf Stream is salt—that of the Mississippi, fresh; and the advocates of this fresh-water theory (§ 74) forgot that just as much salt as escapes from the Gulf of Mexico through this stream, must enter the Gulf through some other channel from the main ocean; for, if it did not, the Gulf of Mexico, in process of time, unless it had a salt bed at the bottom, or was fed with salt springs from below—neither of which is probable—would become a fresh-water basin.

76. Livingston's hypothesis.—The above quoted argument of Captain Livingston, however, was held to be conclusive; and upon the remains of the hypothesis which he had so completely overturned, he set up another, which, in turn, has also been upset. In it he ascribed the velocity of the Gulf Stream as depending "on the motion of the sun in the ecliptic, and the influence he has on the waters of the Atlantic."

77. Franklin's theory.—But the opinion that came to be most generally received and deep-rooted in the mind of seafaring people was the one repeated by Dr. Franklin, and which held that the Gulf Stream is the escaping of the waters that have been forced into the Caribbean Sea by the trade-winds, and that it is the pressure of those winds upon the water which drives up into that sea-head, as it were, for this stream.

78. Objections to it.—We know of instances in which the waters have been accumulated on one side of a lake, or in one end of a canal, at the expense of the other. The pressure of the trade-winds may assist to give the Gulf Stream its initial velocity, but are they of themselves sufficient to send such a stream of water all the way across the ocean, projecting by a single impress a volume of water from the shores of America to the shores of Europe, that exceeds in discharge the mighty Mississippi a thousand times? Reason teaches and examination shows that they are not. With the view of ascertaining the average number of days during the year that the N.E. trade-winds of the Atlantic operate upon the currents between 25° N. and the equator, log-books containing no less than 380,284^[1] observations on the force and direction of the wind in that ocean were examined. The data thus attributed were carefully compared and discussed. The results show that within those latitudes, and on the average, the wind from the N.E. quadrant is in excess of the winds from the S.W. only 111 days out of the 365. During the rest of the year the S.W. counteract the effect of the N.E. winds upon the currents. Now, can the N.E. trades, by blowing for less than one-third of the time, cause the Gulf Stream to run all the time, and without varying its velocity either to their force or their prevalence?

79. Herschel's explanation.—Sir John Herschel maintains"^[2] that they can; that the trade-winds are the sole cause^[3] of the Gulf Stream; not, indeed, by causing "a head of water" in the West Indian seas, but by rolling particles of water before them somewhat as billiard balls are rolled over the table. He denies to evaporation, temperature, salts, and sea-shells, any effective influence whatever upon the circulation of the waters in the ocean. According to him the winds are the supreme current-producing power in the sea.^[4]

80. Objections to it.—This theory would require all the currents of the sea to set with the winds, or when deflected, to be deflected from the shore, as billiard balls are from the cushions of the table, making the littoral angles of incidence and reflection equal. Now, so far from this being the case, ""not" one of the constant currents of the sea either makes such a rebound or sets with the winds. The Gulf Stream sets, as it comes out of the Gulf of Mexico, and for hundreds of miles after it enters the Atlantic, against the trade-winds; for a part of the way it runs right in the "wind's eye." The Japan current, "the Gulf Stream of the Pacific," does the same. The Mozambique current runs to the south, against the S.E. trade-winds, and it changes not with the monsoons. The ice-bearing currents of the north oppose the winds in their course. Humboldt's current has its genesis in the ex-tropical regions of the south, where the "brave west winds" blow with almost if not with quite the regularity of the trades, but with double their force. And this current, instead of setting to the S.E. before these winds, flows north in spite of them. These are the main and constant currents of the sea—the great arteries and jugulars through which its circulation is conducted. In every instance, and regardless of winds, those currents that are warm flow towards the poles, those that are cold set towards the equator. And this they do, not by the force of the winds, but in spite of them, and by the force of those over agencies that make the winds to blow. They flow thus by virtue of those efforts which the sea is continually making to restore that equilibrium to its waters which heat and cold, the forces of evaporation, and the secretion of its inhabitants are everlastingly destroying.

81. The supremacy of the winds disputed.—If the winds make the upper, what makes the under and counter currents? This question is of itself enough to impeach that supremacy of the winds upon the currents, which the renowned philosopher, with whom I am so unfortunate as to differ, travelled so far out of his way to vindicate.^[5] The "bottles" also dispute, in their silent way, the "supremacy of the wands over the currents of the sea. The bottles that are thrown overboard to the currents are partly out of the water. The wind has influence upon them, yet of all those—and they are many—that have been thrown overboard in the trade-wind region of the North Atlantic, or in the Caribbean Sea, where the trade-winds blow, none have been found to drift with the wind: they all drift with the current, and nearly at right angles to the wind. 82. The Bonifaccio current.—That the winds do make currents in the sea no one will have the hardihood to deny: but currents that are born of the winds are as unstable as the winds; uncertain as to time, place, and direction, they are sporadic and ephemeral; they are not the constant currents such as have been already enumerated. Admiral Smyth, in his valuable memoir on the Mediterranean (p.162), mentions that a continuance in the Sea of Tuscany of "gusty gales" from the south-west has been known to raise its surface no less than twelve feet above its ordinary level. This, he says, occasions a strong surface drift through the Strait of Bonifaccio. But in this we have nothing like the Gulf Stream; no deep and narrow channel-way to conduct these waters off like a miniature river even in that sea, but a mere surface flow, such as usually follows the piling up of water in any pond or gulf above the ordinary level. The Bonifaccio current does not flow like a "river in the sea" across the Mediterranean, but it spreads itself out as soon as it passes the Straits, and, like a circle on the water, loses itself by broad spreading as soon as it finds sea-room. As soon as the force that begets it expends itself, the current is done.

83. The heel of the Gulf Stream an ascending plane.—Supposing with Franklin, and those of his school, that the pressure of the waters that are forced into the Caribbean Sea by the trade-winds is the sole cause of the Gulf Stream, that sea and the Mexican Gulf should have a much higher level than the Atlantic. Accordingly, the advocates of this theory require for its support "a great degree of elevation." Major Rennell likens the stream to "an immense river descending from a higher level into a plain." Now we know very nearly the average breadth and velocity of the Gulf Stream in the Florida Pass. We also know, with a like degree of approximation, the velocity and breadth of the same waters off Cape Hatteras. Their breadth here is about seventy-five miles against thirty-two in the "Narrows" of the Straits, and their mean velocity is three knots off Hatteras against four in the "Narrows." This being the case, it is easy to show that the depth of the Gulf Stream off Hatteras is not so great as it is in the "Narrows" of Bemini by nearly 50 per cent., and that, consequently, instead of descending, its bed represents the surface of an inclined plane—inclined downwards from the north towards the south—up which plane the lower depths of the stream must ascend. If we assume its depth off Bemini^[6] to be two hundred fathoms, which are thought to be within limits, the above rates of breadth and velocity will give one hundred and fourteen fathoms for its depth off Hatteras. The waters therefore, which in the Straits are below the level of the Hatteras depth, so far from descending, are actually forced up an inclined plane, whose submarine ascent is not less than ten inches to the mile.

84. The Niagara.—The Niagara is an "immense river descending into a plain." But instead of preserving its character in Lake Ontario as a distinct and well-defined stream for several hundred miles, it spreads itself out, and its waters are immediately lost in those of the lake. Why should not the Gulf Stream do the same? It gradually enlarges itself, it is true; but, instead of mingling with the ocean by broad spreading, as the "immense rivers" descending into the northern lakes do, its waters, like a stream of oil in the ocean, preserve a distinctive character for more than three thousand miles.

85. A current counter to the Gulf Stream.—Moreover, while the Gulf Stream is running to the north from its supposed elevated level at the south, there is a cold current coming down from the north; meeting the warm waters of the Gulf midway the ocean, it divides itself, and runs by the side of them right back into those very reservoirs at the south, to which theory gives an elevation sufficient to send out entirely across the Atlantic a jet of warm water said (§ 75) to be more than three thousand times greater in volume than the Mississippi River. This current from Baffin's Bay has not only no trade-winds to give it a head, but the prevailing winds are unfavourable to it, and for a great part of the valley it is below the surface, and flow beyond the propelling reach of any wind. And there is every reason to believe that this, with other polar currents, is quite equal in volume to the Gulf Stream. Are they not the effects of like causes? If so, what, have the trade-winds to do with the one more than the other?

86. Bottle chart.—It is a custom often practised by seafaring people to throw a bottle overboard, with a paper, stating the time and place at which it is done. In the absence of other information as to currents, that afforded by these mute little navigators is of great value. They leave no tracks behind them, it is true, and their routes cannot be ascertained. But knowing where they were cast, and seeing where they are found, some idea may be formed as to their course. Straight lines may at least be drawn, showing the shortest distance from the beginning to the end of their voyage, with the time elapsed. Captain Becher, R.N., has prepared a chart representing in this way the tracks of more than one hundred bottles. From this chart it appears that the waters from every quarter of the Atlantic tend toward the Gulf of Mexico and its stream. Bottles cast into the sea midway between the Old and the New Worlds, near the coasts of Europe, Africa, and America, at the extreme north or farthest south, have been found either in the West Indies, on the British Isles, or within the well-known range of Gulf Stream waters.

87. Their drift.—Of two cast out together in south latitude on the coast of Africa, one was found on the island of Trinidad; the other on Guernsey, in the English Channel. In the absence of positive information on the subject, the circumstantial evidence that the latter performed the tour of the Gulf is all but conclusive. And there is reason to suppose that some of the bottles of the gallant captain's chart have also performed the tour of the Gulf Stream; then, without being cast ashore, have returned with the drift along the coast of Africa into the inter-tropical region; thence through the Caribbean Sea, and so on with the Gulf Stream again. (Plate VI.) Another bottle, said to be thrown over off Cape Horn by an American ship-master in 1837, was afterwards picked up on the coast of Ireland. An inspection of the chart, and of the drift of the other bottles, seems to force the conclusion that this bottle too went even from that remote region to the so-called higher level of the Gulf Stream reservoir.

88. The Sargasso Sea.—Midway the Atlantic, in the triangular space between the Azores, Canaries, and the Cape de Verd Islands, is the great Sargasso Sea. (Plate VI.) Covering an area equal in extent to the Mississippi Valley, it is so thickly matted over with Gulf weed (Fucus natans) that the speed of vessels passing through it is often much retarded. When the companions of Columbus saw it, they thought it marked the limits of navigation, and became alarmed. To the eye, at a little distance, it seems substantial enough to walk upon. Patches of the weed are generally to be seen floating along the outer edge of the Gulf Stream. The sea-weed always "tails to" a steady or a constant wind, so that it serves the mariner as a sort of marine anemometer, telling him whether the wind as he finds it has been blowing for some time, or whether it has but just shifted, and which way. Columbus first found this weedy sea on his voyage of discovery; there it has remained to this day, moving up and down, and changing its position, like the calms of Cancer, according to the seasons, the storms, and the winds. Exact observations as to its limits and their range, extending back for fifty years, assure us that its mean position has not been altered since that time. That the water which comes through the Florida Pass with the Gulf Stream flows in a circle, going to the north on the western side, and returning to the south on the east side of the Atlantic—sloughing off its drift matter always to the right, is shown not only by the Sargasso and its weeds, but it is indicated also, by our "bottle papers," by the facts developed in Plate VI., and by other sources of information. If, therefore, this be so, why give the endless current a higher level in one part of its course than another?

89. A bifurcation.—Nay, more; at the very season of the year when the Gulf Stream is rushing in greatest volume through the Straits of Florida, and hastening to the north with the greatest rapidity, there is a cold stream from Baffin's Bay, Labrador, and the coasts of the north, running to the south with equal velocity. There is the trade-wind that gives the higher level to Baffin's Bay", or that even presses upon, or assists to put this current in motion? The agency of winds in producing currents in the deep sea must be very partial. These two currents meet off the Grand Banks, where the latter is divided. One part of it underruns the Gulf Stream, as is shown by the icebergs which are carried in a direction tending across its course. The probability is, that this "fork" flows on towards the south, and runs into the Caribbean Sea, for the temperature of the water at a little depth there has been found far below the mean temperature of the earth's crust, and quite as cold as at a corresponding depth off the Arctic shores of Spitzbergen.

90. Winds exercise but little influence constant currents.—More water cannot run from the equator or the pole than to it. If we make the trade-winds to cause the Gulf Stream, we ought to have some other wind to produce the Polar flow; but these currents, for the most part, and for great distances, are submarine, and therefore beyond the influence of winds. Hence it should appear that winds have little to do with the general system of aqueous circulation in the ocean. The other "fork" runs between our shores and the Gulf Stream to the south, as already described. As far as it has been traced, it warrants the belief that it, too, runs up to seek the so-called higher level of the Mexican Gulf.

91. Effects of diurnal rotation upon the Gulf Stream.—The power necessary to overcome the resistance opposed to such a body of water as that of the Gulf Stream, running several thousand miles without any renewal of impulse from the forces of gravitation or any other known cause, is truly surprising. It so happens that we have an argument for determining, with considerable accuracy, the resistance which the waters of this stream meet with in their motion towards the east. Owing to the diurnal rotation, they are carried around with the earth on its axis towards the east, with an hourly velocity of one hundred and fifty-seven^[7] miles greater when they enter the Atlantic than when they arrive off the Banks of Newfoundland; for in consequence of the difference of latitude between the parallels of these two places, their rate of motion around the axis of the earth is reduced from nine hundred and fifteen^[8] to seven hundred and fifty-eight miles the hour. Hence this immense volume of water would, if we suppose it to pass from the Bahamas to the Grand Banks in an hour, meet with an opposing force in the shape of resistance sufficient, in the aggregate, to retard it two miles and a half the minute in its eastwardly rate. If the actual resistance be calculated according to received laws, it will be found equal to several atmospheres. And by analogy, how inadequate must the pressure of the gentle trade-winds be to such resistance, and to the effect assigned them!

92. The Gulf Stream cannot he accounted for by a higher level.—If therefore, in the proposed inquiry, we search for a propelling power nowhere but in the higher level of the Gulf, or in the "billiard-ball" rebound from its shores, we must admit, in the head of water there, the existence of a force capable of putting in motion, and of driving over a plain at the rate of four miles the hour, all the waters, as fast as they can be brought down by three thousand (§ 75) such streams as the Mississippi River—a power at least sufficient to overcome the resistance required to reduce from two miles and a half to a few feet per minute the velocity of a stream that keeps in perpetual motion one-fourth of all the waters in the Atlantic Ocean. Not only so, we must admit the existence of an engine in the Gulf of Mexico, which, being played upon by the gentle forces of the trade-winds, is capable of sending a stream of water from the shores of the New World to the shores of the Old.

93. Nor by the trade-wind theory.—The advocates of the trade-wind theory, whether, with Franklin (§ 77), they make the propelling power to be derived from a head of water in the Gulf, or, with Herschel (§ 79), from the rebound, à la billiard-balls, against its shores, require that the impulse then and there communicated to the waters of the Gulf Stream should be sufficient to send them entirely across the Ocean; for in neither case does their theory provide for any renewal of the propelling power by the wayside. Can this be? Can water flow on any more than cannon-balls can continue their flight after the propelling force has been expended?

94. Illustration.—When we inject water into a pool, be the force never so great, the jet is soon overcome, broken up, and made to disappear. In this illustration the Gulf Stream may be likened to the jet, and the Atlantic to the pool. We remember to have observed as children how soon the mill-tail loses its current in the pool below; or we may now see at any time, and on a larger scale, how soon the Niagara, current and all, is swallowed up in the lake below.

95. Gulf Stream the effect of some constantly operating power.—Nothing but a continually-acting power can keep currents in the sea, any more than cannon-balls in the air or rivers on the land, in motion. But for the forces of gravitation the waters of the Mississippi would remain at its fountain, and but for difference of specific gravity the waters of the Gulf Stream would remain in the caldron, as the inter-tropical parts of the Atlantic Ocean may be called.

96. The production of currents without wind.—For the sake of further illustration, let us suppose a globe of the earth's size, and with a solid nucleus, to be covered all over with water two hundred fathoms deep, and that every source of heat and cause of radiation be removed, so that its fluid temperature becomes constant and uniform throughout. On such a globe, the equilibrium remaining undisturbed, there would be neither wind nor current. Let us now suppose that all the water within the tropics, to the depth of one hundred fathoms, suddenly becomes oil. The aqueous equilibrium of the planet would thereby be disturbed, and a general system of currents and counter-currents would be immediately commenced—the oil, in an unbroken sheet on the surface, running towards the poles, and the water, in an under-current, towards the equator. The oil is supposed, as it reaches the polar basin, to be reconverted into water, and the water to become oil as it crosses Cancer and Capricorn, rising to the surface in the inter-tropical regions, and retuning as before. Thus, without wind, we should have a perpetual and uniform system of tropical and polar currents; though without wind, Sir John Herschel maintains,^[9] we should have no "considerable currents" whatever in the sea. In consequence of the diurnal rotation of the planet on its axis, each particle of oil, were resistance small, would approach the poles on a spiral turning to the east with a relative velocity greater and greater, until, finally, it would reach the pole, and whirl about it at the rate of nearly a thousand miles the hour. Becoming water and losing its velocity, it would approach the tropics by a similar, but reversed spiral, turning towards the west. Owing to the principle here alluded to, all currents from the equator to the poles should have an eastward tendency, and all from the poles towards the equator a westward. Let us now suppose the solid nucleus of this hypothetical globe to assume the exact form and shape of the bottom of our seas, and in all respects, as to figure and size, to represent the shoals and islands of the sea, as well as the coast lines and continents of the earth. The uniform system of currents just described would now be interrupted by obstructions and local causes of various kinds, such as unequal depth of water, contour of shore lines, &c.; and we should have at certain places currents greater in volume and velocity than at others. But still there would be a system of currents and counter-currents to and from either pole and the equator. Now, do not the cold waters of the north, and the warm waters of the gulf, made specifically lighter by tropical heat, and which we see actually preserving such a system of counter-currents, hold, at least in some degree, the relation of the supposed water and oil?

97. Warm currents flow towards the pole, cold towards the equator.—In obedience to the laws here hinted at, there is a constant tendency (Plate IX.) of polar waters towards the tropics and of tropical waters towards the poles. Captain Wilkes, of the United States' Exploring Expedition, crossed one of these hyperborean under-currents two hundred miles in breadth at the equator.

98. Edges of the Gulf Stream a striking feature.—No feature of the Gulf Stream excites remark among seamen more frequently than the sharpness of its edges, particularly along its inner borders. There, it is a streak on the water. As high up as the Carolinas this streak may be seen, like a greenish edging to a blue border—the bright indigo of the tropical contrasting finely (§ 70) with the dirty green of the littoral waters. It is this apparent reluctance of the warm waters of the stream to mix with the cool of the ocean that excites wonder and calls forth remark. But have we not, so to speak, a similar reluctance manifested by all fluids, only upon a smaller scale, or under circumstances less calculated to attract attention or excite remark?

99. Illustrations.—The water, hot and cold, as it is let into the tub for a warm bath, generally arranges itself in layers or sections, according to temperature; it requires violent stirring to break them up, mix, and bring the whole to an even temperature. The jet of air from the blow-pipe, or of gas from the burner, presents the phenomenon still more familiarly; here we have, as with the Gulf Stream, the dividing line between fluids in motion and fluids at rest finely presented. There is a like reluctance for mixing between streams of clear and muddy water. This is very marked between the red waters of the Missouri and the inky waters of the upper Mississippi; here the waters of each may be distinguished for the distance of several miles after these two rivers come together. It requires force to inject, as it were, the particles of one of these waters among those of the other, for mere vis inertia tends to maintain in their statu quo fluids that have already arranged themselves in layers, streaks, or aggregations.

100. How the water of the Gulf Stream differs from the littoral waters.—In the ocean we have the continual heaving of the sea and agitation of the waves to overcome this vis inertia; and the marvel is, that they in their violence do not, by mingling the Gulf and littoral waters together (§ 70), sooner break up and obliterate all marks of a division between them. But the waters of the Gulf Stream differ from the inshore waters not only in colour, transparency, and temperature, but in specific gravity, in saltness (§ 102), and in other properties, I conjecture, also. Therefore they may have a peculiar viscosity, or molecular arrangement of their own, which further tends to prevent mixture, and so preserve their line of demarkation.

101. Action on copper.—Observations made for the purpose in the navy show that ships cruising in the West Indies suffer in their copper sheathing more than they do in any other seas. This would indicate that the waters of the Caribbean Sea and Gulf of Mexico, from which the Gulf Stream is fed, have some peculiar property or other which makes them so destructive upon the copper of cruisers.

102. Saltness of the Gulf Stream.—The story told by the copper and the blue colour (§ 71) indicates a higher point of saturation with salts than sea-water generally has; and the salometer confirms it. Dr. Thomassy, a French savant, who has been extensively engaged in the manufacture of salt by solar evaporation, informs me that on his passage to the United States he tried the saltness of the water with a most delicate instrument: he found it in the Bay of Biscay to contain 3½ per cent, of salt; in the trade-wind region 4 $\scriptstyle {\frac {4}{10}}$ per cent.; and in the Gulf Stream, off Charleston, 4 per cent., notwithstanding the Amazon and the Mississippi, with all the intermediate rivers, and the clouds of the West Indies, had lent their fresh water to dilute the saltness of this basin.

103. Agents concerned.—Now the question may be asked. What should make the waters of the Mexican Gulf and Caribbean Sea salter than the waters in those parts of the ocean through which the Gulf Stream flows? There are physical agents that are known to be at work in different parts of the ocean, the tendency of which is to make the waters in one part of the ocean Salter and heavier, and in another part lighter and less salt than the average of sea-water. These agents are those employed by sea-shells in secreting solid matter for their structures; they are also heat^[10] and radiation, evaporation and precipitation. In the trade-wind regions at sea (Plate VIII.), evaporation is generally in excess of precipitation, while in the extra-tropical regions the reverse is the case; that is, the clouds let down more water there than the winds take up again; and these are the regions in which the Gulf Stream enters the Atlantic. Along the shores of India, where observations have been made, the evaporation from the sea is said to amount to three-fourths of an inch daily. Suppose it in the trade-wind region of the Atlantic to amount to only half an inch, that would give an annual evaporation of fifteen feet. In the process of evaporation from the sea, fresh water only is taken up; the salts are left behind. Now a layer of sea-water fifteen feet deep, and as broad as the trade-wind belts of the Atlantic, and reaching across the ocean, contains an immense amount of salts. The great equatorial current (Plate VI.) which often sweeps from the shores of Africa across the Atlantic into the Caribbean Sea is a surface current; and may it not bear into that sea a large portion of those waters that have satisfied the thirsty trade-winds with saltless vapour? If so—and it probably does—have we not detected here the footprints of an agent that does tend to make the waters of the Caribbean Sea salter, and therefore heavier, than the average of sea-water at a given temperature?

104. Evaporation and precipitation.—It is immaterial, so far as the correctness of the principle upon which this reasoning depends is concerned, whether the annual evaporation from the trade-wind regions of the Atlantic be fifteen, ten, or five feet. The layer of water, whatever be its thickness, that is evaporated from this part of the ocean, is not all poured back by the clouds upon the same spot whence it came. But they take and pour it down in showers upon the extra-tropical regions of the earth—on the land as well as in the sea—and on the land more water is let down than is taken up into the clouds again. The rest sinks down through the soil to feed the springs, and returns through the rivers to the sea. Suppose the excess of precipitation in these extra-tropical regions of the sea to amount to but twelve inches, or even to but two—it is twelve inches or two inches, as the case may be, of fresh water added to the sea in those parts, and which therefore tends to lessen the specific gravity of sea-water there to that extent, and to produce a double dynamical effect, for the simple reason that what is taken from one scale, by being put into the other, doubles the difference. 105. Current into the Caribbean Sea.—Now that we may form some idea as to the influence which the salts left by the vapour that the trade-winds, north-east and south-east, take up from sea-water, is calculated to exert in creating currents, let us make a partial calculation to show how much salt this vapour held in solution before it was taken up, and, of course, while it was yet in the state of sea-water. The north-east trade-wind regions of the Atlantic embrace an area of at least three million square miles, and the yearly evaporation from it is (§ 103), we will suppose, fifteen feet. The salt that is contained in a mass of sea-water covering to the depth of fifteen feet an area of three million square miles in superficial extent, would be sufficient to cover the British islands to the depth of fourteen feet. As this water supplies the trade-winds with vapour, it therefore becomes salter, and as it becomes salter, it becomes heavier; and therefore we may infer that the forces of aggregation among its particles are increased.

106. Amount of salt left by evaporation.—Whatever be the cause that enables these trade-wind waters to remain on the surface, whether it be from the fact just stated, and in consequence of which the waters of the Gulf Stream are held together in their channel; or whether it be from the fact that the expansion from the heat of the torrid zone is sufficient to compensate for this increased saltness; or whether it be from the low temperature and high saturation of the submarine waters of the intertropical ocean; or whether it be owing to all of these influences together that these waters are kept on the surface, suffice it to say, we do know that they go into the Caribbean Sea (§ 103) as a surface current. On their passage to and through it, they intermingle with the fresh waters that are emptied into the sea from the Amazon, the Orinoco, and the Mississippi, and from the clouds, and the rivers of the coasts round about. An immense volume of fresh water is supplied from these sources. It tends to make the sea-water, that the trade-winds have been playing upon and driving along, less briny, warmer, and lighter: for the waters of these large intertropical streams are warmer than sea-water. This admixture of fresh water still leaves the Gulf Stream a brine stronger than that of the extratropical sea generally, but not quite so strong (§ 102) as that of the trade-wind regions.

107. Currents created by storms.—The dynamics of the sea confess the power of the winds in those tremendous currents which storms are sometimes known to create; and that even the gentle trade-winds may have influence and effect upon the currents of the sea has not been denied (§ 82). But the effect of moderate winds, as the trades are, is to cause what may he called the drift of the sea rather than a current. Drift is confined to surface waters, and the trade-winds of the Atlantic may assist in creating the Gulf Stream by drifting the waters which have supplied them with vapour towards the Caribbean Sea. But admit never so much of the water which the trade-winds have played upon to be drifted into the Caribbean Sea, what should make it flow thence with the Gulf Stream to the shores of Europe? It is because there is room for it there; and there is room for it because of the difference in the specific gravity of sea-water in an intertropical sea on one side, as compared with the specific gravity of water in northern seas and frozen oceans on the other.

108. The dynamical force that calls forth the Gulf Stream to be found in the difference as to specific gravity of intertropical and polar waters.—The dynamical forces which are expressed by the Gulf Stream may with as much propriety be said to reside in those northern waters as in the West India seas; for on one side we have the Caribbean Sea, and Gulf of Mexico, with their waters of brine; on the other, the Great Polar basin, the Baltic, and the North Sea, the two latter with waters that are but little more than brackish.^[11] In one set of these sea-basins the water is heavy; in the other it is light. Between them the ocean intervenes; but water is bound to seek its equilibrium as its level; and here, therefore, we unmask one of the agents concerned in causing the Gulf Stream. What is the power of this agent—is it greater than that of other agents, and how much? We cannot say how much; we only know it is one of the chief agents concerned. Moreover, speculate as we may as to all the agencies concerned in collecting these waters, that have supplied the trade-winds with vapour, into the Caribbean Sea, and then in driving them across the Atlantic—we are forced to conclude that the salt which the trade-wind vapour leaves behind in the tropics has to be conveyed away from the trade-wind region, to be mixed up again in due proportion with the other water of the sea—the Baltic Sea and the Arctic Ocean included—and that these are some of the waters, at least, which we see running off through the Gulf Stream. To convey them away is doubtless one of the offices which, in the economy of the ocean, has been assigned to it. But as for the seat of the forces which put and keep the Gulf Stream in motion, theorists may place them exclusively on one side of the ocean with as much philosophical propriety as on the other. Its waters find their way into the North Sea and the Arctic Ocean by virtue of their specific gravity, while water thence, to take their place, is, by virtue of its specific gravity and by counter currents, carried back into the Gulf. The dynamical force which causes the Gulf Stream may therefore be said to reside both in the polar and in the intertropical waters of the Atlantic.

109. Winter temperature of the Gulf Stream.—As to the temperature of the Gulf Stream, there is, in a winter's day, off Hatteras, and even as high up as the Grand Banks of Newfoundland in mid-ocean, a difference between its waters and those of the ocean near by of 20° and even 30°. Water, we know, expands by heat, and here the difference of temperature may more than compensate for the difference in saltness, and leave, therefore, the waters of the Gulf Stream, though salter, yet lighter by reason of their warmth.

110. Top of Gulf Stream roof-shaped.—If they be lighter, they should therefore occupy a higher level than those through which they flow. Assuming the depth off Hatteras to be one hundred and fourteen fathoms, and allowing the usual rates of expansion for sea-water, figures show that the middle or axis of the Gulf Stream there should be nearly two feet higher than the contiguous waters of the Atlantic. Hence the surface of the stream should present a double inclined plane, from which the water would be running down on either side as from the roof of a house. As this runs off at the top, the same weight of colder water runs in at the bottom, and so raises up the cold-water bed of the Gulf Stream, and causes it to become shallower and shallower as it goes north. That the Gulf Stream is therefore roof-shaped, causing the waters on its surface to flow off to either side from the middle, we have not only circumstantial evidence to show, but observations to prove. Navigators, while drifting along with the Gulf Stream, have lowered a boat to try the surface current. In such cases, the boat would drift either to the east or to the west, as it happened to be on one side or the other of the axis of the stream, while the vessel herself would drift along with the stream in the direction of its course: thus showing the existence of a shallow roof-current from the middle towards either edge, which would carry the boat along, but which, being superficial, does not extend deep enough to affect the drift of the vessel.

111. Drift matter sloughed off to the right.—That such is the case (§ 110) is also indicated by the circumstance that the sea-weed and drift-wood which are found in such large quantities along the outer edge of the Gulf Stream, are rarely, even with the prevalence of easterly winds, found along its inner edge—and for the simple reason that to cross the Gulf Stream, and to pass over from that side to this, they would have to drift up an inclined plane, as it were; that is, they would have to stem this roof-current until they reached the middle of the stream. We rarely hear of planks, or wrecks, or of any floating substance which is cast into the sea on the other side of the Gulf Stream being found along the coast of the United States. Drift-wood, trees, and seeds from the West India Islands, are often cast up on the shores of Europe, but rarely on the Atlantic shores of this country.

112. Why so sloughed off.—We are treating now of the effects of physical causes. The question to which I ask attention is, Why does the Gulf Stream slough off and cast upon its outer edge, sea-weed, drift-wood, and all other solid bodies that are found floating upon it? One cause has been shown to be in its roof-shaped current; but there is another which tends to produce the same effect; and because it is a physical agent, it should not, in a treatise of this kind, be overlooked, be its action never so slight. I allude now to the effects produced upon the drift matter of the stream by the diurnal rotation of the earth.

113. Illustration.—Take, for illustration, a railroad that lies north and south in our hemisphere. It is well known to engineers that when the cars are going north on such a road, their tendency is to run off on the east side; but when the train is going south, their tendency is to run off on the west side of the track—i. e., always on the right-hand side. Whether the road be one mile or one hundred miles in length, the effect of diurnal rotation is the same; and, whether the road be long or short, the tendency to run off, as you cross a given parallel at a stated rate of speed, is the same; for the tendency to fly off the track is in proportion to the speed of the train, and not at all in proportion to the length of the road. Now, vis inertæ and velocity being taken into the account, the tendency to obey the force of this diurnal rotation, and to trend to the right, is proportionably as great in the case of a patch of sea-weed as it drifts along the Gulf Stream, as it is in the case of the train of cars as they speed to the north along the iron track of the Hudson River, or the North-Western railway, or any other railway that lies nearly north and south. The rails restrain the cars and prevent them from flying off; but there are no rails to restrain the sea-weed, and nothing to prevent the drift matter of the Gulf Stream from going off in obedience to this force. The slightest impulse tending to turn aside bodies moving freely in water is immediately felt and implicitly obeyed.

114. Drift-wood on the Mississippi.—It is in consequence of this diurnal rotation that drift-wood coming down the Mississippi is so very apt to be cast upon the west or right bank. This is the reverse of what obtains upon the Gulf Stream, for it flows to the north; it therefore sloughs off (§ 111) to the east.

115. Effect of diurnal rotation upon.—The effect of diurnal rotation upon the winds and upon the currents of the sea is admitted by all—the trade-winds derive their easting from it—it must, therefore, extend to all the matter which these currents bear with them, to the largest iceberg as well as to the smallest spire of grass that floats upon the waters, or the minutest organism that the most powerful microscope can detect among the impalpable particles of sea-dust. This effect of diurnal rotation upon drift will be frequently alluded to in the pages of this work.

116. Formation of the Grand Banks.—In its course to the north, the Gulf Stream gradually tends more and more to the eastward, until it arrives off the Banks of Newfoundland, where its course becomes nearly due east. These banks, it has been thought, deflect it from its proper course, and cause it to take this turn. Examination will prove, I think, that they are an effect, certainly not the cause. It is here that the frigid current already spoken of (§ 85), and its icebergs from the north, are met and melted by the warm waters of the Gulf. Of course the loads of earth, stones, and gravel brought down upon these bergs are here deposited. Captain Scoresby, far away in the north, counted at one time five hundred icebergs setting out from the same vicinity upon this cold current for the south. Many of them, loaded with earth, have been seen aground on the Banks. This process of transferring deposits from the north for these shoals, and of snowing down upon them the infusoria and the corpses of "living creatures" that are brought forth so abundantly in the warm waters of the Gulf Stream, and delivered in myriads for burial where the conflict between it and the great Polar current (§ 89) takes place, is everlastingly going on. These agencies, with time, seem altogether adequate to the formation of extensive bars or banks.

117. Deep water near.—The deep-sea soundings that have been made by vessels of the English and American navies (Plate XI.) tend to confirm this view as to the formation of these Banks. The greatest contrast in the bottom of the Atlantic is just to the south of these Banks. Nowhere in the open sea has the water been found to deepen so suddenly as here. Coming from the north, the bottom of the sea is shelving; but suddenly, after passing these Banks, it dips down by a precipitous descent to unknown depths—thus indicating that the débris which forms the Grand Banks comes from the north.

118. The Gulf Stream describes in its course the path of a trajectory.—From the Straits of Bemini the course of the Gulf Stream (Plate VI.) describes (as far as it can be traced over toward the British Islands which are in the midst of its waters) the arc of a great circle nearly. Such a course as the Gulf Stream takes is very nearly the course that a cannon-ball, could it be shot from these straits to those islands, would follow.

119. Its path from Bemini to Ireland.—If it were possible to see Ireland from Bemini, and to get a cannon that would reach that far, the person standing on Bemini and taking aim, intending to shoot at Ireland as a target, would, if the earth were at rest, sight direct, and make no allowance for difference of motion between marksman and target. Its path would lie in the plane of a great circle. But there is diurnal rotation; the earth does revolve on its axis; and since Bemini is nearer to the equator than Ireland is, the gun would be moving in diurnal rotation (§91) faster than the target, and therefore the marksman, taking aim point blank at his target, would miss. He would find, on examination, that he had shot south—that is, to the right (§ 103) of his mark. In other words, that the path actually described by the ball would be a resultant arising from this difference in the rate of rotation and the trajectile force. Like a ray of light from the stars, the ball would be affected by aberration. The ball so shot presents the case of the passenger in the railroad car throwing an apple, as the train sweeps by, to a boy standing by the wayside. If he throw straight at the boy, he will miss, for the apple, partaking of the motion of the cars, will go ahead of the boy, and for the very reason that the shot will pass in advance of the target, for both the marksman and the passenger are going faster than the object at which they aim.

120. Tendency of all currents both in the sea and air to move in great circles a physical law.—Hence we may assume it as a law, that the natural tendency of all currents in the sea, like the natural tendency of all projectiles through the air, is to describe each its curve of flight very nearly in the plane of a great circle. The natural tendency of all matter, when put in motion, is to go from point to point by the shortest distance, and it requires force to overcome this tendency. Light, heat, and electricity, the howling wind, running water, and all substances, whether ponderable or imponderable, seek, when in motion, to obey this law. Electricity may be turned aside from its course, and so may the cannon-ball or running water; but remove every obstruction, and leave the current or the shot free to continue on in the direction of the first impulse, or to turn aside of its own volition, so to speak, and straight it will go, and continue to go—if on a plane, in a straight line; if about a sphere, in the arc of a great circle—thus showing that it has no volition except to obey impulse; and that impulse comes from the physical requirements upon it to take the shortest way to its point of destination.

121. This law recognized by the Gulf Stream.—The waters of the Gulf Stream, as they escape from the Gulf, are bound for the British Islands, to the North Sea, and Frozen Ocean (Plate IX.). Accordingly, they take (§118), in obedience to this physical law, the most direct course by which nature will permit them to reach their destination. And this course, as already remarked, is nearly that of the great circle, and of the supposed cannon-ball.

122. Shoals of Nantucket do not control its course.—Many philosophers have expressed the opinion—indeed, the belief (§ 116) is common among mariners—that the coasts of the United States and the Shoals of Nantucket turn the Gulf Stream towards the east; but if the view I have been endeavouring to make clear be correct, it would appear that the course of the Gulf Stream is fixed and prescribed by exactly the same laws that require the planets to revolve in orbits, the planes of which shall pass through the centre of the sun; and that, were the Nantucket Shoals not in existence, the course of the Gulf Stream, in the main, would be exactly as it is and where it is. The Gulf Stream is bound over to the North Sea and Bay of Biscay partly for the reason, perhaps, that the waters there are lighter than those of the Mexican Gulf; and if the Shoals of Nantucket were not in existence, it could not pursue a more direct route. The Grand Banks, however, are encroaching (§ 116), and cold currents from the north come down upon it: they may, and probably do, assist now and then to turn it aside.

123. Herschel's theory not consistent with known facts.—Now if this explanation as to the course of the Gulf Stream and its eastward tendency hold good, a current setting from the north towards the south should (§ 103) have a westward tendency. It should also move in a circle of trajection, or such as would be described by a trajectile moving through the air without resistance and for a great distance. Accordingly, and in obedience to the propelling powers derived from the rate at which different parallels are whirled around in diurnal motion (§ 91), we find the current from the north, which meets the Gulf Stream on the Grand Banks (Plate IX.), taking a south-westwardly direction, as already described (§114). It runs down to the tropics by the side of the Gulf Stream, and stretches as far to the west as our own shores will allow. Yet, in the face of these facts, and in spite of this force, both Major Rennell and M. Arago would make the coasts of the United States and the Shoals of Nantucket to turn the Gulf Stream towards the east: and Sir John Herschel (§79) makes the trade-winds, which blow from the eastward, drive this stream to the eastward!

124. The Channel of the Gulf Stream shifts with the season.—But there are other forces operating upon the Gulf Stream. They are derived (§ 80) from the effect of changes in the waters of the whole ocean, as produced by changes in their temperature and saltness from time to time. As the Gulf Stream leaves the coasts of the United States, it begins to vary its position according to the seasons; the limit of its northern edge, as it passes the meridian of Cape Race (Plate VI.), being in winter about latitude 40-41°, and in September, when the sea is hottest, about latitude 45-46°. The trough of the Gulf Stream, therefore, may be supposed to waver about in the ocean not unlike a pennon in the breeze. Its head is confined between the shoals of the Bahamas and the Carolinas; but that part of it which stretches over towards the Grand Banks of Newfoundland is, as the temperature of the waters of the ocean changes, first pressed down towards the south, and then again up towards the north, according to the season of the year.

125. The phenomenon thermal in its character.—To appreciate the extent of the force by which it is so pressed, let us imagine the waters of the Gulf Stream to extend all the way to the bottom of the sea, so as completely to separate, by an impenetrable liquid wall, if you please, the waters of the ocean on the right from the waters in the ocean on the left of the stream. It is the height of summer: the waters of the sea on either hand are for the most part in a liquid state, and the Gulf Stream, let it be supposed, has assumed a normal condition between the two divisions, adjusting itself to the pressure on either side so as to balance them exactly and be in equilibrium, Now, again, it is the dead of winter, and the temperature of the waters over an area of millions of square miles in the North Atlantic has been changed many degrees, and this change of temperature has been followed likewise by a change in volume of those waters, amounting, no doubt, in the aggregate, to many hundred millions of tons, over the whole ocean; for sea-water, unlike fresh (§ 103), contracts to freezing, and below. Now is it probable that, in passing from their summer to their winter temperature, the sea-waters to the right of the Gulf Stream should change their specific gravity exactly as much in the aggregate as do the waters in the whole ocean to the left of it? If not, the difference must be compensated by some means. Sparks are not more prone to fly upward, nor water to seek its level, than Nature is sure with her efforts to restore equilibrium in both sea and air whenever, wherever, and by whatever it be disturbed. Therefore, though the waters of the Gulf Stream do not extend to the bottom, and though they be not impenetrable to the waters on either hand, yet, seeing that they have a waste of waters on the right and a waste of waters on the left, to which (§ 70) they offer a sort of resisting permeability, we are enabled to comprehend how the waters on either hand, as their specific gravity is increased or diminished, will impart to the trough of this stream a vibratory motion, pressing it now to the right, now to the left, according to the seasons and the consequent changes of temperature in the sea.

126. Limits of the Gulf Stream in March and September.—Plate VI. shows the limits of the Gulf Stream for March and September. The reason for this change of position is obvious. The banks of the Gulf Stream (§70) are cold water. In winter the volume of cold water on the American, or left side of the stream, is greatly increased. It must have room, and gains it by pressing the warmer waters of the stream farther to the south, or right. In September, the temperature of these cold waters is modified; there is not such an extent of them, and then the warmer waters, in turn, press them back, and so the pendulum-like motion is preserved.

127. Reluctance of layers or patches to mingle.—In the offings of the Balize, sometimes as far out as a hundred miles or more from the land, puddles or patches of Mississippi water may be observed on the surface of the sea with little or none of its brine mixed with it. This anti-mixing property in water has already (§ 98) been remarked upon. It may be observed from the gutters in the street to the rivers in the ocean, and everywhere, wherever two bodies of water that differ in colour are found in juxtaposition. The patches of white, black, green, yellow, and reddish waters so often met with at sea are striking and familiar examples. We have seen, also, that a like proclivity exists (§ 99) between bodies or streams of water that differ in temperature or velocity. This peculiarity is often so strikingly developed in the neighbourhood of the Gulf Stream, that persons have been led to suppose that the Gulf Stream has forks in the sea, and that these are they.

128. Streaks of warm and cool.—Now, if any vessel will take up her position a little to the northward of Bermuda, and steering thence for the Capes of Virginia, will try the water-thermometer all the way at short intervals, she will find its readings to be now higher, now lower; and the observer will discover that he has been crossing streak after streak of warm and cool water in regular alternations. He will then cease to regard them as bifurcations of the Gulf Stream, and view them rather in the light of thermal streaks of water which have, in the plan of oceanic circulation and in the system of unequal heating and cooling, been brought together.

129. Waters of the ocean kept in motion by thermo-dynamical means.—The waters of the Gulf Stream form by no means the only body of warm water that the thermo-dynamical forces of the ocean keep in motion. Nearly all that portion of the Atlantic which lies between the Gulf Stream and the island of Bermuda has its surface covered with water which a tropical sun and tropical winds have played upon—with water, the specific gravity of which has been altered by their action, and which is now drifting to more northern climes in the endless search after lost equilibrium. This water, moreover, as well as that of the Gulf Stream, cools unequally. It would be surprising if it did not: for by being spread out over such a large area, and then drifting for so great a distance, and through such a diversity of climates, it is not probable that all parts of it should have been exposed to like vicissitudes by the way, or even to the same thermal conditions: therefore all of the water over such a surface cannot be heated alike; radiation here, sunshine there; clouds and rain one day, and storms the next; the unequal depths; the breaking up of the fountains below, and the bringing their cooler or their warmer waters to the surface by the violence of the waves, may all be expected, and are well calculated, to produce unequal heating in the torrid and unequal cooling in the temperate zone; the natural result of which would be streaks and patches of water differing in temperature. Hence it would be surprising if, in crossing this drift and stream (Plate VI.) with the water-thermometer, the observer should find the water all of one temperature. By the time it has reached the parallel of Bermuda or "the Capes" of the Chesapeake, some of this water may have been ten days, some ten weeks, and some perhaps longer on its way from the "caldron" at the south. It has consequently had ample time to arrange itself into those differently-tempered streaks and layers (§ 127) which are so familiar to navigators, and which have been mistaken for "forks of the Gulf Stream."

130. Fig. A, Plate VI.—Curves showing some of these variations of temperature have been projected by the Coast Survey on a chart of engraved squares (Fig. A, Plate VI.). These curves show how these waters have sometimes arranged themselves off the Capes of Virginia into a series of thermal elevations and depressions. 131. The high temperature and drift in the western half of North Atlantic and Pacific Oceans.—In studying the Gulf Stream, the high temperature and drift of the waters to the east of it are worthy of consideration. The Japan current (§ 80) has a like drift of warm water to the east of it also (Plates VI. and IX.). In the western half, reaching up from the equator to the Gulf Stream, both of the North Atlantic and North Pacific, the water is warmer, parallel for parallel, than it is in the eastern half. On the west side, where the water is warm, the flow is to the north; on the east side, where the temperature is lower, the flow is to the south—making good the remark (§ 80) that, when the waters of the sea meet in currents, the tendency of the warm is to seek cooler latitudes; and of the cool, warmer.

132. Gulf Stream in each.—The Gulf Stream of each ocean has its genesis on the west side, and in its course it skirts the coast along; leaving the coast, it strikes off to the eastward in each case, losing velocity and spreading out. Between each of these Gulf Streams and its coasts there is a current of cool water setting to the south. On the outside, or to the east of each stream, and coming up from the tropics, is a broad sheet of warm water; it covers an area of thousands of square miles, and its drift is to the north. Between the northern drift on the one side of the ocean and the southern set on the other, there is in each ocean a Sargasso (§ 88), into which all drift matter, such as wood and weeds, finds its way. In both oceans the Gulf Streams sweep across to the eastern shores, and so, bounding these seas, interpose a barrier between them and the higher parallels of latitude, which this drift matter cannot pass. Such are the points of resemblance between the two oceans and in the circulation of their waters.

133. Their connection with the Arctic Ocean.—A prominent point for contrast is afforded by the channels or water-ways between the Arctic and these two oceans. with the Atlantic they are divers and large; with the Pacific there is but one, and it is both narrow and shallow. In comparison with that of the Atlantic, the Gulf Stream of the Pacific is sluggish, ill-defined and irregular. Were the water-ways between the Atlantic and the Arctic Ocean no larger than Behring's Straits, our Gulf Stream would fall for below that of the Pacific in majesty and grandeur.

134. The sargassos show the feeble power of the trade-winds upon currents.—Here I am reminded to turn aside and call attention to another fact that militates against the vast current-begetting power that has been given by theory to the gentle trade-winds. In both oceans these weedy seas lie partly within the trade-wind region; but in neither do these winds give rise to any current. The weeds are partly out of water, and the wind has therefore more power upon them than it has upon the water itself; they tail to the wind. And if the supreme power over the currents of the sea reside in the winds, as Sir John Herschel would have it, then of all places in the trade-wind region, we should have here the strongest currents. Had there been currents here, these weeds would have been borne away long ago; but so far from it, we simply know that they have been in the Sargasso Sea (§ 88) of the Atlantic since the first voyage of Columbus. But to take up the broken thread:—

135. The drift matter confined to sargassos by currents.—The water that is drifting north, on the outside of the Gulf Stream, turns, with the Gulf Stream, to the east also. It cannot reach the high latitudes (§ 80), for it cannot cross the Gulf Stream. Two streams of water cannot cross each other, unless one dip down and underrun the other; and if this drift water do dip down, as it may, it cannot carry with it its floating matter, which, like its weeds, is too light to sink. They, therefore, are cut off from a passage into higher latitudes.

136. Theory as to the formation of sargassos.—According to this view, there ought to be a sargasso sea somewhere in the sort of middle ground between the grand equatorial flow and reflow which is performed by the waters of all the great oceans. The place where the drift matter of each sea would naturally collect would be in this sort of pool, into which every current, as it goes from the equator, and again as it returns, would slough off its drift matter. The forces of diurnal rotation would require this collection of drift to be, in the northern hemisphere, on the right-hand side of the current, and, in the southern, to be on the left. (See Chap. XVIII. and Plate IX.)

137. Sargassos of southern seas to the left of the southern, to the right of the great polar and equatorial flow and reflow.—Thus, with the Gulf Stream of the Atlantic, and the "Black Stream" of the Pacific, their sargassos are on the right, as they are also on the right of the returning and cooler currents on the eastern side of each one of those northern oceans. So, also, with the bique current, which runs south along the east coast of Africa from the Indian Ocean, and with the cooler current setting to the north on the Australian side of the same sea. Between these there is a sargasso on the left; for it is in the southern hemisphere.

138. Their position conforms to the theory.—Again, there is in the South Pacific a flow of equatorial waters to the Antarctic on the east of Australia, and of Antarctic waters (Humboldt's current) to the north, along the western shores of South America; and, according to this principle, there ought to be another sargasso somewhere between New Zealand and the coast of Chili. (See Plate IX.)

139. The discovery of a new sargasso.—To test the correctness of this view, I requested Lieut. Warley to overhaul our sea-journals for notices of kelp and drift matter on the passage from Australia to Cape Horn and the Chincha Islands. He did so, and found it abounding in small patches, with "many birds about," between the parallels of 40° and 60° south, the meridians of 140° and 178° west. This sargasso is directly south of the Georgian Islands, and is, perhaps, less abundantly supplied with drift matter, less distinct in outline, and less permanent in position than any one of the others.

140. One in the South Atlantic.—There is no warm current, or if one, a very feeble one, flowing out of the South Atlantic. Most of the drift matter borne upon the ice-bearing current into that sea finds its way to the equator, and then into the veins which give volume to the Gulf Stream, and supply the sargasso of the North Atlantic with extra quantities of drift. The sargassos of the South Atlantic are therefore small. The formations and physical relations of sargassos will be again alluded to in Chapter XVIII.

141. The large volume of warm water outside of the Gulf Stream.—Let us return (§ 129) to this great expanse of warm water which, coming from the torrid zone on the south-western side of the Atlantic, drifts along to the north on the outside of the Gulf Stream. Its velocity is slow, not sufficient to give it the name of current; it is a drift, or what sailors call a "set." By the time this water reaches a parallel of 35° or 40° it has parted with a good deal of its intertropical heat: consequent upon this change in temperature is a change in specific gravity also, and by reason of this change, as well as by the difficulties of crossing the Gulf Stream, its progress to the north is arrested. It now turns to the east with the Gulf Stream, and, yielding to the force of the westerly winds of this latitude, is (§ 107) by them slowly drifted along: losing temperature by the way, these waters reach the southward flow on the east side with their specific gravity so altered that, disregarding the gentle forces of the wind, they heed the voice of the sea, and proceed to unite with this cool flow, and to set south in obedience to those dynamical laws that derive their force in the sea from differing specific gravity.

142. The resemblance between the currents in the North Atlantic and the North Pacific.—The Thermal Charts of the North Atlantic afford for these views other illustrations which, when compared with the charts of the North Pacific now in the process of construction, will make still more striking the resemblance of the two oceans in the general features of their systems of circulation. We see how, in accordance with this principle (§ 132), the currents necessary for the formation of thickly-set sargassos are generally wanting in southern oceans. How closely these two seas of the north resemble each other; and how, on account of the large openings between the Atlantic and the Frozen Ocean, the flow of warm waters to the north and of cold waters to the south is so much more active in the Atlantic than it is in the Pacific. Ought it not so to be?

143. A cushion of cool water protects the bottom of the deep sea from abrasion by its currents.—As a rule, the hottest water of the Gulf Stream is at or near the surface; and as the deep-sea thermometer is sent down, it shows that these waters, though still far warmer than the water on either side at corresponding depths, gradually become less and less warm until the bottom of the current is reached. There is reason to believe that the warm waters of the Gulf Stream are nowhere permitted, in the oceanic economy, to touch the bottom of the sea. There is everywhere a cushion of cool water between them and the solid parts of the earth's crust. This arrangement is suggestive, and strikingly-beautiful. One of the benign offices of the Gulf Stream is to convey heat from the Gulf of Mexico, where otherwise it would become excessive, and to dispense it in regions beyond the Atlantic for the amelioration of the climates of the British Islands and of all Western Europe. Now cold water is one of the best non-conductors of heat, and if the warm water of the Gulf Stream was sent across the Atlantic in contact with the solid crust of the earth—comparatively a good conductor of heat—instead of being sent across, as it is, in contact with a cold, non-conducting cushion of cool water to fend it from the bottom, much of its heat would be lost in the first part of the way, and the soft climates of both France and England would be, as that of Labrador, severe in the extreme, ice-bound, and bitterly cold.

144. Why should the Gulf Stream take its rise in the Gulf of Mexico?—That there should be in the North Atlantic Ocean a constant and copious flow and reflow of water between that ocean and the Arctic is (§ 107) not so strange, for there are abundant channel-ways between the two oceans. In one, water is to be found nearly at blood heat; in the other, as cold as ice. A familiar experiment shows that if two basins of such water be brought in connection by opening a water-way between them, the warm will immediately commence to flow to the cold, and the cold to seek the place of the warm. But why this warm flow in the Atlantic Ocean should seem to issue from the Gulf of Mexico, as if by pressure, is not so clear.

145. The trade-winds as a cause of the Gulf Stream.—To satisfy ourselves that the trade-winds have little or nothing to do in causing the Gulf Stream, we may by a process of reasoning, which ignores all the facts and circumstances already adduced, show that they cannot create a current to run when or where they do not blow. The north-east trade-winds of the Atlantic blow between the parallel of 25° and the equator; the Gulf Stream flows between the parallel of 25° and the North Pole.

146. Gulf Stream impelled by a constantly acting force.—A constantly acting power, such as the force of gravitation, is as necessary (§ 95) to keep fluids as it is to keep solids in motion. In either case the projectile force is soon overcome by resistance; and unless it be renewed, the current in the sea will cease to flow onward, as surely as a cannon-ball will stop its flight through the air when its force is spent. When the waters of Niagara reach Lake Ontario, they are no longer descending an inclined plane; there, gravity ceases to act as a propelling force, and the stream ceases to flow on, notwithstanding the impulse it derived from the falls and rapids above. A propelling power, having its seat only in the Gulf of Mexico, or the trade-wind region, could (§ 92) no more drive a jet of water across the ocean, than any other single impulse could send any other projectile that distance through either air or water. The power that conveys the waters of the Gulf Stream across the ocean is acting upon them (§ 95) every moment, like gravity upon the current of the Mississippi river; with this difference, however, the Mississippi runs down hill, the Gulf Stream on the dead level of the sea. But if we appeal (§ 80) to salt and vapour, to heat and cold, and to the secreting powers of the insects of the sea, we shall find just such sources of everlasting changes and just such constantly acting forces as are required (§ 108) to keep up and sustain, not only the Gulf Stream, but the endless round of currents in the sea, which run from the equator to the poles, and from the poles back to the equator; and these forces are derived from difference in specific gravity between the flowing and reflowing water.

147. The true cause of the Gulf Stream.—The waters of the Gulf as they go from their fountain have their specific gravity in a state of perpetual alteration in consequence of the change of saltness, and in consequence also of the change of temperature. In these changes, and not in the trade-winds, resides the power which makes the great currents of the sea.

1. Nautical Monographs, Washington Observatory, No. 1.
2. Article "Physical Geography," 8th edition Encyclopædia Britannica.
3. "The dynamics of the Gulf Stream have of late, in the work of Lieutenant Maury, already mentioned, been made the subject of much (we cannot but think misplaced) wonder, as if there could be any possible ground for doubting that it owes its origin entirely to the trade-winds."—Art. 57, Phys. Geography, 8th edition Encyc. Brit.
4. Art. 65, Phys. Geography, Encyc. Brit.
5. "We have, perhaps, been more diffuse on the subject of oceanic currents than the nature of this article may seem to justify; but some such detail seemed necessary to vindicate to the winds their supremacy in the production of currents, without calling in the feeble and ineffective aid of heated water, or the still more insignificant influence of insect secretion, which has been pressed into the service as a cause of buoyancy in the regions occupied by coral formations."—Art. 65, Phys. Geography, Encyc. Brit.
6. Navy officers of the United States Coast Survey have sounded with the deep-sea lead, and ascertained its depth here to be 370 fathoms (January 1856).
7. In this calculation the earth is treated as a perfect sphere, with a diameter of 7923.56 miles.
8. Or, 915.26 to 758.60. On the latter parallel the current has an east set of about one and a half mile the hour, making the true velocity to the east, and on the axis of the earth, about seven hundred and sixty miles an hour at the Grand Banks.
9. "If there were no atmosphere, there would be no Gulf Stream or any other considerable oceanic current (as distinguished from a mere surface drift whatever."—Art. 37, Physical Geography, 8th ed. Encyclop. Brit.
10. According to Dr. Marcet, sea-water contracts down to 28°; my own to about 25.6.
11. The Polar basin has a known water area of 3,000,000 square miles, and an unexplored area, including land and water, of 1,500,000 square miles. Whether the water in this basin be more or less salt than that of the intertropical seas, we know it is quite different in temperature, and difference of temperature will beget currents quite as readily as difference in saltness, for change in specific gravity follows either.