Chapter X: Part 10

Increasing quickly in bulk, and spreading out more and more on both sides of the equator, it flows rapidly due west toward the coast of South America. At the eastern point of South America, Cape St. Roque, the equatorial current splits into two, and one portion trends southward to deflect the isotherms of 21°, 15°.5, 10°, and 4°.5 C. into loops upon our maps, thus carrying a scrap of comfort to the Falkland Islands and Cape Horn; while the northern portion follows the northeast coast of South America, gaining continually in temperature under the influence of the tropical sun. Its speed has now increased to sixty-eight miles in twenty-four hours, and by the union with it of the waters of the river Amazon, it rises to one hundred miles (6.5 feet in a second), but it soon falls off again when it gets into the Caribbean Sea. Flowing slowly through the whole length of this sea, it reaches the Gulf of Mexico through the Strait of Yucatan, when a part of it sweeps immediately round Cuba; but the main stream, “having made the circuit of the Gulf of Mexico, passes through the Strait of Florida; thence it issues as the ‘Gulf Stream’ in a majestic current upward of thirty miles broad, two thousand two hundred feet deep, with an average velocity of four miles an hour, and a temperature of 86° Fahr. (30° C.).” The hot water pours from the strait with a decided though slight northeasterly impulse on account of its great initial velocity. Mr. Croll calculates the Gulf Stream as equal to a stream of water fifty miles broad and a thousand feet deep flowing at a rate of four miles an hour; consequently conveying 5,575,680,000,000 cubic feet of water per hour, or 133,816,320,000,000 cubic feet per day. This mass of water has a mean temperature of 18° C. as it passes out of the gulf, and on its northern journey it is cooled down to 4°.5, thus losing heat to the amount of 13°.5 C. The total quantity of heat therefore transferred from the equatorial regions per day amounts to something like 154,959,300,000,000,000,000 foot-pounds.

This is nearly equal to the whole of the heat received from the sun by the Arctic regions, and, reduced by a half to avoid all possibility of exaggeration, it is still equal to one-fifth of the whole amount received from the sun by the entire area of the North Atlantic. The Gulf Stream, as it issues from the Strait of Florida and expands into the ocean on its northward course, is probably the most glorious natural phenomenon on the face of the earth. The water is of a clear crystalline transparency and an intense blue, and long after it has passed into the open sea it keeps itself apart, easily distinguished by its warmth, its color, and its clearness; and with its edges so sharply defined that a ship may have her stem in the clear blue stream while her stern is still in the common water of the ocean.

Setting aside the wider question of the possibility of a general oceanic circulation arising from heat, cold, and evaporation, I believe that Captain Maury and Dr. Carpenter are the only authorities who of late years have disputed this source of the current which we see and can gauge and measure as it passes out of the Strait of Florida; for it is scarcely necessary to refer to the earlier speculations that it is caused by the Mississippi River, or that it flows downward by gravitation from a “head” of water produced by the trade-winds in the Caribbean Sea.

Captain Maury writes that “the dynamical force that calls forth the Gulf Stream is 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 which are little more than brackish. 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 and to maintain its level; and here, therefore, we unmask one of those 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 can not 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 vapor, into the Caribbean Sea, and then in driving them across the Atlantic, we are forced to conclude that the salt which the trade-wind vapor leaves behind it 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.”

Dr. Carpenter attributes all the great movements of ocean water to a general convective circulation, and of this general circulation he regards the Gulf Stream as a peculiarly modified case. Dr. Carpenter states that “the Gulf Stream constitutes a peculiar case, modified by local conditions,” of “a great general movement of equatorial water toward the polar area.” I confess I feel myself compelled to take a totally different view. It seems to me that the Gulf Stream is the one natural physical phenomenon on the surface of the earth whose origin and principal cause, the drift of the trade-winds, can be most clearly and easily traced.

The further progress and extension of the Gulf Stream through the North Atlantic in relation to influence upon climate has been, however, a fruitful source of controversy. The first part of its course, after leaving the strait, is sufficiently evident, for its water long remains conspicuously different in color and temperature from that of the ocean, and a current having a marked effect on navigation is long perceptible in the peculiar Gulf Stream water. “Narrow at first, it flows round the peninsula of Florida, and, with a speed of about 70 or 80 miles, follows the coast at first in a due north, afterward in a northeast direction. At the latitude of Washington it leaves the North American coast altogether, keeping its northeastward course; and to the south of the St. George’s and Newfoundland banks it spreads its waters more and more over the Atlantic Ocean, as far as the Azores. At these islands a part of it turns southward again toward the African coast. The Gulf Stream has, so long as its waters are kept together along the American coast, a temperature of 26°.6 C.; but, even under north latitude 36°, Sabine found it 23°.3 C. at the beginning of December, while the sea-water beyond the stream showed only 16°.9 C. Under north latitude 40-41° the water is, according to Humboldt, at 22°.5 C. within, and 17°.5 C. without the stream.”

Opposite Tortugas, passing along the Cuban coast, the stream is unbroken and the current feeble; the temperature at the surface is about 26°.7 C. Issuing from the Strait of Bemini the current is turned nearly directly northward by the form of the land; a little to the north of the strait, the rate is from three to five miles an hour. The depth is only 325 fathoms, and the bottom, which in the Strait of Florida was a simple slope and counter-slope, is now corrugated. The surface temperature is about 26°.5 C., while the bottom temperature is 4°.5; so that in the moderate depth of 325 fathoms the equatorial current above and the polar counter-current beneath have room to pass one another, the current from the north being evidently tempered considerably by mixture. North of Mosquito inlet the stream trends to the eastward of north, and off St. Augustine it has a decided set to the eastward. Between St. Augustine and Cape Hatteras the set of the stream and the trend of the coast differ but little, making 5° of easting in 5° of northing. At Hatteras it curves to the northward, and then runs easterly. In the latitude of Cape Charles it turns quite to the eastward, having a velocity of from a mile to a mile and a half in the hour.

A brief account of one of the sections will best explain the general phenomena of the stream off the coast of America. I will take the section following a line at right angles to the coast off Sandy Hook. From the shore out, for a distance of about 250 miles, the surface temperature gradually rises from 21° to 24° C.; at 10 fathoms it rises from 19° to 22° C.; and at 20 fathoms it maintains, with a few irregularities, a temperature of 19° C. throughout the whole space; while at 100, 200, 300, and 400 fathoms it maintains in like manner the respective temperatures of 8°.8, 5°.7, 4°.5, and 2°.5 C. This space is, therefore, occupied by cold water, and observation has sufficiently proved that the low temperature is due to a branch of the Labrador current creeping down along the coast in a direction opposite to that of the Gulf Stream. In the Strait of Florida this cold stream divides--one portion of it passing under the hot Gulf Stream water into the Gulf of Mexico, while the remainder courses round the western end of Cuba. Two hundred and forty miles from the shore the whole mass of water takes a sudden rise of about 10° C. within 25 miles, a rise affecting nearly equally the water at all depths, and thus producing the singular phenomenon of two masses of water in contact--one passing slowly southward and the other more rapidly northward, at widely different temperatures at the same levels. This abutting of the side of the cold current against that of the Gulf Stream is so abrupt that it has been aptly called by Lieutenant George M. Bache the “cold wall.” Passing the cold wall, we reach the Gulf Stream, presenting all its special characters of color and transparency and of temperature. In the section which we have chosen as an example, upward of 300 miles in length, the surface temperature is about 26°.5 C., but the heat is not uniform across the stream, for we find that throughout its entire length, as far south as the Cape Canaveral section, the stream is broken up into longitudinal alternating bands of warmer and cooler water. Off Sandy Hook, beyond the cold wall, the stream rises to a maximum of 27°.8 C., and this warm band extends for about 60 miles. The temperature then falls to a minimum of 26°.5 C., which it retains for about 30 miles, when a second maximum of 27°.4 succeeds, which includes the axis of the Gulf Stream, and is about 170 miles wide. This is followed by a second minimum of 25°.5 C., and this by a third maximum, when the bands become indistinct. It is singular that the minimum bands correspond with valley-like depressions in the bottom, which follow in succession the outline of the coast and lodge deep southward extensions of the polar indraught.

The last section of the Gulf Stream surveyed by the American hydrographers extends in a southeasterly direction from Cape Cod, lat. 41° N., and traces the Gulf Stream, still broken up by its bands of unequal temperature, spreading directly eastward across the Atlantic; its velocity has, however, now become inconsiderable, and its limits are best traced by the thermometer.

The course of the Gulf Stream beyond this point has given rise to much discussion. I again quote Professor Buff for what may be regarded as the view most generally received among physical geographers:

“A great part of the warm water is carried partly by its own motion, but chiefly by the prevailing west and northwest winds, toward the coast of Europe and even beyond Spitzbergen and Nova Zembla; and thus a part of the heat of the south reaches far into the Arctic Ocean. Hence, on the north coast of the Old Continent, we always find driftwood from the southern regions, and on this side the Arctic Ocean remains free from ice during a great part of the year, even as far up as 80° north latitude; while on the opposite coast (of Greenland) the ice is not quite thawed even in summer.” The two forces invoked by Professor Buff to perform the work are thus the _vis à tergo_ of the trade-wind drift and the direct driving power of the anti-trades, producing what has been called the anti-trade drift. This is quite in accordance with the views here advocated. The proportion in which these two forces act, it is undoubtedly impossible in the present state of our knowledge to determine.

Mr. A. G. Findlay, a high authority on all hydrographic matters, read a paper on the Gulf Stream before the Royal Geographical Society, reported in the 13th volume of the Proceedings of the Society. Mr. Findlay, while admitting that the temperature of Northern Europe is abnormally ameliorated by a surface-current of the warm water of the Atlantic which reaches it, contends that the Gulf Stream proper--that is to say, the water injected, as it were, into the Atlantic through the Strait of Florida by the impulse of the trade-winds--becomes entirely thinned out, dissipated, and lost opposite the Newfoundland banks about lat. 45° N. The warm water of the southern portion of the North Atlantic basin is still carried northward; but Mr. Findlay attributes this movement solely to the anti-trades--the southwest winds--which by their prevalence keep up a balance of progress in a northeasterly direction in the surface layer of the water.

Dr. Carpenter entertains a very strong opinion that the dispersion of the Gulf Stream may be affirmed to be complete in about lat. 45° N. and long. 35° W. Dr. Carpenter admits the accuracy of the projection of the isotherms on the maps of Berghaus, Dove, Petermann, and Keith Johnston, and he admits likewise the conclusion that the abnormal mildness of the climate on the northwestern coast of Europe is due to a movement of equatorial water in a northeasterly direction. “What I question is the correctness of the doctrine that the northeast flow is an extension or prolongation of the Gulf Stream, still driven on by the _vis à tergo_ of the trade-winds--a doctrine which (greatly to my surprise) has been adopted and defended by my colleague, Professor Wyville Thomson. But while these authorities attribute the whole or nearly the whole of this flow to the true Gulf Stream, _I_ regard a large part, if not the whole, of that which takes place along our own western coast, and passes north and northeast between Iceland and Norway toward Spitzbergen, as quite independent of that agency; so that it would continue if the North and South American Continents were so completely disunited that the equatorial currents would be driven straight onward by the trade-winds into the Pacific Ocean, instead of being embayed in the Gulf of Mexico and driven out in a northeast direction through the ‘narrows’ off Cape Florida.” Dr. Carpenter does not mean by this to indorse Mr. Findlay’s opinion that the movement beyond the 54th parallel of latitude is due solely to the drift of the anti-trades; he says, “On the view I advocate, the northeasterly flow is regarded as due to the _vis à fronte_ originating in the action of cold upon the water of the polar area, whereby its level is always tending to depression.” The amelioration of the climate of northwestern Europe is thus caused by a “modified case” of the general oceanic circulation, and neither by the Gulf Stream nor by the anti-trade drift.

Although there are, up to the present time, very few trustworthy observations of deep-sea temperatures, the surface temperature of the North Atlantic has been investigated with considerable care. The general character of the isothermal lines, with their singular loop-like northern deflections, has long been familiar through the temperature charts of the geographers already quoted, and of late years a prodigious amount of data have been accumulated.

In 1870, Dr. Petermann, of Gotha, published an extremely valuable series of temperature charts, embodying the results of the reduction of upward of 100,000 observations.

Dr. Petermann has devoted the special attention of a great part of his life to the distribution of heat on the surface of the ocean, and the accuracy and conscientiousness of his work in every detail are beyond the shadow of a doubt.

In the North Atlantic every curve of equal temperature, whether for the summer, for the winter, for a single month, or for the whole year, instantly declares itself as one of a system of curves which are referred to the Strait of Florida as a source of heat, and the flow of warm water may be traced in a continuous stream--indicated when its movement can no longer be observed by its form--fanning out from the neighborhood of the Strait across the Atlantic, skirting the coasts of France, Britain, and Scandinavia, rounding the North Cape, and passing the White Sea and the Sea of Kari, bathing the western shores of Nova Zembla and Spitzbergen, and finally coursing round the coast of Siberia, a trace of it still remaining to find its way through the narrow and shallow Behring’s Strait into the North Pacific.

Now, it seems to me that if we had only these curves upon the chart, deduced from an almost infinite number of observations which are themselves merely laboriously multiplied corroborations of many previous ones, without having any clew to their rationale, we should be compelled to admit that whatever might be the amount and distribution of heat derived from a general oceanic circulation--whether produced by the prevailing winds of the region, by convection, by unequal barometric pressure, by tropical heat, or by arctic cold--the Gulf Stream, the majestic stream of warm water whose course is indicated by the deflections of the isothermal lines, is sufficiently powerful to mask all the rest, and, broadly speaking, to produce of itself all the abnormal thermal phenomena.

The deep-sea temperatures taken in the _Porcupine_ have an important bearing upon this question, since they give us the depth and volume of the mass of water which is heated above its normal temperature, and which we must regard as the softener of the winds blowing on the coasts of Europe. In the Bay of Biscay, after passing through a shallow band superheated by direct radiation, a zone of warm water extends to the depth of 800 fathoms, succeeded by cold water to a depth of nearly two miles. In the Rockall channel the warm layer has nearly the same thickness, and the cold underlying water is 500 fathoms deep. Off the Butt of Lewis the bottom temperature is 5°.2 C. at 767 fathoms, so that there the warm layer evidently reaches to the bottom. In the Faroe channel the warm water forms a surface layer, and the cold water underlies it, commencing at a depth of 200 fathoms--567 fathoms above the level of the bottom of the warm water off the Butt of Lewis. The cold water abuts against the warm--there is no barrier between them. Part of the warm water flows over the cold indraught, and forms the upper layer in the Faroe channel. What prevents the cold water from slipping, by virtue of its greater weight, under the warm water of the Butt of Lewis? It is quite evident that there must be some force at work keeping the warm water in that particular position, or, if it be moving, compelling it to follow that particular course. The comparatively high temperature from 100 fathoms to 900 fathoms I have always attributed to the northern accumulation of the water of the Gulf Stream. The amount of heat derived directly from the sun by the water as it passes through any particular region, must be regarded, as I have already said, as depending almost entirely upon latitude. Taking this into account, the surface temperatures in what we were in the habit of calling the “warm area” coincided precisely with Petermann’s curves indicating the northward path of the Gulf Stream.

Showing the Tendency to take the Form of Six-Pointed Figures]

The North Atlantic and Arctic seas form together a _cul de sac_ closed to the northward, for there is practically no passage for a body of water through Behring’s Strait. While, therefore, a large portion of the water, finding no free outlet toward the northeast, turns southward at the Azores, the remainder, instead of thinning off, has rather a tendency to accumulate against the coasts bounding the northern portions of the trough. We accordingly find that it has a depth off the west coast of Iceland of at least 4,800 feet, with an unknown lateral extension. Dr. Carpenter, discussing this opinion, says: “It is to me physically inconceivable that this surface film of _lighter_ (because warmer) water should collect itself together again--even supposing it still to retain any excess of temperature--and should burrow downward into the ‘trough,’ _displacing colder and heavier water_, to a depth much greater than that which it possesses at the point of its greatest ‘glory’--its passage through the Florida Narrows. The upholders of this hypothesis have to explain how such a recollection and dipping-down of the Gulf Stream water is to be accounted for on physical principles.” I believe that, as a rule, experimental imitations on a small scale are of little use in the illustration of natural phenomena; a very simple experiment will, however, show that such a process is possible. If we put a tablespoonful of cochineal into a can of hot water, so as to give it a red tint, and then run it through a piece of India-rubber tube with a considerable impulse along the surface of a quantity of cold water in a bath, we see the red stream widening out and becoming paler over the general surface of the water till it reaches the opposite edge, and very shortly the rapidly heightening color of a band along the opposite wall indicates an accumulation of the colored water where its current is arrested. If we now dip the hand into the water of the centre of the bath, a warm bracelet merely encircles the wrist; while at the end of the bath opposite the warm influx, the hot water, though considerably mixed, envelops the whole hand.

The North Atlantic forms a basin closed to the northward. Into the corner of this basin, as into a bath--with a northeasterly direction given to it by its initial velocity, as if the supply pipe of the bath were turned so as to give the hot water a definite impulse--this enormous flood is poured, day and night, winter and summer. When the basin is full--and not till then--overcoming its northern impulse, the surplus water turns southward in a southern eddy, so that there is a certain tendency for the hot water to accumulate in the northern basin, to “bank down” along the northeastern coasts.

It is scarcely necessary to say that for every unit of water which enters the basin of the North Atlantic, and which is not evaporated, an equivalent must return. As cold water can gravitate into the deeper parts of the ocean from all directions, it is only under peculiar circumstances that any movement having the character of a current is induced; these circumstances occur, however, in the confined and contracted communication between the North Atlantic and the Arctic Sea. Between Cape Farewell and North Cape there are only two channels of any considerable depth, the one very narrow along the east coast of Iceland, and the other along the east coast of Greenland. The shallow part of the sea is entirely occupied, at all events during summer, by the warm water of the Gulf Stream, except at one point, where a rapid current of cold water, very restricted and very shallow, sweeps round the south of Spitzbergen and then dips under the Gulf Stream water at the northern entrance of the German Ocean.

This cold flow, at first a current, finally a mere indraught, affects greatly the temperature of the German Ocean; but it is entirely lost, for the slight current which is again produced by the great contraction at the Strait of Dover has a summer temperature of 7°.5 C. The path of the cold indraught from Spitzbergen may be readily traced by the depressions in the surface isothermal lines, and in dredging by the abundance of gigantic amphipodous and isopodous crustaceans, and other well-known Arctic animal forms.

From its low initial velocity the Arctic return current, or indraught, must doubtless tend slightly in a westerly direction, and the higher specific gravity of the cold water may probably even more powerfully lead it into the deepest channels; or possibly the two causes may combine, and in the course of ages the currents may hollow out deep southwesterly grooves. The most marked is the Labrador current, which passes down inside the Gulf Stream along the coasts of Carolina and New Jersey, meeting it in the strange abrupt “cold wall,” dipping under it as it issues from the Gulf, coming to the surface again on the other side, and a portion of it actually passing under the Gulf Stream, as a cold counter-current, into the Gulf of Mexico.

Fifty or sixty miles out from the west coast of Scotland, I believe the Gulf Stream forms another, though a very mitigated, “cold wall.” In 1868, after our first investigation of the very remarkable cold indraught into the channel between Shetland and Faroe, I stated my belief that the current was entirely banked up in the Faroe Channel by the Gulf Stream passing its gorge. Since that time I have been led to suspect that a part of the Arctic water oozes down the Scottish coast, much mixed, and sufficiently shallow to be affected throughout by solar radiation. About sixty or seventy miles from shore the isothermal lines have a slight but uniform deflection. Within that line types characteristic of the Scandinavian fauna are numerous in shallow water, and in the course of many years’ use of the towing net I have never met with any of the Gulf Stream pteropods, or of the lovely Polycystina and Acanthometrina which absolutely swarm beyond that limit. The difference in mean temperature between the east and west coasts of Scotland, amounting to about 1° C., is almost somewhat less than might be expected if the Gulf Stream came close to the western shore.

While the communication between the North Atlantic and the Arctic Sea--itself a second _cul de sac_--is thus restricted, limiting the interchange of warm and cold water in the normal direction of the flow of the Gulf Stream, and causing the diversion of a large part of the stream to the southward, the communication with the Antarctic basin is as open as the day; a continuous and wide valley upward of 2,000 fathoms in depth stretching northward along the western coasts of Africa and Europe.

That the southern water wells up into this valley there could be little doubt from the form of the ground; but here again we have curious corroborative evidence in the remarkable reversal of the curves of the isotherms. The temperature of the bottom water at 1,230 fathoms off Rockall is 3°.22 C., exactly the same as that of water at the same depth in the serial sounding, lat. 47° 38′ N., long. 12° 08′ W. in the Bay of Biscay, which affords a strong presumption that the water in both cases is derived from the same source; and the bottom water off Rockall is warmer than the bottom water in the Bay of Biscay (2°.5 C.), while a cordon of temperature soundings drawn from the northwest of Scotland to a point on the Iceland shallow gives no temperature lower than 6°.5 C. This makes it very improbable that the low temperature of the Bay of Biscay is due to any considerable portion of the Spitzbergen current passing down the west coast of Scotland; and as the cold current to the east of Iceland passes southward considerably to the westward, as indicated by the successive depressions in the surface isotherms, the balance of probability seems to be in favor of the view that the conditions of temperature and the slow movement of this vast mass of moderately cold water, nearly two statute miles in depth, are to be referred to an Antarctic rather than to an Arctic origin.

The North Atlantic Ocean seems to consist first of a great sheet of warm water, the general northerly reflux of the equatorial current. Of this the greater part passes through the Strait of Florida, and its northeasterly flow is aided and maintained by the anti-trades, the whole being generally called the Gulf Stream. This layer is of varying depths, apparently from the observations of Captain Chimmo and others, thinning to a hundred fathoms or so in the mid-Atlantic, but attaining a depth of 700 to 800 fathoms off the west coasts of Ireland and Spain. Secondly, of a “stratum of intermixture” which extends to about 200 fathoms in the Bay of Biscay, through which the temperature falls rather rapidly; and, thirdly, of an underlying mass of cold water, in the Bay of Biscay 1,500 fathoms deep, derived as an indraught falling in by gravitation from the deepest available source, whether Arctic or Antarctic. It seems at first sight a startling suggestion, that the cold water filling deep ocean valleys in the Northern Hemisphere may be partly derived from the southern; but this difficulty, I believe, arises from the idea that there is a kind of diaphragm at the equator between the northern and southern ocean basins, one of the many misconceptions which follow in the train of a notion of a convective circulation in the sea similar to that in the atmosphere. There is undoubtedly a gradual elevation of an intertropical belt of the underlying cold water, which is being raised by the subsiding of still colder water into its bed to supply the place of the water removed by the equatorial current and by excessive evaporation; but such a movement must be widely and irregularly diffused and excessively slow, not in any sense comparable with the diaphragm produced in the atmosphere by the rushing upward of the northeast and southeast trade-winds in the zone of calms. Perhaps one of the most conclusive proofs of the extreme slowness of the movement of the deep indraught is the nature of the bottom. Over a great part of the floor of the Atlantic a deposit is being formed of microscopic shells. These with their living inhabitants differ little in specific weight from the water itself, and form a creamy flocculent layer, which must be at once removed wherever there is a perceptible movement. In water of moderate depth, in the course of any of the currents, this deposit is entirely absent, and is replaced by coarser or finer gravel.

It is only on the surface of the sea that a line is drawn between the two hemispheres by the equatorial current, whose effect in shedding a vast intertropical drift of water on either side as it breaks against the eastern shores of equatorial land may be seen at a glance on the most elementary physical chart.

The Gulf Stream loses an enormous amount of heat in its northern tour. At a point 200 miles west of Ushant, where observations at the greatest depths were made on board the _Porcupine_, a section of the water of the Atlantic shows three surfaces at which interchange of temperature is taking place. First, the surface of the sea--that is to say, the upper surface of the Gulf Stream layer--is losing heat rapidly by radiation, by contact with a layer of air which is in constant motion and being perpetually cooled by convection, and by the conversion of water into vapor. As this cooling of the Gulf Stream layer takes place principally at the surface, the temperature of the mass is kept pretty uniform by convection. Secondly, the band of contact of the lower surface of the Gulf Stream water with the upper surface of the cold indraught. Here the interchange of temperature must be very slow, though that it does take place is shown by the slight depression of the surface isotherms over the principal paths of the indraught. But there is a good deal of intermixture extending through a considerable layer. The cold water being beneath, convection in the ordinary sense can not occur, and interchange of temperature must depend mainly upon conduction and diffusion, causes which in the case of masses of water must be almost secular in their action, and probably to a much greater extent upon mixture produced by local currents and by the tides. The third surface is that of contact between the cold indraught and the bottom of the sea. The temperature of the crust of the earth has been variously calculated at from 4° to 11° C., but it must be completely cooled down by anything like a movement and constant renewal of cold water. All we can say, therefore, is that contact with the bottom can never be a source of depression of temperature. As a general result the Gulf Stream water is nearly uniform in temperature throughout the greater part of its depth; there is a marked zone of intermixture at the junction between the warm water and the cold, and the water of the cold indraught is regularly stratified by gravitation; so that in deep water the contour lines of the sea-bottom are, speaking generally, lines of equal temperature. Keeping in view the enormous influence which ocean currents exercise in the distribution of climates at the present time, I think it is scarcely going too far to suppose that such currents--movements communicated to the water by constant winds--existed at all geological periods as the great means, I had almost said the sole means, of producing a general oceanic circulation, and thus distributing heat in the ocean. They must have existed, in fact, wherever equatorial land interrupted the path of the drift of the trade-winds. Wherever a warm current was deflected to north or south from the equatorial belt a polar indraught crept in beneath to supply its place; and the ocean consequently consisted, as in the Atlantic and doubtless in the Pacific at the present day, of an upper warm stratum and a lower layer of cold water becoming gradually colder with increasing depth.

I must repeat that I have seen as yet no reason to modify the opinion which I have consistently held from the first, that the remarkable conditions of climate on the coasts of Northern Europe are due in a broad sense solely to the Gulf Stream. That is to say that, although movements, some of them possibly of considerable importance, must be produced by differences of specific gravity, yet the influence of the great current which we call the Gulf Stream, the reflux of the great equatorial current, is so paramount as to reduce all other causes to utter insignificance.

THE PHOSPHORESCENCE OF THE SEA
--G. HARTWIG

He who still lingers on the shore after the shades of evening have descended not seldom enjoys a most magnificent spectacle; for lucid flashes burst from the bosom of the waters, as if the sea were anxious to restore to the darkened heavens the light it had received from them during the day. On approaching the margin of the rising flood to examine more closely the sparkling of the breaking wave, the spreading waters seem to cover the beach with a sheet of fire.

Each footstep over the moist sands elicits luminous star-like points and a splash in the water resembles the awakening of slumbering flames. The same wonderful and beauteous aspect frequently gladdens the eye of the navigator who plows his way through the wide deserts of ocean, particularly if his course leads him through the tropical seas.

“When a vessel,” says Humboldt, “driven along by a fresh wind, divides the foaming waters, one never wearies of the lovely spectacle their agitation affords; for, whenever a wave makes the ship incline sidewise, bluish or reddish flames seem to shoot upward from the keel. Beautiful beyond description is the sight of a troop of dolphins gamboling in the phosphorescent sea. Every furrow they draw through the waters is marked by streaks of intense light. In the Gulf of Cariaco, between Cumana and the peninsula of Maniquarez, this scene has often delighted me for hours.”

But even in the colder oceanic regions the brilliant phenomenon appears from time to time in its full glory. During a dark and stormy September night, on the way from the Sealion Island, Saint George, to Unalashka, Chamisso admired as beautiful a phosphorescence of the ocean as he had ever witnessed in the tropical seas. Sparks of light, remaining attached to the sails that had been wetted by the spray, continued to glow in another element. Near the south point of Kamtchatka, at a water temperature hardly above freezing point, Ermann saw the sea no less luminous than during a seven months’ sojourn in the tropical ocean. This distinguished traveler positively denies that warmth decidedly favors the luminosity of the sea.

At Cape Colborn, one of the desolate promontories of the desolate Victoria Land, the phosphoric gleaming of the waves, when darkness closed in, was so intense that Simpson assures us he had seldom seen anything more brilliant. The boats seemed to cleave a flood of molten silver, and the spray, dashed from their bows before the fresh breeze, fell back in glittering showers into the deep.

Mr. Charles Darwin paints in vivid colors the magnificent spectacle presented by the sea while sailing in the latitude of Cape Horn on a very dark night. There was a fresh breeze, and every part of the surface, which during the day is seen as foam, now glowed with a pale light. The vessel drove before her bows two billows of liquid phosphorus, and in her wake she was followed by a milky train. As far as the eye reached, the crest of every wave was bright, and the sky above the horizon, from the reflected glare of these livid flames, was not so utterly obscure as over the rest of the heavens.

While _La Venus_ was at anchor before Simon’s Town, the breaking of the waves produced so strong a light that the room in which the naturalists of the expedition were seated was illumined as by sudden flashes of lightning. Although more than fifty paces from the beach when the phenomenon took place, they tried to read by this wondrous oceanic light, but the successive glimpses were of too short duration to gratify their wishes.

Thus we see the same nocturnal splendor which shines forth in the tropical seas and gleams along our shores burst forth from the Arctic waters, and from the waves that bathe the southern promontories of the Old and the New Worlds.

But what is the cause of the beautiful phenomenon so widely spread over the face of the ocean? How comes it that at certain times flames issue from the bosom of an element generally so hostile to their appearance?

Without troubling the reader with the groundless surmises of ancient naturalists, or repeating the useless tales of the past, I shall at once place myself with him on the stage of our actual knowledge of this interesting and mysterious subject.

It is now no longer a matter of doubt that many of the inferior marine animals possess the faculty of secreting a luminous matter, and thus adding their mite to the grand phenomenon. When we consider their countless multitudes, we shall no longer wonder at such magnificent effects being produced by creatures individually so insignificant.

In our seas it is chiefly a minute gelatinous animal, the _Noctiluca miliaris_, most probably an aberrant member of the infusorial group, which, as it were, repeats the splendid spectacle of the starry heavens on the surface of the ocean. In form it is nearly globular, presenting on one side a groove, from the anterior extremity of which issues a peculiar curved stalk or appendage marked by transverse lines, which might seem to be made use of as an organ of locomotion. Near the base of this tentacle is placed the mouth, which passes into a dilatable digestive cavity, leading, according to Mr. Huxley, to a distinct anal orifice. From the rather firm external coat proceed thread-like prolongations through the softer mass of the body, so as to divide it into irregular chambers. This little creature, which is just large enough to be discerned by the naked eye when the water in which it may be swimming is contained in a glass jar exposed to the light, seems to feed on diatoms, as this loricæ may frequently be detected in its interior. It multiplies by spontaneous fission, and the rapidity of this process may be inferred from the immensity of its numbers. A single bucket of luminous sea-water will often contain thousands, while for miles and miles every wave breaking on the shore expands in a sheet of living flame. It was first described by Forster in the Pacific Ocean; it occurs on all the shores of the Atlantic; and the Polar Seas are illumined by its fairy light. “The nature of its luminosity,” says Dr. Carpenter, “is found by microscopic examination to be very peculiar; for what appears to the eye to be a uniform glow is resolvable under a sufficient power into a multitude of evanescent scintillations, and these are given forth with increased intensity whenever the body of the animal receives any mechanical shock.”

The power of emitting a phosphorescent light is widely diffused, both among the free-swimming and the sessile _Cœlenterata_. Many of the _Physophoridæ_ are remarkable for its manifestation, and a great number of the jelly-fishes are luminous. Our own _Thaumantias lucifera_, a small and by no means rare medusid, displays the phenomenon in a very beautiful manner, for, when irritated by contact of fresh water, it marks its position by a vivid circlet of tiny stars, each shining from the base of a tentacle. A remarkable greenish light, like that of burning silver, may also be seen to glow from many of our Sertularians, becoming much brighter under various modes of excitation.

Among the _Ctenophora_ the large _Cestum Veneris_ of the Mediterranean is specially distinguished for its luminosity, and while moving beneath the surface of the water gleams at night like a brilliant band of flame.

The Sea-pens are eminently phosphorescent, shining at night with a golden-green light of a most wonderful softness. When touched, every branchlet above the shock emits a phosphoric glow, while all the polyps beneath remain in darkness. When thrown into fresh water or alcohol, they scatter sparks about in all directions, a most beautiful sight; dying, as it were, in a halo of glory.

But of all the marine animals the Pyrosomas, doing full justice to their name (fire bodies), seem to emit the most vivid coruscations. Bibra relates in his _Travels to Chili_ that he once caught half a dozen of these remarkable light-bearers, by whose phosphorescence he could distinctly read their own description in a naturalist’s vade-mecum. Although completely dark when at rest, the slightest touch sufficed to elicit their clear blue-green light. During a voyage to India, Mr. Bennett had occasion to admire the magnificent spectacle afforded by whole shoals of Pyrosomas. The ship, proceeding at a rapid rate, continued during an entire night to pass through distinct but extensive fields of these mollusks, floating and glowing as they floated on all sides of her course. Enveloped in a flame of bright phosphorescent light, and gleaming with a greenish lustre, the Pyrosomas, in vast sheets, upward of a mile in breadth, and stretching out till lost in the distance, presented a sight the glory of which may be easily imagined. The vessel, as it chased the gleaming mass, threw up strong flashes of light, as if plowing through liquid fire, which illuminated the hull, the sails, and the ropes with a strange, unearthly radiance.

In his memoir on the Pyrosoma, M. Péron describes with lively colors the circumstances under which he first made its discovery, during a dark and stormy night, in the tropical Atlantic. “The sky,” says this distinguished naturalist, “was on all sides loaded with heavy clouds; all around the obscurity was profound; the wind blew violently; and the ship cut her way with rapidity. Suddenly we discovered at some distance a great phosphorescent band stretched across the waves, and occupying an immense tract in advance of the ship. Heightened by the surrounding circumstances, the effect of this spectacle was romantic, imposing, sublime, riveting the attention of all on board. Soon we reached the illuminated tract, and perceived that the prodigious brightness was certainly and only attributable to the presence of an innumerable multitude of largish animals floating with the waves. From their swimming at different depths they took apparently different forms--those at the greatest depths were very indefinite, presenting much the appearance of great masses of fire, or rather enormous, red-hot cannon-balls; while those more distinctly seen near the surface perfectly resembled incandescent cylinders of iron.

“Taken from the water, these animals entirely resembled each other in form, color, substance, and the property of phosphorescence, differing only in their sizes, which varied from three to seven inches. The large, longish tubercles with which the exterior of the Pyrosomas was bristled were of a firmer substance, and more transparent than the rest of the body, and were brilliant and polished like diamonds. These were the principal scene of phosphorescence. Between these large tubercles, smaller ones, shorter and more obtuse, could be distinguished; these also were phosphorescent. Lastly, in the interior of the substance of the animal, could be seen, by the aid of the transparency, a number of little, elongated, narrow bodies (viscera), which also participated in a high degree in the possession of phosphoric light.”

In the Pholades or Lithodomes, that bore their dwellings in hard stone, as other shell-fish do in the loose sands, the whole mass of the body is permeated with light. Pliny gives us a short but animated description of the phenomenon in the edible date-shell of the Mediterranean (_Pholas dactylus_):

“It is in the nature of the pholades to shine in the darkness with their own light, which is the more intense as the animal is more juicy. While eating them, they shine in the mouth and on the hands, nay, even the drops falling from them upon the ground continue to emit light, a sure proof that the luminosity we admire in them is associated with their juice.”

Milne-Edwards found this observation perfectly correct, for, wishing to place some living pholades in alcohol, he saw a luminous matter exude from their bodies, which, on account of its weight, sank in the liquid, covering the bottom of the vessel, and there forming a deposit as shining as when it was in contact with the air.

Several kinds of fishes likewise possess the luminous faculty. The sunfish, that strange deformity emits a phosphoric gleam; and a species of Gunard (_Trigla lucerna_) is said to sparkle in the night, so as to form fiery streams through the water.

With regard to the luminosity of the larger marine animals, Ermann, however, remarks that he so often saw small luminous crustacea in the abdominal cavity of the transparent _Salpa pinnata_ that it may well be asked whether the phosphorescence of the larger creatures is not in reality owing to that of their smaller companions.

According to Mr. Bennett--_Whaling Voyage Round the Globe_--a species of shark first discovered by himself is distinguished by an uncommonly strong emission of light. When the specimen, taken at night, was removed into a dark apartment, it afforded a very interesting spectacle. The entire inferior surface of the body and head emitted a vivid and greenish phosphorescent gleam, imparting to the creature by its own light a truly ghastly and terrific appearance. The luminous effect was constant, and not perceptibly increased by agitation or friction. When the shark expired (which was not until it had been out of the water more than three hours), the luminous appearance faded entirely from the abdomen, and more gradually from other parts, lingering longest around the jaws and on the fins.

The only part of the under surface of the animal which was free from luminosity was the black collar round the throat; and while the inferior surface of the pectoral, anal, and caudal fins shone with splendor, their superior surface (including the upper lobe of the tail fin) was in darkness, as were also the dorsal fins and the back and summit of the head.

Mr. Bennett is inclined to believe that the luminous power of this shark resides in a peculiar secretion from the skin. It was his first impression that the fish had accidentally contracted some phosphorescent matter from the sea, or from the net in which it was captured; but the most rigid investigation did not confirm this suspicion, while the uniformity with which the luminous gleam occupied certain portions of the body and fins, its permanence during life, and decline and cessation upon the approach and occurrence of death, did not leave a doubt in his mind but that it was a vital principle essential to the economy of the animal. The small size of the fins would seem to denote that this fish is not active in swimming; and, since it is highly predaceous and evidently of nocturnal habits, we may perhaps indulge in the hypothesis that the phosphorescent power it possesses is of use to attract its prey, upon the same principle as the Polynesian islanders and others employ torches in night-fishing.

Some of the lower sea-plants also appear to be luminous. Thus, over a space of more than 600 miles (between lat. 8° N. and 2° S.), Meyen saw the ocean covered with phosphorescent _Oscillatoria_, grouped together into small balls or globules, from the size of a poppy-seed to that of a lentil.

But if the luminosity of the ocean generally proceeds from living creatures, it sometimes also arises from putrefying organic fibres and membranes, resulting from the decomposition of these living light bearers. “Sometimes,” says Humboldt, “even a high magnifying power is unable to discover any animals in the phosphorescent water, and yet light gleams forth wherever a wave strikes against a hard body and dissolves in foam. The cause of this phenomenon lies then most likely in the putrefying fibres of dead mollusks, which are mixed with the waters in countless numbers.”

Summing up the foregoing in a few words, it is thus an indisputable fact that the phosphorescence of the sea is by no means an electrical or magnetic property of the water, but exclusively bound to organic matter, living or dead. But although thus much has been ascertained, we have as yet only advanced one step toward the unraveling of the mystery, and its prominent cause remains an open question. Unfortunately, science is still unable to give a positive answer, and we are obliged to be content with a more or less plausible hypothesis.

We know as little of what utility marine phosphorescence may be. Why do the countless myriads of Mammariæ gleam and sparkle along our coasts? Is it to signify their presence to other animals, and direct them to the spot where they may find abundance of food? So much is certain, that so grand and widespread a phenomenon must necessarily serve some end equally grand and important.

As the phosphorescence of the sea is owing to living creatures, it must naturally show itself in its greatest brilliancy when the ocean is at rest; for during the daytime we find the surface of the waters most peopled with various animals when only a slight zephyr glides over the sea. In stormy weather, the fragile or gelatinous world of the lower marine creatures generally seek a greater depth, until the elementary strife has ceased, when it again loves to sport in the warmer or more cheerful superficial waters.

In the tropical zone, Humboldt saw the sea most brilliantly luminous before a storm, when the air was sultry and the sky covered with clouds. In the North Sea we observe the phenomenon most commonly during fine, tranquil autumnal nights; but it may be seen at every season of the year, even when the cold is most intense. Its appearance is, however, extremely capricious; for, under seemingly unaltered circumstances, the sea may one night be very luminous and the next quite dark. Often months, even years, pass by without witnessing it in full perfection. Does this result from a peculiar state of the atmosphere, or do the little animals love to migrate from one part of the coast to another?

It is remarkable that the ancients should have taken so little notice of oceanic phosphorescence. The _Periplus_ of Hanno contains, perhaps, the only passage in which the phenomenon is described.