Encyclopædia Britannica, Ninth Edition/Atlantic Ocean
THE designation Atlantic Ocean, originally given to the i sea that lies beyond the great range of Atlas in North-western Africa, has come to be applied, with the extension of geographical knowledge, to the whole of that vast ocean which occupies the wide and deep trough that separates the New from the Old World. Its limits are variously defined; some geographers regarding it as extending from pole to pole, whilst others consider it as bounded at its northern and southern extremities by the Arctic and Antarctic circles respectively. As the peculiarity of the physical conditions of the Polar Seas renders it on every account more appropriate to describe them under a separate head (POLAR REGIONS), the Atlantic will be here treated as bounded at the north by the Arctic circle, which nearly corresponds with the natural closing-in of its basin by the approach of the coasts of Norway and Greenland i with Iceland lying between them ; while at the south, where ! the basin is at its widest, its only boundary is the Antarctic ! circle. The line which separates its southern extension from the Indian Ocean may be considered to be the meridian of Cape Agulhas, the southernmost point of the African continent ; whilst the boundary between the South Atlantic and South Pacific would be formed in like manner by the meridian of Cape Horn. Although the Baltic and the Mediterranean are commonly regarded as appendages to the Atlantic, yet their physical conditions are so peculiar as to require separate treatment. (See BALTIC and MEDI TERRANEAN.) Every physical geographer who has written upon the Atlantic has noticed the curious parallelism between its eastern and its western borders, their salient and retiring angles corresponding very closely to each other. Thus, beginning at the north we see that the projection formed by the British Islands (which extends much further westwards at 100 fathoms below the surface than it does above the sea-level), answers to the wide entrance to Baffin s Bay ; whilst, on the other hand, the projection of the American coast at Newfoundland answers to the Bay of Biscay. Further south, the great rounded prominence of Northern Africa corresponds with the vast bay that stretches from Nova Scotia to St Thomas ; whilst the angular projection of South America towards the east corresponds with that receding portion of the mid-African coast-line which is known as the Gulf of Guinea. This correspondence suggested to Humboldt the idea that the Atlantic basin was originally excavated by a very violent rush of water from the south, which, being repulsed by the mountain ranges of Brazil, was directed by them towards the coast of Africa, and formed the Gulf of Guinea; being there checked and turned to the west by the mountains of Upper Guinea, the stream excavated the Caribbean Sea and the Gulf of Mexico ; and issuing thence, it ran between the mountains of North America and Western Europe, until it gradually diminished in velocity and force, and at length subsided. Another writer speaks of the basin of the Atlantic as an immense rift, made by some terrible force, which rent the surface-land asunder, but left the edges of the ravine to show by their form that they had once been connected. For neither of these specula tions, however, is there the smallest foundation in fact. What has to be accounted for, indeed, in regard to either of the great areas at present covered by water, is not so much the excavation of its sea-bed, as its segregation from an ocean originally universal by the boundaries that now enclose it ; in other words, not so much the depression of the bottom of its basin as the elevation of its sides. Not only is the proportion of the laud-surface of the globe to its water- surface scarcely more than one-third (being as 1 to 2 78), but the entire mass of the land which thus covers little more than one-fourth of the surface of the globe is quite insignificant in comparison with that of the water which covers the remaining three-fourths. For whilst the average elevation of the whole land is certainly less than one-fifth of a mile, giving from 9 to 10 millions of cubic miles as the total mass of land that rises above the sea-level, the average depth of the sea (so far as at present known) may be taken at about 2 miles, giving a total of nearly 290 millions of cubic miles of water, which is therefore about thirty times the mass of the land. From the computation of Keith Johnston, it appears that, " if we conceive an equalising line, which, passing around the globe, would leave a mass of the earth s crust above it, just sufficient to fill up the hollow which would be left below it, this line would then fall nearly a mile below the present level of the sea." This is tantamount to saying that, if the solid crust of the earth could be conceived to be smoothed down to one uniform level, its entire surface would be covered with water to the depth of about a mile. Hence it is obvious that as the elevation of that crust into land over certain areas must be accompanied by a corresponding depression of the sea-bed over other areas, such depression, augmenting in those areas the previous depth of the aqueous covering of the globe, would be quite sufficient to account for the existence of the great oceanic basins, without any excavating action. And a confirmation of this view is found in the fact, ascertained by recent soundings, that the deepest local depressions of the sea-bed are met with in the neighbourhood of islands that have been raised by volcanic agency. Further, as the quantity of solid mat ter that must have been removed (on Humboldt s hypo thesis) in the excavation of the Atlantic valley must have been nearly four times as great as that which forms the whole known land of the globe, and as it is impos sible to conceive of any mode in which such a mass can have been disposed of, we may dismiss that hypothesis as not only untenable in regard to the Atlantic basin, but as equally inapplicable to any other valley of similar width and depth. 1 The general direction of geological opinion, indeed, has of late been, on physical grounds, towards the high anti quity of the great oceanic basins, not exactly as at present bounded, but as areas of depression having the same rela tion as they have now to the areas of elevation which form the great continents. Thus Sir Charles Lyell was strongly impressed by the fact that the mean depth of the sea is not improbably fifteen times as great as the mean height of the land ; and that depressions of the sea-bottom to a depth of three miles or more extend over wide areas, whilst elevations of the land to similar height are confined to a few peaks and narrow ridges. Hence, he remarked, " while the effect of vertical movements equalling 1000 feet in both directions, upward and downward, is to cause a vast transposition of land and sea in those areas which are now continental, and adjoining to which there is much sea not exceeding 1000 feet in depth, movements of equal amount would have no tendency to produce a sensible alteration in the Atlantic or Pacific Oceans, or to cause the oceanic and continental areas to change places. Depressions of 1000 feet would submerge large areas of the existing land; but fifteen times as much movement would be required to convert such land into an ocean of average depth, or to cause an ocean three miles deep to replace any one of the existing continents." 2 And Professor Dana, who, more than any other geologist, has studied the structure of the existing continents and the succession of changes concerned in their elevation, has been led, by the consideration of the probable direction of the forces by which that elevation was effected, to conclude that the defining of the present con tinental and oceanic areas began with the commencement of the solidification of the earth s crust. " The continental areas are the areas of least contraction, and the oceanic basins those of the greatest, the former having earliest had a solid crust. After the continental part was thus stiffened, and rendered comparatively unyielding, the oceanic part went on cooling, solidifying, and contracting throughout ; consequently, it became depressed, with the sides of the depression somewhat abrupt. The formation of the oceanic basins and continental areas was thus due to unequal radial contraction. " In the opinion of Professor Dana, there has never been any essential change in the relations of these great features. "It is hardly possible," he says, "to conceive of any conditions of the contracting forces that should have allowed of the continents and oceans in after time changing places, or of oceans, as deep nearly as exist ing oceans, being made where are now the continental areas; although it is a necessary incident to the system of things that the continental plateaus should have varied greatly in their outline and outer limits, and perhaps thousands of feet in the depths of some portions of the overlying seas, and also that the oceans should have varied in the extent of their lands." ..." The early defining, even in Archsean times, of the final features of North America, ;nid the con formity to one system visibly marked out in every event through the whole history in the positions of its outlines and the formations of its rocks, in the character of its oscillations, and the courses of the mountains from time to time raised sustain the statement that the American con tinent is a regular growth. The same facts also make it evident that the oceanic areas between which the continent 1 The case of such a shallow trough as that of the English Channel, of the former continuity of whose sides there is ample evidence, whilst its bottom is nowhere 500 feet beneath the surface, is obviously alto gether different. The extraordinary depth of the Mediterranean basin, on the other hand, affords strong reason for regarding it as, like the Atlantic, a portion of the original area of depression, cirsumscribed by the elevation of its borders. 2 Principles of Geology, llth ed. vol. i. p. 2G9.
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lies have been chief among the regions of the earth s crust that have used the pent-up force in the contracting sphere to carry forward the continental developments. If this was true of the North American continent, the same in principle was law for all continents." 1
Dimensions of the Atlantic. The length of the Atlantic basin, considered as extending from the Arctic to the Antarctic circle, is nearly 8000 geographical miles. The nearest approach of its boundaries is between Greenland and Norway, whose coasts are only about 800 miles apart. They thence recede from each other towards the south, as far as the parallel of 30 N. lat., where, between the peninsula of Florida and the western coast of Marocco, there is an interval of 70 of longitude, or about 3600 geographical miles. The channel then rapidly narrows as it passes southward, so that between Cape St Roque in Brazil (5 S. lat.) and the coast of Sierra Leone (between 5 and 8 N. lat.) the African and American continents approach within 1500 miles of each other. The sudden eastward recession of the African coast as it approaches the equator, and the westward trend of the South American coast-line between Cape St Roque and Cape Horn, widen out the South Atlantic basin to the same breadth as that of the North Atlantic in the parallel of 30 N., the interval between the Cape of Good Hope and the estuary of La Plata, in the parallel of 35 S., being no less than 73|- of longitude, or about 3GOO geographical miles. The depth of the North Atlantic has been more care fully and systematically examined than that of any other oceanic basin ; and the general contours of its undulating sea-bed may now be regarded as pretty well determined. Putting aside the older soundings as utterly untrust worthy, and accepting only those taken by the modern methods, whose reliability has been amply tested by the accordance of diversified experiences, we can now assert with confidence that scarcely any portion of its floor has a depth exceeding 3000 fathoms, or about 3 "4 miles, the greatest depth determined by the recent "Challenger" sound ings, which was that of a limited depression about a hundred miles to the north of St Thomas, having been 3875 fathoms, or about 4 4 miles. Except in the neigh bourhood of its coast-lines, and in certain shallower areas to be presently specified, the floor of the basin at its widest part seems to lie at a depth of from 2000 to 3000 fathoms, its slopes being extremely gradual. The central portion of the principal basin of the North Atlantic, however, is occupied by a plateau of irregular shape, of which a considerable part lies at a less depth than 2000 fathoms. Of this plateau the Azores may be regarded as the culmination ; and that group being taken as its centre, it may be said to extend to the north as far as lat. 50, and to the south-west as far as the tropic of Cancer. The northern extension of this plateau narrows out into a sort of isthmus, which connects it with the plateau that occupies a great part of the Atlantic basin to the north of 50 N. lat. ; and it is across this isthmus, and along the bottom of the deep narrow valley on either side of it, that the telegraph cables are laid between Ireland and Newfound land. Whether its south-western prolongation, known as the "Dolphin Rise" (fig. 1, infra) extends to the equator, so as to become continuous with the elevated area which cul minates in St Paul s rocks, and by a further southward ex tension becomes continuous either with the volcanic elevation of St Helena and Ascension Island, or with the elevation in the middle of the South Atlantic which culminates in the island of Tristan da Cunha (fig. 2), has not yet been ascertained. According to the view already suggested as to the formation of the Atlantic basin, the plateau might " On some Results of the Earth s Contraction trom Cooling," in Amer. Journ. of Science, June 1873. be regarded as representing the original sea-bed (from which the Azores have been lifted up by volcanic action), whilst the deep valleys on either side of it are " areas of subsidence " answering to the " areas of elevation " of the land that borders them. Generally speaking, the depths of these valleys increase pretty rapidly with the distance from the shore-line, so that th contour-lines of one and two miles follow the shore-lines pretty closely. But there are two localities in which shallow water extends to a much greater distance from land than it appears to do elsewhere. One of these lies in the neighbourhood of the British Isles. For a dis tance of about 230 miles to the westward of Ireland there is a slope of only about 6 feet in a mile ; but in the next 20 miles there is a fall of 9000 feet, after which there is little change of level for 1200 miles. Hence as the depth of the sea immediately surrounding the British Isles is nowhere 100 fathoms (so that an elevation of their whole area to that amount would unite these islands not only to each other but also to the continent of Europe), it is obvious that the platform on which they rest is really, although now submerged, a part of the land-mass of Europe. Another of these extensive shallows is that of which the Banks of Newfoundland form the highest part ; and of the existence of this a probable explanation may be found in the accumulation of the rock-masses that are brought down by icebergs every summer from the coasts of Greenland and Labrador. For it is now generally admitted that these icebergs are really parts of glaciers, that were originally formed on the mountain-slopes of Greenland and Labrador, and then descended valleys which open out on their coasts, so as, on arriving at the mouths of these valleys, to detach themselves and float away, being borne southwards by the Polar Current to be presently described. Most Arctic icebergs of which a near view can be obtained are observed to have upon them a considerable number of pieces of rock, sometimes of a very considerable size ; and. these are of course deposited on the sea-bed when the icebergs melt (which they usually do on the borders of the Gulf Stream), thus forming a vast conglomerate bed, to which parallels are not improbably to be found in various geological epochs.
Geological Age of the Atlantic Basin. Guided by the principle that great oceanic basins are to be considered rather as original marine areas that have been limited by the elevation of their boundaries, than as having been formed by the excavation of terrestrial areas, we have to inquire what evidence there is that the basin of the Atlantic has undergone any considerable change within a compara tively recent period. As has been pointed out by Prof. Wyville Thomson (Depths of the Sea, p. 473), it is difficult to show that any oscillations have occurred in the north of Europe since the termination of the Secondary period, to a greater extent than from 4000 to 5000 feet, this being the extreme vertical depth between the base of the Tertiaries and the highest point at which Tertiary or post-Tertiary shells are found on the slopes and ridges of mountains. Such oscillations, while considerably modifying the boundaries of the Atlantic, would not seriously affect the condition of the deeper parts of its sea-bed ; and hence it may be concluded that the two deep valleys, one on the European side of the modern volcanic platform of the Azores, and the other on the American, each having a width of 600 or 700 miles, and an average depth of 15,000 feet, could neither have been formed by such oscillations, nor could, when once formed, have been converted into dry land. It will be presently shown that this idea of the existence of an Atlantic basin correspond ing generally to that now existing, as far back as the later Secondary period, is strongly supported by the evidence III. 3 recently obtained of the continuity of animal life on the Atlantic sea-bed from the Cretaceous epoch to the present time. Important information as to the changes which the sea bed of the Atlantic has undergone within the later geolo gical periods, may be gathered from the structure of the islands which lift themselves above its surface. Along its eastern border, at no considerable distance from the coast of North Africa, there are three principal groups, the Madeiras, Canaries, and Cape Verd, all of which have an evidently volcanic origin, and rise up from the eastern slope of the basin, where it is progressively shallowing towards its continental shore-line. Further out, in mid- ocean, lies the group of the Azores, which also is volcanic, and rises from the plateau already spoken of ; but between this area and the slope from which the Madeiras and Canaries are based is a very deep channel, ranging down wards to at least 15,000 feet; and a like depth is also found between the Azores and the coast of Portugal. The structure of all these groups of islands gives obvious indications of their formation by separate igneous eruptions in a sea of great depth ; and the earliest of these eruptions seems to have taken place in the later Miocene period. As soon as the first solid lavas raised their heads above water, and were thus exposed to the action of the waves, fragments were detached and rounded on the shore ; and these being swept off, with the debris resulting from their attrition, formed deposits of various kinds upon the slope of the cone, in which corals, shells, <tc., were embedded. These fossiliferous deposits have been subse quently elevated to heights of from 1500 to 2000 feet above the level of the sea, showing a rise of the base of the craters; progressive additions have been made to their upper part by the piling up of basaltic andtrachytic lavas. 1 That this state of activity still continues is proved by the fact that in 1811 a new island was temporarily formed in the Azores group, off St Michael, by the throwing-up of ashes, and the formation of a cone about 300 feet high, with a crater in the centre. This island, to which the name Sabrina was given, was soon washed away by the waves. And only a few years since, another submarine eruption in this neighbourhood was indicated by earth quakes, jets of steam and columns of smoke, and floating masses of scoriae. All these considerations concur (as Sir Charles Lyell, loc. cit., justly urges) to negative on geolo gical grounds the hypothesis which has been advocated by some eminent naturalists, that the Azores, Madeiras, and Canaries are the last remaining fragments of a continuous area of land which once connected them with the west of Europe and North Africa. Proceeding to the south of the equator, we meet with similar evidence of volcanic activity in the structure of the only two islands, Ascension and St Helena, which lie near the line stretching from the Cape Verd group to the Cape of Good Hope ; and these also arise from a plateau of considerably less depth than the circumjacent area whose eastern slope gradually shallows to the coast of South Africa. This plateau stretches in a north-westerly direction towards the equator, so as to meet it in from 20 to 22 W. long.; and here indications of volcanic activity earthquakes, troubled water, floating scoriae, and columns of smoke have been several times observed since the middle of the last century, betokening the probable formation of an island or an archipelago in that locality. Nearly midway between the southern prolongations of the African and American continents, the solitary peak of Tristan da Cunha (fig. 2) lifts itself above the ocean : this also is volcanic, and seems to rise from a broad base 1 See Sir C. Lyell s account of them in his Principles of Geology, llth ed. p. 407, 22- of general elevation, resembling the plateau of the North Atlantic. The entire chain of the Greater and Lesser Antilles, which stretches from the delta of the Orinoco to the peninsula of Florida, and forms the eastern boundary of the Caribbean Sea, seems to have been in like manner elevated by vol canic action. That this elevation, like that of the groups of islands on the eastern side of the Atlantic, took place for the most part during the later Tertiary period, is shown by the occurrence of shells, corals, &c., of upper Miocene age, in the upraised sedimentary beds of several of the islands ; while the presence of " fringing reefs " of coral around the shores of many of the West India islands is an indication that they lie in an area in which elevation is still proceeding. The channels by which they are separated are so deep as to render it very unlikely that there was ever a continuity of land between them ; and the occa sional recurrence of earthquakes and volcanic eruptions at different points of this " line of fire," shows that the plutonic action by which the islands were raised is still going on beneath. The case is very different, however, in regard to the Bermuda group, which constitutes a singular exception to the general fact of the absence in the Atlantic of those coral islands that are so numerous in the Pacific. This group consists of about 300 islands, of which, however, only five are of any considerable size ; and these rise from a shoal or platform of about 23 miles long by 13 miles broad, the channels between the islands being very shallow, while at a small distance from the edge of the shoal, the bottom rapidly deepens to 15,000 feet. The islands are entirely composed of upraised beds of coral, shells, &c. (the highest elevation being only about 180 feet above the sea-level) ; and the shoal itself appears to have the like structure throughout, no traces of any other rock than a limestone formed by the metamorphoses of coral being anywhere met with. Hence, as this insular platform proves to be the summit of a submarine column of 15,000 feet high, rising from a very small base, and as nothing we know of the structure of mountains volcanic or other would justify us in supposing that a column of such a height could be formed in any other way than by coral growth, the structure of the Bermuda group would seem to indicate a progressive subsidence of the bed of this part of the Atlantic during its formation, corresponding to that which (according to the well-known views of Mr Darwin) is at present in progress over a large area of the Pacific. It is probable that this coral growth was deter mined in the first instance by the existence of a submarine mountain, of which the summit lay near the surface, or lifted itself above it ; that as soon as this came to be submerged, the coral formation commenced ; and that by its continued growth at the summit, at a rate equal to that of the subsidence of its base, the platform has been kept up to the sea-level. The slight elevation which has raised its highest portion above that level may not impro bably have taken place in connection with the much larger recent elevations already referred to. Thus, then, we have evidence of considerable recent local modifications in the level of the Atlantic sea-bed, without any such change as would affect its general cha racter as an ocean basin ; while all geological probability seems in favour of the remoteness of the principal depres sion of the Atlantic area, even if we do. not regard it as dating back to the period when the surface of the globe was first undergoing solidification. Currents of the Atlantic. By the term " current" will be here meant that sensible movement of ocean water in particular directions which can be generally traced, directly or indirectly, to the action of wind upon its surface. A
current thus directly impelled by wind is termed a " drift- current," whilst a current whose onward movement is sustained by the vis a tergo of a drift-current is called a "stream-current." But there is another source of current- movement, which has been overlooked by most writers on this subject, namely, the indraught which necessarily takes place to keep up the level of any area from which the surface-water is constantly being drifted away. Such cur rents, which may be designated as " indraught " or " supply currents," complete the " horizontal circulation " that must necessarily take place in any oceanic area of which one part is subjected to the action of a wind almost constantly blowing in the same direction. Of such a. circulation we have a very characteristic example in the South Atlantic, the principal currents of which we shall see to be very easily accounted for. The initial movement of the current-system, alike of the North and of the South Atlantic, is given by the trade- winds, which are continually driving the water of the inter- tropical region from the African towards the American side of the basin, so as to produce what is known as the Equa torial Current. The position of the northern and southern boundaries of this current shifts, like the area of the trade- winds, in accordance with the northward and southward declination of the sun ; a steady westward drift being generally met with to the north of the tropic of Cancer in the summer of the northern hemisphere, and to the south of the tropic of Capricorn in the summer of the southern, whilst in the winter of each hemisphere the border of the drift lies within the tropic of that hemisphere. But as the thermal equator lies from two to three degrees to the north of the geographical equator, the entire zone of the trade- winds, and of the Equatorial Current propelled by them, is wider on the northern than on the southern side of the latter ; and while the northerly trade often reaches 30 N. in July, and rarely extends south in January within 2 or 3 of the geographical equator, the southerly trade does not extend farther than 25 S. in January, and generally crosses the equator in July, even extending occasionally as far as 5 N. As between the northerly and southerly trades there is a region of " equatorial calms," so there is a corresponding interval between the northern and southern divisions of the Equatorial Current ; and in this interval there is a counter-current (resembling the " back-water " often to be noticed in a stream that is flowing rapidly past some obstacle, such as a vessel at anchor, or a projecting angle of a river-bank), that runs eastwards, sometimes with considerable velocity, towards the Bight of Biafra, which may be considered the " head- water " of the Equatorial Current. From the recent observations of Capt. ISTares in the "Challenger," it appears that the Equatorial Current, like other drift-currents, is very shallow, its depth being not much greater than 50 fathoms. He estimates its rate at the surface to be about 75 miles per hour, or 18 miles per day, whilst at 50 fathoms it only moves at about half that rate. 1 Its surface temperature generally ranges between 75 and 80 ; but the thermometer falls to 60 at a depth of little more than 100 fathoms, the temperature of this belt of water, as will be hereafter shown, being kept down by the continual rising of polar water from below. The Equatorial Current passes directly across the Atlantic towards the chain of the Antilles and the coast of South America ; and as not only the whole of the northern divi sion, but a considerable part of the southern, strikes the American coast-line to the north of the salient angle of Cape St Roque (about 5 S. lat.), the portion of the current which is deflected into the northern hemisphere is much greater than that which is turned to the southward. It is 1 Mr Laughton, however, states the average velocity to be between 20 and 30 miles per day. a general fact, that where a current encounters any partial obstruction, such as a coast-line meeting it obliquely, a narrowing of its channel, the lateral pressure of another current, or even that of a mass of stationary water. its velocity increases ; and so the portion of the Equatorial Current that is pressed to the northward by the coast-line between Cape St Eoque and the mouth of the Orinoco (known in the first part of its course as the Cape St Roque Current, and afterwards as the Guiana Current) acquires a greatly augmented rate, running ordinarily at the rate of from 30 to 50 miles, and occasionally at a rate of 80 miles, in the 24 hours. Entering the Carib bean Sea, it is reinforced by the portion of the Equatorial Current which flows in between the Lesser Antilles ; and it then passes westwards along the northern coast of South America, until it is deflected northwards by the coast-line of Central America, and driven between the peninsula of Yucatan and the western extremity of Cuba into the Gulf of Mexico, at the rate of from 30 to 60 miles per day. A por tion of it passes direct to the KE. along the northern shore of Cuba; but by far the larger part sweeps round the gulf, following the course of its coast-line, and approaches the coast of Cuba from the N.W. as a broad deep stream of no great velocity, seldom running at more than 30 miles per day. The reunited current, being met by the Equatorial Current from the outside, which is pressing to the west along the north coast of Cuba and between the Bahama isles, is deflected northwards through the passage termed the Florida Channel, which is bounded on the one side by the southern extremity of the peninsula of Florida, and on the other by the coast of Cuba and the Bahamas. The rate of movement of the powerful current that flows through this channel, henceforth known as the Gulf Stream, is con siderably augmented in its narrowest part, which is also its shallowest ; but although its velocity sometimes reaches 4 (nautical) miles per hour, or even more, its average rate through the whole year may be confidently stated at not more than 2 miles per hour, or 48 miles per day. 2 The Gulf Stream current, however, does not by any means occupy the whole of the sectional area of the Florida Channel ; for it is separated from the American coast by a band of cold water, which occupies about three-eighths of its total breadth of 40 miles, and which also dips under the outflowing current. The movement of the cold superficial band is perceptibly inwards, and that of the cold under stratum is presumably so ; and it is the opinion of the American surveyors that the depth of the warm outward current is not more than one-third of that of the channel through which it flows. It is probable that the rate of movement decreases from the surface downwards ; but upon this point we have as yet no certain information. The meaning of the cold inflow will hereafter become apparent. The course taken by the Gulf Stream in the first in stance is nearly parallel to the line of the United States coast, from which it is everywhere separated by a band of cold water, the boundary line between the two being so distinct as to be known as the " cold wall. " It does not show for some time any great disposition to spread itself out laterally, though a division into alternate bands of warmer and colder water, the cause of which seems to lie in the contour of the bottom of the Florida Channel, be comes perceptible before it reaches Charleston, and is very marked off Cape Hatteras. The Stream there presents the form of a fan, its three warm bands spreading out over the Atlantic surface to an aggregate breadth of 167 miles, 2 This statement, which is much lower than that adopted by most writers on the Gulf Stream, is based on the entire aggregate of observa tions collected by the Meteorological Department, which further show that, for sii months of the year, the monthly mean averages only 1 4 miles per hour, or 34 miles per day, whilst for the other six months it only averages 2 niiles an hour, or 60 miles per day. whilst two cold bands of an aggregate breadth of 52 miles are interposed between them. The innermost warm band is the one which exhibits the highest temperature and greatest rate of flow, its velocity being greatest where it is pressed on laterally by the Arctic Current, so that a rate of 4 miles per hour is occasionally observed. Capt. Nares estimates the depth of the Stream in this part of its course at about 100 fathoms, and its rate of flow in the line of most rapid movement at 3 miles per hour. The outermost band, on the other hand, graduates insensibly, both as to temperature and rate of movement, into the general sur face-water of the Atlantic. It is when passing Sandy Hook that the Gulf Stream takes its decided turn east wards, this change in its direction, being partly due to the eastward bend of the United States coast-line, and partly to the excess of easterly momentum which it brings from the lower latitude in which it issued from the Florida Channel. Its general rate of flow past Nantucket seems not to exceed 1 mile per hour, and to be frequently less ; but several degrees to the eastward of this, the current has been found occasionally running at the rate of 4 miles an hour, this acceleration being probably due to the lateral pressure of the Arctic Current, which, during the early months of the year, is driven southwards at the rate of 10 or 12 miles per day by the N. and N.W. winds then pre vailing along the coast of Labrador, and which, turning westwards round the south of Newfoundland, keeps close to the coast of the United States (being left behind in the rotation of the earth, in consequence of its deficiency of easterly momentum), and follows it southwards, every where separating it from the Gulf Stream. By the gradual thinning-out and expansion of the Gulf Stream after passing the Banks of Newfoundland, by the progressive reduction of its rate of movement, and by the loss of that excess of temperature which previously distinguished it, as well as of its peculiar blue colour (which probably depends on its holding in suspension the finest particles of the river-silt brought down by the Mississippi), this remarkable current so far loses all its special attri butes, as to be no longer recognisable to the east of the meridian of 30 3 W. long., there degenerating into the general easterly drift of that region of the Atlantic which is kept up by the prevalence of westerly winds, some times called " anti-trades." Where the Florida Current or true Gulf Stream can last be distinctly recognised, it forms a stratum not mora than 50 fathoms in thickness ; and it is there flowing almost due east, at a rate which would require about 100 days to bring it to the Land s End. The only valid evidence of the extension of any part of it to the western shores of Europe (the ameliora tion of their temperature being otherwise accounted for, while the transport of trunks of trees, drift-timber, fruits, shells, &c., to the Western Hebrides, the Orkney, Shetland, and Faroe islands, and the coast of Norway, may be fairly set down to the surface-drift sustained by the pre valence of S.W. winds) is afforded by the variable current known as Eennell s, which, flowing eastwards into the southern part of the Bay of Biscay, is deflected in a N.W. direction by the trend of its coast-line, so as to cross the British Channel towards the Scilly Islands, whence it passes to the S.W. coast of Ireland, its strength mainly depending on the continued prevalence of the westerly anti-trades. (See Plate I.) Of the whole mass of water, on the other hand, that is brought into the mid- Atlantic by the Gulf Stream, it may be stated with confidence that the larger proportion turns southward to the east of the Azores, and helps to form the North African Current ; the other tributary of which may be considered as originating as far north as Cape Finisterre, under the influence of the northerly winds which prevail along the coast of Portugal. As this current flows past the entrance to the Strait of Gibraltar, a part of it, forming what is known as the Gibraltar Current, is drawn in to keep up the level of the Mediterranean, which would other wise be reduced by the excess of evaporation from its sur face ; but the greater part keeps its course southwards along the Marocco coast, reinforcing the south-flowing extension of the Gulf Stream. On arriving at the border of the northerly trade, the North African Current divides into two parts, the western division being at once carried into the course of the equatorial drift, whilst the eastern, which may be considered as essentially an indraught or supply current, follows the African coast-line, and turns eastward into the Gulf of Guinea, forming the Guinea Current, which, coalescing with the eastward " back-water " already mentioned, flows pretty constantly, sometimes with con siderable rapidity, towards the Bight of Biafra. There it meets the South African Current, which forms the other great feeder of the Equatorial Current; and the circulation thus completed may be considered as recommencing from this " head-water." The large area of comparatively still water which lies in the interior of this North Atlantic circulation is called the Sargasso Sea, a corruption of the name (Mar de Sargago) which it received from Columbus and the early Spanish navigators, on account of the quantity of sea-weed that floats on its surface. The boundaries of this area, which is of an irregularly elliptical shape, and nearly equals that of Continental Europe, are somewhat variable; but it may be considered to lie between the parallels of 20 and 35 N., and between the meridians of 30 and GO W. Into it is collected a large propor tion of the drift or wreck which floats about the North Atlantic. Proceeding now to the South Atlantic, we meet with a circulation of the same kind, uncomplicated by any embaying of the Equatorial Current. The smaller division of this current which strikes the coast of South America to the south of Cape St Koque flows along the coast of Brazil at the rate of from 12 to 20 miles a day, forming the Brazil Current, which, however, is separated from the land by an intervening band of lower temperature, that has, during the winter months, a distinct flow towards the equator. The Brazil Current can be traced southwards, by its temperature rather than by its movement, as far as the estuary of the La Plata, before reaching which, however, a great part of it takes an easterly direction, and crosses the Atlantic towards the Cape of Good Hope, forming what is known as the Southern Connecting Current. The easterly movement of this current seems to be partly due to the westerly anti-trades, and partly to the excess of easterly momentum which is retained by the Brazil Current in its southward course from Cape St Roque ; whilst it partly depends also on the junction of an Antarctic current that flows N.E. from Cape Horn, meeting the Brazil Current off the estuary of La Plata, just as the Arctic Current meets the Gulf Stream off Newfoundland, dense fogs being produced, in the one case as in the other, through the precipitation of the vapour overlying the Equatorial Current, by the colder air that overlies the Polar. On meeting the coast of South Africa, the Southern Connecting Current turns northwards, and runs towards the Bight of Biafra, forming the South African Current, the movement of which is partly sustained by the southerly winds which prevail along that coast, but is partly attributable to the indraught set up to supply the efflux of the Equatorial Current. In its passage thither, however, the part of it most distant from the land is draughted westwards by the southern trade, forming the most southerly portion of the equatorial drift. Between this and the Southern Con necting Current is a central space, lying between the
parallels of 20 and 30 S., and the meridians of and 25 W., over which there are no regular currents ; and to this the name Sargasso Sea is sometimes applied by analogy, although its surface has no covering of sea-weed. (See Plate I.)
Temperature of the Atlantic. The distribution of surface temperature over the area of the Atlantic has now been made out with considerable accuracy ; and it corresponds closely with what has been already stated as the course of the surface currents. There is, of course, a seasonal change, alike in its northern and in its southern division, this change being more and more marked as we recede from the equator. Following the course of the mean annual isotherms, however, we find that they cross the South Atlantic at nearly regular intervals, in an east and west direction, the principal departure from that direction being shown at their western end in the bend they take towards the south under the influence of the warm Brazil Current, and at their eastern in the still stronger bend they take towards the north under the influence of the cold South African Current, which reduces to about 75 the temperature of the southern equatorial that flows alongside the Guinea Current, whose temperature is 82. In the North Atlantic, however, the influence of the movement of oceanic water on the surface-temperature is very much more marked. The annual isotherms, which cross the Sargasso Sea with nearly regular parallelism, and on the African side tend somewhat to the south, where they meet the colder water of the North African Current, show a strong northward bend on the American side, along the early course of the Gulf Stream ; but as its excess of temperature above that of the Atlantic generally diminishes as we trace it towards the Banks of Newfoundland, this northward deflection progres sively becomes less. The marked contrast in temperature which is often there exhibited between two contiguous bands of water, a thermometer hanging from a ship s bow show ing a temperature of 70, whilst another hanging from the stern shows only 40, is due not so much to the elevation produced by the Gulf Stream as to the depression produced by the Arctic Current. This depression manifests itself in the southward bend given, on the American side, alike to the summer and the winter isotherms (see Plate), beyond the summer isotherm of 70 and the winter isotherm of 60, which may be considered as having nearly their normal position; whilst the northward tendency of these same isotherms on the European side not less conspicuously indicates a flow of warm water towards the western coasts of the British Isles, Norway, and even Iceland and Spitzbergen. It has been customary to regard this flow as an extension of the Gulf Stream ; but if that term be limited (as it ought) to the current that issues from the Gulf of Mexico through the Florida Channel, the hypothesis is found to be untenable so soon as the thermal phenomena of that current are carefully examined. For, in the first place, the popular idea that the Gulf Stream retains its high temperature with little diminution during its passage first northwards and then eastwards is clearly disproved by observation, as is shown by the following table of average temperatures taken at different seasons in the warmest of its bands : 3 "a _" ti t/T Summer | Florida Channel 25 N. 77 7S 83 82 Off Charlestown 33 N. 75 77 82 81 Off Cape Hatteras 35 N. 72 73 80 76 S.E. of Nantucket Shoals S. of Nova Scotia 35 X. 35 N. 67 62 68 67 80 78 72 69 From this it appeal s that, while the high surface-tempera ture with which the Gulf Stream leaves the Florida Channel is retained in summer with only 5 reduction as far as Nova Scotia, there is a reduction of 5 in winter during its north ward passage to Cape Hatteras, and a further reduction of no less than 10 during its eastward passage from Cape Hatteras to Nova Scotia, making a total reduction of 15. In spring, again, there is a total reduction of 11, and in autumn of 13; and in both cases the reduction during the eastward flow under the parallel of 35 N. is greater than the reduction in the northward flow from 25 N. to 35 N. The explanation of this is plainly to be found in the fact that in the early part of the course of the Gulf Stream its superheated stratum is a thick one, so that when its superficial film is cooled down by a superincumbent atmosphere of lower temperature, it is replaced by the uprising of a deeper stratum having nearly its original temperature. But as the stream spreads out superficially, its superheated stratum becomes proportionally thinner, and will consequently be more and more rapidly cooled down by the superincumbent atmosphere. Even supposing, therefore, that it were not subjected to any special cooling influence, it appears certain that, as the rate of the current slackens and its depth diminishes, the cooling process must continue at an increased rate, so as to bring down the surface-temperature of the stream to the normal isotherm of the locality, long before it could reach the shores of Europe. But it has been shown that when it passes Newfoundland the Gulf Stream is subjected to a special cooling influence that of the Labrador Current with its fleet of icebergs, which melt away when borne into it ; and this produces such an immediate reduction of its surface- temperature, that it thenceforth shows very little excess, although its sub-surface stratum still appears to be warmer than that of the ocean through which it flows. But, further, the Gulf Stream, where it is last recog nisable as a current, is flowing due east, and its southern portion turns first south-east and then south, whilst, on the other hand, the course of the isothermal lines (see Plate) clearly shows that the flow of warm water which carries them northward spreads across the whole breadth of the Atlantic, from the British Isles to Labrador, even extending up to the west of north into Baffin s Bay. When we contrast this immense body of north-moving water with the thinned-out film of what is by comparison a mere rivulet, it becomes obvious (1) that its northward flow cannot be attributable to the vis a tergo of the Florida Current, whilst (2) its convection of heat to the Arctic Sea cannot be accounted for by any amount of excess of temperature that is limited to a small depth, since the temperature of such a stratum, moving north-east at a rate of (at most) 4 or 5 miles per day, must soon be brought down to that of the atmosphere above it. Influenced by these considerations, several eminent hydrographers, both British and American, have been disposed to deny, not only that the temperature of the North Atlantic is modified in any considerable degree by the true Gulf Stream, but that any other agency than that of warm S.W. winds is concerned in producing the climatic amelioration popularly attributed to it. They maintained, in fact, that the surface-temperature of the North Atlantic and Arctic Seas follows that of the superincumbent air, the atmospheric temperature not being in any degree raised by that of warmer water beneath. This doctrine, however, is found to be inconsistent with the results of careful com parisons recently instituted between marine and atmospheric temperatures along the western coasts of Scotland, the Orkney, Shetland, and Faroe Islands, and especially with those obtained along the western coast of Norway. For it is found that during the winter months there is a constant excess of sea-temperature above that of the air, averaging 6 2 Fahr. along the western coast of Scotland and its islands, and rising to 14 5 at Fruholm near the North Cape. And it is also a very significant fact (ascertained by the care ful inquiries of Mr Buchan), that while the summer isotherms cross the British Islands nearly east and west (the tempera ture diminishing pretty regularly from south to north), the winter isotherms traverse them nearly north and south (the temperature diminishing from west to east) ; whilst in Ireland the isotherms seem to envelope the islands in their folds, which increase in warmth from the centre of the island to its sea-board. So in Norway the isothermal lines run parallel to the coast-line, and this alike in summer and in winter, the temperature falling in winter, and rising in summer, with the increase of distance from the sea. Nothing could prove more conclusively than such facts as these (taken in connection with the absence of ice in the harbours of Norway, even as far north as Hammerfest, through the whole winter) the dependence of the mild winter climate of the north-western coasts of Europe upon the proximity of a sea which is warmer than the superincumbent atmosphere ; and we have now to inquire how this great N". E. movement of a stratum of warm water sufficiently thick to retain a surface-temperature con siderably higher than that of the air above it is to be accounted fur. The solution of the problem seems to be afforded by the doctrine of a General Oceanic Circulation, sustained by opposition of temperature only, which was first distinctly propounded in 1845 by Professor Lenz of St Petersburg, on the basis of observations made by him during the second voyage of Kotzebue (1825-1828). Others had been previously led to surmise that "Polar Currents" flow along the floors of the great oceans, even as far as the equator, balancing the superficial counter-currents which are observable in the opposite direction. But Lenz was led to conclude that the whole of the deeper portion of the great ocean-basins in communication with the polar areas is occupied by polar water, which is constantly, though slowly, flowing towards the equator ; whilst conversely the whole upper stratum of equatorial water is as con stantly, though slowly, flowing towards one or both of the poles. And he particularly dwelt on the existence of a belt of water under the equator, colder than that which lies either north or south of it, as an evidence that polar water is there continually rising from beneath towards the surface, a phenomenon which, he considered, admits of no other explanation. He further adduced the low salinity of equatorial water (previously noticed by Hum- boldt, and confirmed by his own observations), compared with that of tropical water, as evidence that the equatorial water of the surface is derived from the polar underflow. And he attributed the maintenance of this circulation to the continually renewed disturbance of equilibrium between the polar and equatorial columns, the greater lateral (because downward) pressure of the former 1 causing a bottom outflow of polar water in the direction of the latter, whilst the reduction of level thus occasioned will produce a surface indraught from the warmer towards the colder areas. The doctrine of Lenz, so far from meeting with the general acceptance to which it had a fair claim, alike on theoretical grounds and from its accordance with the facts ascertained by careful observation, seems to have been put aside and forgotten, a preference being given to the doc trine of the prevalence of a uniform deep-sea temperature of 39, which was supposed to be established by the 1 It must be borne in mind that sea water does not expand like fresh -water in cooling below 39 0- 2, but continues to contract down to its freezing point, which lies between 27 and 25 Fahr., according as it is still or agitated. thermometric observations made in the voyages of D Urvillo and Sir James Ross. No such precaution was taken, however, in these observations as that to which Lenz had recourse, to obviate the effects of the tremendous pressure (1 ton per square inch for every 800 fathoms of depth) to which deep-sea thermometers are exposed ; and it is now certain that the temperatures at great depths recorded by D Urville and Ross were several degrees too high. It was in entire ignorance of the doctrine of Lenz, and under the influence of that of D Urville and Ross, which, had been stamped with the great weight of Sir John Herschel s weight of authority, 2 that Dr Carpenter com menced in 18G8 (in concert with Professor Wyville Thom son) a course of inquiry into the thermal condition of the deep sea, which at once convinced him of the fallacy of the uniform 39 doctrine, and led him to conclusions essentially accordant with those of Lenz. For in the channel of from 500 to GOO fathoms depth between the north of Scotland and the Faroe Islands, they found the deeper half to be occupied by a stratum of glacial water, whose temperature ranged downwards from 32 to 29 0> 5 ; Avhilst the upper half was occupied by a stratum warmer than the normal temperature of the latitudes. This phenomenon was interpreted by Carpenter as indicat ing a deep glacial flow from N.E. to S.W., and a warm upper flow from S.W. to N.E.; and finding that to the west of this channel, on the border of the deep Atlantic basin, the excess of warmth extended to a depth of more than 500 fathoms, he came to the conclusion that the north-moving stratum which brought it could not be an extension of the true Gulf Stream, but must be urged on by some much more general force. A series of tempera ture-soundings taken along the west of Ireland, the Bay of Biscay, and the coast of Portugal, confirmed him in this view, by showing that the division between an upper warm stratum and a cold under-stratum exists in the North Atlantic at a depth of from 700 to 900 fathoms, the whole mass of water below this having either flowed into the basin from the polar area, or having had its temperature brought down to from 39 to 3G 0- 5 by mixture with the polar inflow. And this conclusion was confirmed by the result of temperature-soundings taken at corresponding depths and under the same parallels of latitude in the Mediterranean ; for as they showed a uniform temperature of from 54 to 56, from beneath the stratum of 100 fathoms that was superheated by direct insolation, to the very bottom, it became clear that depth per se could have no effect in reducing the bottom-temperature; and that the cause of the excess of temperature in the mass of water occupying the Mediterranean basin above that of Atlantic water at the same depths, lies in the seclusion of the former from the polar underflow which brings down the deep temperature of the latter. This conclusion having received marked confirmation from temperature-soundings taken in the Eastern seas, was put forward by Carpenter as justify ing the doctrine of a vertical oceanic circulation sustained by opposition of temperature only, quite independent of and distinct from the horizontal circulation produced by wind, which doctrine he expressed in terms closely cor responding with those that had been used by Lenz. And the collection of data for the establishment or confutation of this doctrine was one of the objects of the " Challenger" expedition, which has already made, in the determination of the thermal stratification of the Atlantic between 38 N. lat. and 38 S. lat,, what may be fairly characterised as the grandest single contribution ever yet made to terrestrial physics. The following are the most important of the facts thus 2 See his Physical Geography of the Globe, originally published in the eighth edition of this Encyclopaedia.
established : Of the water which fills the deep trough of the North Atlantic (fig. 1) between Teneriffe (lat. 28 u N.) and St Thomas (lat. 18 N.), divided by the "Dolphin rise into an eastern and western basin, by far the larger
mass has a temperature ranging from 40 downwards, in the eastern basin, to a bottom-temperature of 35^, whilst in the western basin apparently under the influence of the Antarctic underflow the bottom-temperature sinks to 34 4. A tolerably regular descent is shown in this sec tion, from a surface-temperature rising near St Thomas to 75, to the bathymetrical isotherm of 45, which lies be tween 400 and COO fathoms depth ; there is then a stratum between 45 and 40, of which the thickness varies from about 250 to 450 fathoms, the isotherm of 40 lying at between 750 and 1000 fathoms depth, while below this, down to the bottom at between 2000 and 3000 fathoms, the further reduction to 34 4 is very gradual. The same general condition prevails in the South Atlantic (fig. 2), between Abrolhos Island (lat. 18 S.) on the coast of Brazil, and the Cape of Good Hope (lat. 34-J- S.), this trough also being divided into two basins by the elevation of the bottom which culminates in the island of Tristan da Cunha. The temperature of the water that occupies it, however, is lower through its whole vertical range than that of the North Atlantic. The stratification is nearly uniform from the surface downwards to the isotherm of 40, which lies at from 300 to 450 fathoms depth, the isotherms of 39 and 38 also lying within about 500 fathoms; there is then a slower reduction down to the isotherm of 35, which lies between 1400 and 1800 fathoms; while the whole sea-bed is covered by a stratum of about 600 fathoms thickness, whose tempera ture ranges downwards from 35 to 33. The whole of this deepest stratum is colder than any water that is found in the corresponding portion of the North Atlantic, except near St Thomas.
It is not a little remarkable that the sub-surface stratum of water, having a temperature above 40, is thinner under the equator than it is in any other part of the Atlantic from the Faroe Islands to the Cape of Good Hope. Not withstanding the rise of the surface-temperature to 76-80, the thermometer descends in the first 300 fathoms more rapidly than anywhere else; so that polar water is met with, as shown in fig. 3, at a much less depth than in the North Atlantic (fig. 1), and 100 fathoms nearer to the surface than even in the colder South Atlantic (fig. 2) ; whilst the temperature of the bottom is but little above 32. Thus the influence of the polar underflow is more pronounced under the equator than it is elsewhere ; as is distinctly seen in the section shown in fig. 4, which is taken in a north and south direction so as to exhibit the relation of the thermal stratification of the North to that of the South Atlantic, and of both to that of the equatorial depth of about 700 fathoms, gradually rises as the equator is approached and it is between the equator and 7 S., where the surface-temperature rises to nearly 80, that cold water is soonest reached, the isotherm of 40 rising to within 300 fathoms of the surface, while that of 55, which in lat. 38 N. lies at nearly 400 fathoms depth, and in lat. 22 N. at about 250 fathoms, actually comes up under the equator within 100 fathoms of the surface. At the same time, while the bottom-temperature under the equator is the lowest anywhere met with, namely, 32 4, 1 the thickness of the stratum beneath the isotherm of 35 is not less than GOO fathoms. In passing southwards, the superficial iso therms are observed to separate again from each other, partly by the reduction of the surface-temperature, and partly by the descent of the isotherm of 40 to a depth of something less than 400 fathoms, which it keeps with little reduction as far south as the Cape of Good Hope. The significance of these facts becomes more remarkable, when we consider that if a portion of the oceanic area under the
belt. The isotherm of 40, which in lat, 22 N. lies at a equator were to be secluded, like the Mediterranean or the
Red Sea, from all but local influences, the temperature of its water from the sub-surface stratum downwards to the bottom whatever its depth would be its isochcimal or mean winter-temperature, which, in the equatorial zone, would be certainly not below 75. Nothing, Dr Carpenter contends, could more conclu sively support the general doctrine of a Vertical Oceanic Circulation sustained by opposition of temperature, than the precise conformity of the facts thus determined by observation to the predictions which his confidence in the theory had led him to put forth. These predictions were essentially as follows : "1. That instead of the local depressions of bottom-temperature imputed by previous writers to polar currents, the temperature of every part of the deep sea-bed in communication with either of the polar areas would be not many degrees above that of the polar areas themselves. " 2. That this general depression of bottom-temperature would be found to depend, not upon such a shallow glacial stream as might be maintained to be a return from the polar areas of water propelled towards them by wind-currents, but upon a creeping flow of the whole under-stratum, having a thickness of from 1000 to 2000 fathoms. "3. That as the depression of bottom-temperature in any part of the general oceanic basin would be proportional to the freedom of communication between its deeper portion and that of one or other of the polar areas, the bottom-temperature of the South Atlan tic would probably range downwards to 32, while that of the North Atlantic would not be below 35, except where it first receives the Arctic flow, or comes under the influence of the Antarctic underflow, which would very probably extend itself to the north of the equator. "4. That as the Arctic and Antarctic underflows must meet at or near the equator, whilst the surface-stratum is there continually being draughted off thence towards either pole, there would be a continual ascent of glacial water under the line, showing itself by a nearer approach of cold water to the surface in the ZHfcr-tropical than in the eatfra-tropical zone." It was further pointed out by Lenz, and more recently (in ignorance of his doctrine) by Carpenter, that additional evidence of such ascent is furnished by the low salinity of the surface-water of the equatorial belt corresponding with that of polar water. For, as was originally observed by Humboldt, then by Lenz himself, and subsequently by many other voyagers, the specific gravity of the surface- water of the Atlantic gradually increases as either tropic is approached from the polar side of its own. hemisphere, reaches its maximum a little nearer the equator, and then rapidly diminishes, coming down under the equator to the standard of polar water. Thus a mean of eight observations taken in the "Challenger" expedition between Bermuda (32 N.) and St Thomas (18| N.) gave 1027-2 as the sp. gr. of surface-water, whilst a mean of seventeen observa tions between the Cape Verd Islands (16| N.) and Bahia (13 S.) gave a sp. gr." of only 1026 3. Now, since between St Thomas and Bermuda the eight " Challenger" observa tions of bottom (polar) water gave a mean sp. gr. of 102 6 3, whilst between Cape Verd and Bahia the mean sp. gr. of the bottom-water was even slightly lower (the results being 1 That the bottom-temperature beneath the equator was lower than any that was met with in the South Atlantic, is attributable to the cir cumstance that, in consequence of unfavourable weather, the tempera ture-soundings were taken at intervals too wide to detect the deep channel through which the coldest Antarctic water doubtless flowed to wards the equator.
in all cases expressed according to a common standard of temperature), such a close conformity subsists between the salinity of the equatorial water of the surface and that of the polar waters of the bottom, as can scarcely be accounted for in any other way than by the continual and tolerably rapid ascent of the latter. Another indication of this ascent is given by the moder ation of the surface-temperature of oceanic water, even under the equator. If there were no ascent of colder water from beneath, there seems no reason why the constant powerful insolation to which equatorial water is subjected should not raise the temperature of its surface to the highest possible elevation. The limit to that elevation, which is obviously set by the cooling influence of evapora tion, is probably that which is met with in the Red Sea, where the monthly average for August rises to 86| and for September to 88, whilst the maxima rise much higher, temperatures of 100, 106, 100, and 96 having been noted on four consecutive days. Moreover, along the Guinea Coast, and especially in the Bight of Biafra, the surface- temperature is stated to range as high as 90. But in these cases there is no reduction of surface-temperature by the upward movement of polar water ; for this is altogether excluded from the Red Sea by the shallowness of the Strait of Babelmandeb, whilst the depth of the bottom along the Guinea Coast is too small to allow of its being overflowed by the glacial stratum. Now, over the deeper parts of the equatorial Atlantic the surface-temperature usually ranges between 75 and 80; and this is its ordi nary range in the Mediterranean during the months of August and September. That the temperature of an equa torial ocean should be thus kept down to that of a sea of which the greater part lies between the parallels of 40 and 35, can scarcely be accounted for in any other way than by the continual uprising of polar waters from beneath. The same principle, once admitted, fully accounts for that amelioration of the cold of north-western Europe, which (as already shown) cannot be fairly attributed to the Florida Current or true Gulf Stream. For it is obvious that a continual efflux of the lower stratum from the polar areas towards the equatorial must involve a continual indraught of the upper stratum towards the polar areas ; and this indraught will be much more marked in the Northern than in the Southern Atlantic, on account of the progressive narrowing of the former, whilst the latter progressively widens out. Of such a slow northerly set of a stratum of water, extending downwards to a depth of at least 600 fathoms, we have evidence in a comparison of the temperature-soundings taken in the " Porcupine " expedi-
tions of 1869 arid 1870, between the coast of Portugal (34 N.) and the Faroe Islands (59^- N.), from which the section fig. 5, has been worked out. For it is there seen that, although the surface-temperature is reduced by the thinning-out of the superficial stratum, there is but a slight change in the position of the bathymetrical isotherms of 45 and 40 ; so that there is an obvious continuity of a stratum of many hundred fathoms thick ness between these two points, notwithstanding their sepa ration by 2o of latitude. The contrast between the position of the isotherm of 40 at 800 fathoms depth off the Faroes, and its position at less than 300 fathoms depth under the equator, is most remarkable. We have seen that the isocheimal in the latter area would not be below 75, and yet we find water colder than 40 lying at within 300 fathoms of the surface; whilst, on the other hand, the normal isocheimal at 59| N. would certainly be below 40 (probably no more than 35), and yet we find water above 43 extending downwards to 600 fathoms, and water above 40 D to 800 fathoms. Thus the vertical oceanic circulation carries a vast mass of water which is beloiv the normal off the coast of Portugal, into a region where it is above the normal, with very little loss of heat by the way, except in its surface-film ; and a little consideration will show that such a movement must be much more effectual as a heater than a corresponding move ment of a thin stratum of much warmer water. For the latter, when it passes beneath an atmosphere much colder than itself, will soon be brought down to a like standard, not having warmer water from below to take its place when it has been cooled down ; whilst in the former, each sur face-layer, when cooled below the temperature of the warmer stratum beneath, will sink and be replaced by it. Now since the true Gulf Stream, when we last know it, has been so thinned out that it could not long retain any excess of temperature, it seems inconceivable that it should exert any decided effect on the temperature of the Faroes and the coast of Norway, unless (as supposed by Dr Petermann and Professor Wyville Thomson) its thickness undergoes an increase from less than 100 fathoms to 600. But since the course of Dr Petermann s isotherms shows that the north ward flow extends across the whole breadth of the Atlantic between Newfoundland and the British Isles a distance of about 2000 miles we are required to believe that a rivulet (for such it is by comparison) of 60 miles breadth and 100 fathoms depth (see section, fig. 5), of which the greater part turns southwards round the Azores, and of which the remainder is flowing due east when we last recognise it, is able to impart a northerly movement to a stratum of 2000 miles in breadth, and at least 600 fathoms depth. On the other hand, the eastward set of this stratum, considered as a northward indraught into the polar area, is readily accounted for by the excess of easterly momentum which it derives from the earth s rotation, this being only half as rapid in lat. 60 as it is under the equator ; and since there is a still more rapid reduction in the rate of this rotation in yet higher latitudes, the con tinually increasing excess of easterly momentum will give to the northward flow a progressively stronger eastward set. On the other hand, the deficiency of easterly momentum in the cold underflow coming from the pole towards the equator will tend to produce a lagging-behind, or westward set of that underflow ; and this has been shown by the " Challenger" temperature-soundings to be the case, the cold deep strata of the Western Atlantic surging upwards along the slope of the North American coast-line, as is shown in fig. 6, where we see not only the bathymetrical isotherms of 60, 55, and 50, but the yet deeper isotherms of 45 and 40, successively rising to the surface as we approach the land ; while at a depth of only 83 fathoms, a temperature of 35 was encountered, which, at no great distance to the south, would only be found at a depth of 2000 fathoms. That the cold water should thus run up- Ill 4 hill is quite conformable to what we see in other cases, in which a heavier under-strstnm has a definite set towards a si ope; and whilst the existence of i such a westerly set is, ex hypo- thesi, a necessary consequence of the southerly < < movement of the C0 c Arctic under- | soo flow, no other | explanation of 1 it has been sug- * 2 c gested. We now 1 1 4 o c see that the cold | , c Labrador Cur- z 1 . " 1 S rent overlies a band of water us " n cold as itself; 2200 and the south- ,400 ward extension a c of this cold band, far beyond that of any definite current - move- Fr(J ment, and its ><s ofikCMf Stream!;
entrance into the Gulf of Mexico, through the Florida Channel, at the side of and beneath the outflowing Gulf Stream, are thus accounted for. The remarkable accordance of so many facts of actual observation, in the Atlantic area, with the probabilities deducible from a theory whose soundness can scarcely be disputed, seems now to justify the admission of the general (vertical) oceanic circulation, sustained by opposition of temperature as an accepted doctrine of terrestrial physics.
Distribution of Organic Life. All that will be attempted under this head will be to indicate the general conditions that seem, from recent researches, to have the greatest influence on the distribution of plants and animals through this great oceanic basin. The distribution of marine plants seems mainly deter mined by light, temperature, and depth, a further influence being exerted by the character of the shores. The diminu tion of liyht iu its passage through sea-water is so rapid, that the quantity which penetrates to a depth of 250 or 300 fathoms may be regarded as almost infinitesimal ; and in conformity with this we find a very rapid diminution of Algal life below the depth of 100 fathoms. The upper stratum is occupied for the most part by the larger and coarser forms of the Fucaceoe, or olive-green sea-weeds, whilst the more delicate Ceramiactce, or red sea-weeds, fre quent deeper waters ; and, as it appears from experiments made in aquaria that the latter do not flourish in full light, but grow well in shadow, it may be concluded that their preference for a moderate depth is rather for reduced light and stillness than for depth per se. At a depth of 150 fathoms very few ordinary sea-weeds maintain their ground; and below this we seldom find any Algae, save the Coral lines and Nulliporcs consolidated by calcareous deposit. The distribution of particular types over different parts of the Atlantic area appears to be mainly regulated by tem perature ; and this would seem to be remarkably the case with the floating Dlatomacece, which, though they form green bands in the surface-water of polar seas, have not been encountered in like abundance in the Atlantic, and do not contribute largely, by the subsidence of their siliceous lories, to the composition of its bottom-deposit. Although it is the habit of the larger Algre to grow from a base of attachment (their roots serving no other purpose however, than that of anchorage), the enormous mass of Gulf -weed found in the Sargasso Sea seems quite independent of any such attachment. It was at one time supposed that this originally grew on the Bahama and Florida shores, and was torn thence by the powerful current of the Gulf Stream ; but it seems certain that if such was its original source, the " Gulf-weed " now lives and propagates whilst freely floating on the ocean-surface, having become adapted by various modifications to its present mode of existence. The distribution of the animals that habitually live in that upper stratum of the ocean whose degree of warmth varies with the latitude, seems mainly determined by temperature. Thus the "right whale" of Arctic seas, and its representative in the Antarctic, seems never to enter the inter-tropical area, generally keeping away from even the temperate seas, whilst, on the other hand, the sperm-whale ranges through the parts of the ocean where the " right whales " are never seen. The distribution of fishes seems generally to follow the same rule ; as does also that of floating mollusks. Thus the little Clio (a Pteropod mollusk), which is a principal article of the food of the " right whales " in polar seas, is rarely met with in the Atlantic, where, however, other pteropods, as Hyalcea, present themselves in abundance. On the other hand, the warmer parts of. its area swarm with Salpa-chains, which are not frequent in higher latitudes ; and the few representatives of the Nautiloid Cephalopods, that were so abundant in Cretaceous seas, are now restricted to tropical or sub-tropical areas. And the distribution of the mollusks, echinoderms, and corals, which habitually live on the bottom, seems to be determined, within certain limits at least, by temperature rather than by depth. The bathymetrical range to which animal life of any higher type than the Ehkopodal might extend, was until recently quite unknown; but the researches initiated by Prof. Wyville Thomson and Dr Carpenter in 1868, and since prosecuted by the "Challenger" expedition, have fully established the existence of a varied and abundant fauna in ocean-depths ranging downwards to 2000 fathoms. And these researches have further established that the distribu tion of this fauna is mainly determined by the tempera ture of the sea-bed ; so that whilst in the channel between the north of Scotland and the Faroes there were found at the same depths, and within a few miles of each other, two faunas almost entirely distinct one a boreal and the other a warmer-temperate on sea-beds having respectively the temperatures of 30 and 43, various types to which a low temperature is congenial are traceable continuously along the whole abyssal sea-bed that intervenes between those northern and southern polar areas within which they present themselves at or near the surface. And hence it becomes clear that, since glacial types are even now being embedded in the strata which are in process of formation beneath the equator, no inferences as to terrestrial climate can be drawn from the character of marine deposits. One very remarkable feature which presents itself over a large proportion of the Atlantic basin is the abund ance of the minute Globigerince and other Foraminifera, the accumulation of whose shells, and of their disintegrated remains, is giving rise to a calcareous deposit of unknown thickness, that corresponds in all essential particulars to Chalk. This deposit, in some parts of the North Atlantic, is replaced by an Arctic drift of fine sand, whilst in other parts there is a mixture of arenaceous and of calcareous components, such as is found in certain beds of the Cretaceous formation. Now on the surface of this deposit there have been found so many living types, especially belonging to the groups of Echinoderms, Corals, Siliceous Sponges, and Foraminifera, which closely correspond with types hitherto regarded as characteristic of the Cretaceous, epoch, that the question naturally suggests itself whether the existing are not the lineal descendants of the fossil types, the differences they present being not greater than may be fairly attributed to the prolonged action of differences of temperature, food, pressure, &c. And when these facts are taken in connection with those previously stated as to the probable remoteness of the period when (if ever) the present sea-bed of the Atlantic was dry land, the doctrine first put forth by Prof. Wyville Thomson, that there has been a continuous formation of Globigerina-mud on the bottom of the Atlantic from the Cretaceous epoch to the present time or, in other words, that the formation of chalk on the sea-bed of the Atlantic did not cease with the elevation of the European area, but has been going on through the whole Tertiary period, must be admitted as (to say the least) a not improbable hypothesis. That some considerable change took place at the conclusion of the Cretaceous epoch, by which the temperature of the upper stratum was lowered, so as to be no longer compatible with the existence of the fishes and chambered cephalopods characteristic of the Cretaceous fauna, may be fairly assumed from their disap pearance ; but this would not so much affect the deeper part of the basin, in which those lower types that seem more capable of adapting themselves to changes in external conditions would continue to hold their ground. That the like conditions had prevailed also through long previous geo logical periods, may be surmised from the persistence, over various parts of the Atlantic sea-bed, of the Apiocrinite type, which carries us back to the Oolitic formation, and of i the Pentacri,nus type, which has come down with very little i alteration from the Liassic ; whilst many existing Tere- bratulidce do not differ more from Oolitic types than the latter differ among each other. Going back still further, we find in the persistence of certain Foraminiferal types from the Carboniferous limestone to the present time, and in the character of its deep-sea beds, a strong indication that they originated in a Foraminiferal deposit, represent ing in all essential particulars that which is now going on ; while the persistence of the Lingula from the early j Silurian strata to the present time suggests the question I whether certain oceanic areas may not have remained in the condition of deep sea throughout the whole subsequent succession of geological changes.
BIBLIOGRAPHY. In addition to the ordinary sources of information, the following publications may be specially referred to for recent information in regard to the physical geography of the Atlantic : " Reports of the Deep-Sea Explorations carried on inH.M. Steam-vessels Lightning/ Porcupine, and Shearwater, " in Proceedings of Hie Roi/al Society for 1868, 1869, 1870, and 1872; " On the Gibraltar Current, the Gulf Stream, and the General Oceanic Circula tion," in the Journal of the Royal Geographical Society for 1871; and "Further Inquiries on Oceanic Circulation" (containing a summary of the " Challenger" Temperature Survey of the Atlantic), in the same journal for 1874 ; Currents and Surface-Temperature of the North and South Atlantic, published by the Meteorological Committee ; and The Depths of the Sea, by Prof. Wyville Thomson. (W.B.C.)