Page images

lies have been chief among the regions of the earth's crust be regarded as representing the original sea-bed (from that have used the pent-up force in the contracting sphere which the Azores have been lifted up by volcanic action), to carry forward the continental developments. If this whilst the deep valleys on either side of it are areas of was true of the North American continent, the same in subsidence" answering to the “areas of elevation of the principle was law for all continents.”l

land that borders them, Dimensions of the Atlantic.--The length of the Atlantic Generally speaking, the depths of these valleys increase basin, considered as extending from the Arctic to the pretty rapidly with the distance from the shore-line, so Antarctic circle, is nearly 8000 geographical miles. The that the contour-lines of one and two miles follow the nearest approach of its boundaries is between Greenland shore-lines pretty closely. But there are two localities in and Norway, whose coasts are only about 800 miles apart. which shallow water extends to a much greater distance They thence recede from each other towards the south, from land than it appears to do elsewhere. One of these as far as the parallel of 30° N. lat., where, between the lies in the neighbourhood of the British Isles. For a dispeninsula of Florida and the western coast of Marocco, tance of about 230 miles to the westward of Ireland there there is an interval of 70° of longitude, or about 3600 is a slope of only about 6 feet in a mile ; but in the next geographical miles. The channel then rapidly narrows as 20 miles there is a fall of 9000 feet, after which there is it passes southward, so that between Cape St Roque in little change of level for 1200 miles. Hence as the depth Brazil (5° S. lat.) and the coast of Sierra Leone (between of the sea immediately surrounding the British Isles is 5° and 8° N. lat.) the African and American continents nowhere 100 fathoms (so that an elevation of their whole approach within 1500 miles of each other. The sudden area to that amount would unite these islands not only to eastward recession of the African coast as it approaches the each other but also to the continent of Europe), it is equator, and the westward trend of the South American obvious that the platform on which they rest is really, coast-line between Cape St Roque and Cape Horn, widen although now submerged, a part of the land-mass of out the South Atlantic basin to the same breadth as that Europe. Another of these extensive shallows is that of of the North Atlantic in the parallel of 30° N.,—the which the Banks of Newfoundland form the highest part ; interval between the Cape of Good Hope and the estuary and of the existence of this a probable explanation may of La Plata, in the parallel of 35° S., being no less than be found in the accumulation of the rock-masses that are 731° of longitude, or about 3600 geographical miles. brought down by icebergs every summer from the coasts

The depth of the North Atlantic has been more care of Greenland and Labrador. For it is now generally fully and systematically examined than that of any other admitted that these icebergs are really parts of glaciers, that oceanic basin ; and the general contours of its undulating were originally formed on the mountain-slopes of Greenland sea-bed may now be regarded as pretty well determined. and Labrador, and then descended valleys which open out Putting aside the older soundings as utterly untrust- on their coasts, so as, on arriving at the mouths of these worthy, and accepting only those taken by the modern valleys, to detach themselves and float away, being borne methods, whose reliability has been amply tested by the southwards by the Polar Current to be presently described. accordance of diversified experiences, we can now assert Most Arctic icebergs of which a near view can be obtained with confidence that scarcely any portion of its floor has are observed to have upon them a considerable number of a depth exceeding 3000 fathoms, or about 3-4 miles, the pieces of rock, sometimes of a very considerable size ; and greatest depth determined by the recent“Challenger” sound these are of course deposited on the sea-bed when the ings, which was that of a limited depression about a icebergs melt (which they usually do on the borders of the hundred miles to the north of St Thomas, having been Gulf Stream), thus forming a vast conglomerate bed, to 3875 fathoms, or about 4:4 miles. Except in the neigh-which parallels are not improbably to be found in various bourhood of its coast-lines, and in certain shallower areas geological epochs. to be presently specified, the floor of the basin at its Geological Age of the Atlantic Basin.—Guided by the widest part seems to lie at a depth of from 2000 to 3000 principle that great oceanic basins are to be considered fathoms, its slopes being extremely gradual. The central rather as original marine areas that have been limited by portion of the principal basin of the North Atlantic, the elevation of their boundaries, than as having been however, is occupied by a plateau of irregular shape, of formed by the excavation of terrestrial areas, we have to which a considerable part lies at a less depth than 2000 inquire what evidence there is that the basin of the Atlantic fathoms. Of this plateau the Azores may be regarded as has undergone any considerable change within a comparathe culmination; and that group being taken as its centre, tively recent period. it may be said to extend to the north as far as lat. 50°, and As has been pointed out by Prof. Wyville Thomson to the south-west as far as the tropic of Cancer. The (Depths of the Sea, p. 473), it is difficult to show that any northern extension of this plateau narrows out into a sort oscillations have occurred in the north of Europe since the of isthmus, which connects it with the plateau that occupies termination of the Secondary period, to a greater extent than a great part of the Atlantic basin to the north of 50°N. from 4000 to 5000 feet,—this being the extreme vertical lat

. ; and it is across this isthmus, and along the bottom depth between the base of the Tertiaries and the highest of the deep narrow valley on either side of it, that the point at which Tertiary or post-Tertiary shells are found on telegraph cables are laid between Ireland and Newfound the slopes and ridges of mountains. Such oscillations, while land. Whether its south-western prolongation, known as the considerably modifying the boundaries of the Atlantic, would "Dolphin Rise" (fig. 1, infra) extends to the equator, so as not seriously affect the condition of the deeper parts of its to become continuous with the elevated area which cul- sea-bed; and hence it may be concluded that the two deep minates in St Paul's rocks, and by a further southward ex- valleys, one on the European side of the modern volcanic tension becomes continuous either with the volcanic elevation platform of the Azores, and the other on the American, of St Helena and Ascension Island, or with the elevation each having a width of 600 or 700 miles, and an average in the middle of the South Atlantic which culminates in depth of 15,000 feet, could neither have been formed by the island of Tristan da Cunha (fig. 2), has not yet been such oscillations, nor could, when once formed, have been ascertained. According to the view already suggested as converted into dry land. It will be presently shown that to the formation of the Atlantic basin, the plateau might this idea of the existence of an Atlantic basin correspond

"On some Results of the Earth s Contraction from Cooling," in ing generally to that now existing, as far back as the later Amer. Journ. of Science, June 1873.

Secondary period, is strongly supported by the evidence recently obtained of the continuity of animal life on the of general elevation, resembling the plateau of the North Atlantic sea-bed from the Cretaceous epoch to the present Atlantic. time,

The entire chain of the Greater and Lesser Antilles, which Important information as to the changes which the sea-stretches from the delta of the Orinoco to the peninsula bed of the Atlantic has undergone within the later geolo- of Florida, and forms the eastern boundary of the Caribbean gical periods, may be gathered from the structure of the Sea, seems to have been in like manner elevated by vol. islands which lift themselves above its surface. Along its canic action. That this elevation, like that of the group's eastern border, at no considerable distance from the coast of islands on the eastern side of the Atlantic, took place of North Africa, there are three principal groups, -the for the most part during the later Tertiary period, is shown Madeiras, Canaries, and Cape Verd, -all of which have an by the occurrence of shells, corals, &c., of upper Miocene evidently volcanic origin, and rise up from the eastern age, in the upraised sedimentary bevis of several of the slope of the basin, where it is progressively shallowing islands ; while the presence of “ fringing reefs ” of coral towards its continental shore-line. Further out, in mid- around the shores of many of the West India islards is an ocean, lies the group of the Azores, which also is volcanic, indication that they lie in an area in which elevation is and rises from the plateau already spoken of; but between still proceeding. The channels by which they are separated this area and the slope from which the Madeiras and are so deep as to render it very unlikely that there was Canaries are based is a very deep channel, ranging down- ever a continuity of land between them; and the occawards to at least 15,000 feet; and a like depth is also sional recurrence of earthquakes and volcanic eruptions at found between the Azores and the coast of Portugal. The different points of this “ line of fire,” shows that the plutonic structure of all these groups of islands gives obvious action by which the islands were raised is still going on indications of their formation by separate igneous eruptions beneath. in a sea of great depth; and the earliest of these eruptions The case is very different, however, in regard to the seems to have taken place in the later Miocene period. | Bermuda group, which constitutes a singular exception to As soon as the first solid lavas raised their heads above the general fact of the absence in the Atlantic of those water, and were thus exposed to the action of the coral islands that are so numerous in the Pacific. This waves, fragments were detached and rounded on the group consists of about 300 islands, of which, however, shore; and these being swept off, with the débris resulting only five are of any considerable size ; and these rise from from their attrition, formed deposits of various kinds upon a shoal or platform of about 23 miles long by 13 miles the slope of the cone, in which corals, shells, &c., were broad, the channels between the islands being very shallow, embedded. These fossiliferous deposits have been subse while at a small distance from the edge of the shoal, the quently elevated to heights of from 1500 to 2000 feet bottom rapidly deepens to 15,000 feet. The islands are above the level of the sea, showing a rise of the base of the entirely composed of upraised beds of coral, shells, dc. craters; progressive additions have been made to their (the highest elevation being only about 180 feet above the upper part by the piling up of basaltic and trachytic lavas." sea-level); and the shoal itself appears to have the like That this state of activity still continues is proved by the structure throughout, no traces of any other rock than a fact that in 1811 a new island was temporarily formed in limestone formed by the metamorphoses of coral being the Azores group, off St Michael, by the throwing up of anywhere met with. Hence, as this insular platform ashes, and the formation of a cone about 300 feet high, proves to be the summit of a submarine column of 15,000 with a crater in the centre. This island, to which the feet high, rising from a very small base, and as nothing we name Sabrina was given, was soon washed away by the know of the structure of mountains--volcanic or otherwaves. And only a few years since, another submarine would justify us in supposing that a column of such a eruption in this neighbourhood was indicated by earth- height could be formed in any other way than by coral quakes, jets of steam and columns of smoke, and floating growth, the structure of the Bermuda group would seem masses of scoriæ. All these considerations concur (as Sir to indicate a progressive subsidence of the bed of this Charles Lyell, loc. cit., justly urges) to negative on geolo- part of the Atlantic during its formation, corresponding gical grounds the hypothesis which has been advocated by to that which (according to the well-known views of Mr some eminent naturalists, that the Azores, Madeiras, and Darwin) is at present in progress over a large area of the Canaries are the last remaining fragments of a continuous Pacific. It is probable that this coral growth was deterarea of land which once connected them with the west of mined in the first instance by the existence of a submarine Europe and North Africa.

mountain, of which the summit lay near the surface, or Proceeding to the south of the equator, we meet with lifted itself above it; that as soon as this came to be similar evidence of volcanic activity in the structure of the submerged, the coral formation commenced ; and that by only two islands, Ascension and St Helena, which lie near its continued growth at the summit, at a rate equal to the line stretching from the Cape Verd group to the Cape that of the subsidence of its base, the platform has been of Good Hope ; and these also arise from a plateau of kept up to the sea-level. The slight elevation which has considerably less depth than the circumjacent area whose raised its highest portion above that level may not improeastern slope gradually shallows to the coast of South Africa. bably have taken place in connection with the much larger This plateau stretches in a north-westerly direction towards recent elevations already referred to. the equator, so as to meet it in from 20° to 22° W. long.; Thus, then, we have evidence of considerable recent and here indications of volcanic activity — earthquakes, local modifications in the level of the Atlantic sea-bed, troubled water, floating scoriæ, and columns of smoke-without any such change as would affect its general chahave been several times observed since the middle of the racter as an ocean basin ; while all geological probability last century, betokening the probable formation of an island seems in favour of the remoteness of the principal depresor an archipelago in that locality.

sion of the Atlantic area, even if we do not regard it as Nearly midway between the southern prolongations of dating back to the period when the surface of the globo the African and American continents, the solitary peak was first undergoing solidification. of Tristan da Cunha (fig. 2) lifts itself above the ocean ; Currents of the Atlantic.- By the term “ current" will this also is volcanic, and seems to rise from a broad base be here meant that sensible movement of ocean water in

See Sir C. Lyell's account of them in his Principles of Geology, 11th particular directions which can be generally traced, directly od. p. 407, 899.

or indirectly, to the action of wind upon its surface. A current thus directly impelled by wind is termed a "drift. a general fact, that where a current encounters any partial current,” whilst a current whose onward movement is obstruction, such as a coast-line meeting it obliquely, a sustained by the vis a tergo of a drift-current is called a narrowing of its channel, the lateral pressure of another "stream-current.” But there is another source of current current, or even that of a mass of stationary water,-its movement, which has been overlooked by most writers on velocity increases; and so the portion of the Equatorial this subject, namely, the indraught which necessarily takes Current that is pressed to the northward by the coast-line place to keep up the level of any area from which the between Cape St Roque and the mouth of the Orinoco surface-water is constantly being drifted away. Such cur- (known in the first part of its course as the Cape St rents, which may be designated as “indraught" or "supply Roque Current, and afterwards as the Guiana Current) currents," complete the "horizontal circulation” that must acquires a greatly augmented rate, running ordinarily at necessarily take place in any oceanic area of which one part the rate of from 30 to 50 miles, and occasionally at is subjected to the action of a wind almost constantly a rate of 80 miles, in the 24 hours. Entering the Caribblowing in the same direction. Of such a circulation we bean Sea, it is reinforced by the portion of the Equatorial have a very characteristic example in the South Atlantic, Current which flows in between the Lesser Antilles ; and it the principal currents of which we shall see to be very then passes westwards along the northern coast of South easily accounted for.

America, until it is deflected northwards by the coast-line The initial movement of the current-system, alike of the of Central America, and driven between the peninsula of North and of the South Atlantic, is given by the trade- | Yucatan and the western extremity of Cuba into the Gulf of winds, which are continually driving the water of the inter- Mexico, at the rate of from 30 to 60 miles per day. A por tropical region from the African towards the American side tion of it passes direct to the N.E. along the northern shore of the basin, so as to produce what is known as the Equa- of Cuba; but by far the larger part sweeps round the gulf, torial Current. The position of the northern and southern following the course of its coast-line, and approaches the boundaries of this current shifts, like the area of the trade- coast of Cuba from the N.W. as a broad deep stream of no winds, in accordance with the northward and southward great velocity, seldom running at more than 30 miles per declination of the sun ;-a steady westward drift being day. The reunited current, being mot by the Equatorial generally met with to the north of the tropic of Cancer in Current from the outside, which is pressing to the west the summer of the northern hemisphere, and to the south along the north coast of Cuba and between the Bahama of the tropic of Capricorn in the summer of the southern, isles, is deflected northwards through the passage termed whilst in the winter of each hemisphere the border of the the Florida Channel, which is bounded on the one side by drift lies within the tropic of that hemisphere. But as the the southern extremity of the peninsula of Florida, and on thermal equator lies from two to three degrees to the north the other by the coast of Cuba and the Bahamas. The rate of the geographical equator, the entire zone of the trade of movement of the powerful current that flows through winds, and of the Equatorial Current propelled by them, is this channel, henceforth known as the Gulf Stream, is conwider on the northern than on the southern side of the siderably augmented in its narrowest part, which is also latter; and while the northerly trade often reaches 30° | its shallowest ; but although its velocity sometimes reaches N. in July, and rarely extends south in January within 2° | 4 (nautical) miles per hour, or even more, its average rate or 3° of the geographical equator, the southerly trade does through the whole year may be confidently stated at not not extend farther than 25° S. in January, and generally more than 2 miles per hour, or 48 miles per day.2 crosses the equator in July, even extending occasionally as The Gulf Stream current, however, does not by any far as 5o N. As between the northerly and southerly means occupy the whole of the sectional area of the Florida trades there is a region of “equatorial calms,” so there is a Channel ; for it is separated from the American coast by a corresponding interval between the northern and southern band of cold water, which occupies about three-eighths of divisions of the Equatorial Current; and in this interval | its total breadth of 40 miles, and which also dips under the there is a counter-current (resembling the “ back-water” outflowing current. The movement of the cold superficial often to be noticed in a stream that is flowing rapidly past band is perceptibly inwards, and that of the cold undersome obstacle, such as a vessel at anchor, or a projecting stratum is presumably so; and it is the opinion of the angle of a river-bank), that runs eastwards, sometimes with American surveyors that the depth of the warm outward considerable velocity, towards the Bight of Biafra, which current is not more than one-third of that of the channel may be considered the “head-water” of the Equatorial through which it flows. It is probable that the rate of Current. From the recent observations of Capt. Nares in movement decreases from the surface downwards ; but upon the “Challenger,” it appears that the Equatorial Current, this point we have as yet no certain information. The like other drift-currents, is very shallow, its depth being not meaning of the cold inflow will hereafter become apparent. much greater than 50 fathoms. He estimates its rate at The course taken by the Gulf Stream in the first inthe surface to be about 0·75 iniles per hour, or 18 miles per

stance is nearly parallel to the line of the United States day, whilst at 50 fathoms it only moves at about half that coast, from which it is everywhere separated by a band of rate. Its surface temperature generally ranges between cold water,—the boundary line between the two being so 75° and 80°; but the thermometer falls to 60° at a depth distinct as to be known as the “cold wall.” It does not of little more than 100 fathoms,—the temperature of this show for some time any great disposition to spread itself belt of water, as will be hereafter shown, being kept down out laterally, though a division into alternate bands of by the continual rising of polar water from below.

warmer and colder water, the cause of which seems to lie The Equatorial Current passes directly across the Atlantic in the contour of the bottom of the Florida Channel, betowards the chain of the Antilles and the coast of South comes perceptible before it reaches Charleston, and is very America ; and as not only the whole of the northern divi- marked off Čape Hatteras. The Stream there presents the sion, but a considerable part of the southern, strikes the form of a fan, its three warm bands spreading out over the American coast-line to the north of the salient angle of Atlantic surface to an aggregate breadth of 167 miles, Cape St Roque (about 5° S. lat.), the portion of the current

This statement, which is much lower than that adopted by most which is deflected into the northern hemisphere is much writers on the Gulf Stream, is based on the entire aggregate of observagreater than that which is turned to the southward. It is tions collected by the Meteorological Department, which further show whilst two cold bands of an aggregate breadth of 52 miles | along the coast of Portugal. As this current flows past the are interposed between them. The innermost warm band entrance to the Strait of Gibraltar, a part of it, forming is the one which exhibits the highest temperature and what is known as the Gibraltar C'urrent, is drawn in to greatest rate of flow, its velocity being greatest where it is keep up the level of the Mediterranean, which would otherpressed on laterally by the Arctic Current, so that a rate of wise be reduced by the excess of evaporation from its sur4 miles per hour is occasionally observed. Capt. Nares face; but the greater part keeps its course southwards along estimates the depth of the Stream in this part of its course the Marocco coast, reinforcing the south-flowing extension at about 100 fathoms, and its rate of flow in the line of of the Gulf Stream. On arriving at the border of the most rapid movement at 3 miles per hour. The outermost northerly trade, the North African Current divides into two band, on the other hand, graduates insensibly, both as to parts,-the western division being at once carried into the temperature and rate of movement, into the general sur- course of the equatorial drist, whilst the eastern, which face-water of the Atlantic. It is when passing Sandy may be considered as essentially an indraught or supply Hook that the Gulf Stream takes its decided turn east current, follows the African coast-line, and turns eastward wards,—this change in its direction being partly due to the into the Gulf of Guinea, forming the Guinea Current, eastward bend of the United States coast-line, and partly which, coalescing with the eastward " back-water" already to the excess of easterly momentum which it brings from mentioned, flows pretty constantly, sometimes with conthe lower latitude in which it issued from the Florida siderable rapidity, towards the Bight of Biafra. There it Channel. Its general rate of flow past Nantucket seems meets the South African Current, which forms the other not to exceed 1 mile per hour, and to be frequently less ; great feeder of the Equatorial Current; and the circulation but several degrees to the eastward of this, the current has thus completed may be considered as recommencing from been found occasionally running at the rate of 4 miles an | this “ head-water.” The large area of comparatively still hour,—this acceleration being probably due to the lateral water which lies in the interior of this North Atlantic pressure of the Arctic Current, which, during the early circulation is called the Sargasso Sca,-a corruption of the months of the year, is driven southwards at the rate of 10 name (Mar de Sargaço) which it received from Columnbus or 12 miles per day by the N. and N. W. winds then pre- and the early Spanish navigators, on account of the quantity vailing along the coast of Labrador, and which, turning of sea-weed that floats on its surface. The boundaries of westwards round the south of Newfoundland, keeps close this area, which is of an irregularly elliptical shape, and to the coast of the United States (being left behind in the nearly equals that of Continental Europe, are somewhat rotation of the earth, in consequence of its deficiency of variable; but it may be considered to lie between the easterly momentum), and follows it southwards, every- parallels of 20° and 35° N., and between the meridians where separating it from the Gulf Stream.

that, for six months of the year, the monthly mean averages only 1.4 1 Mr Laughton, however, states the average velocity to be between miles per hour, or 34 miles per day, whilst for the other six months it 20 and 30 milos per day.

only averages 24 miles an hour, or 60 miles per day.

of 30° and 60° W. Into it is collected a large proporBy the gradual thinning-out and expansion of the Gulf tion of the drift or wreck which floats about the North Stream after passing the Banks of Newfoundland, by the Atlantic. progressive reduction of its rate of movement, and by Proceeding now to the South Atlantic, we meet with a the loss of that excess of temperature which previously circulation of the same kind, uncomplicated by any distinguished it, as well as of its peculiar blue colour (which embaying of the Equatorial Current. The smaller division probably depends on its holding in suspension the finest of this current which strikes the coast of South America particles of the river-silt brought down by the Mississippi), to the south of Cape St Roque flows along the coast of this remarkable current so far loses all its special attri- Brazil at the rate of from 12 to 20 miles a day, forming butes, as to be no longer recognisable to the east of the the Brazil Current, which, however, is separated from the meridian of 30° W. long.,—there degenerating into the land by an intervening band of lower temperature, that general easterly drift of that region of the Atlantic which has, during the winter months, a distinct flow towards the is kept up by the prevalence of westerly winds, some equator. The Brazil Current can be traced southwards, by times called “anti-trades." Where the Florida Current its temperature rather than by its movement, as far as the or true Gulf Stream can last be distinctly recognised, it estuary of the La Plata, before reaching which, however, forms a stratum not more than 50 fathoms in thickness ; a great part of it takes an easterly direction, and crosses and it is there flowing almost due east, at a rate which the Atlantic towards the Cape of Good Hope, forining would require about 100 days to bring it to the Land's what is known as the Southern Connecting Current. The End. The only valid evidence of the extension of any easterly movement of this current seems to be partly due part of it to the western shores of Europe (the ameliora- to the westerly anti-trades, and partly to the excess of tion of their temperature being otherwise accounted for, easterly momentum which is retained by the Brazil Current while the transport of trunks of trees, drift-timber, fruits, in its southward course from Cape St Roque; whilst it shells, &c., to the Western Hebrides, the Orkney, Shetland, partly depends also on the junction of an Antarctic current and Farve islands, and the coast of Norway, may be that flows N.E. from Cape Horn, meeting the Brazil fairly set down to the surface-drift sustained by the pre- Current off the estuary of La Plata, just as the Arctic valence of S.W. winds) is afforded by the variable current Current meets the Gulf Stream off Newfoundland,--dense known as Rennell's, which, flowing eastwards into the fogs being produced, in the one case as in the other, through southern part of the Bay of Biscay, is deflected in a N.W. the precipitation of the vapour overlying the Equatorial direction by the trend of its coast-line, so as to cross the Current, by the colder air that overlies the Polar. On British Channel towards the Scilly Islands, whence it meeting the coast of South Africa, the Southern Connecting passes to the S.W. coast of Ireland, its strength mainly Current turns northwards, and runs towards the Bight of depending on the continued prevalence of the westerly Biafra, forming the South African Current, the movement anti-trades. (See Plate I.)

of which is partly sustained by the southerly winds which Of the whole mass of water, on the other hand, that is prevail along that coast, but is partly attributable to the brought into the mid-Atlantic by the Gulf Stream, it may indraught set up to supply the efflux of the Equatorial be stated with confidence that the larger proportion turns Current. In its passage thither, however, the part of it southward to the east of the Azores, and helps to form the most distant from the land is draughted westwards by the North African Current ; the other tributary of which may southern trade, forming the most southerly portion of the be considered as originating as far north as Cape Finisterre, equatorial drift. Between this and the Southern Conunder the influence of the northerly winds which prevail necting Current is a central space, lying between the

little excess,

parallels of 20° and 30° S., and the meridians of 0° and ture with which the Gulf Stream leaves the Florida Channel 25° W., over which there are no regular currents; and to is retained in summer with only 5° reduction as far as Nova this the name Sargasso Sea is sometimes applied by analogy, Scotia, there is a reduction of 5° in winter during its northalthough its surface has no covering of sea-weed. (See ward passage to Cape Hatteras, and a further reduction Plate I.)

of no less than 10° during its eastward passage from Cape Temperature of the Atlantic.—The distribution of surface Hatteras to Nova Scotia, making a total reduction of 15°. temperature over the area of the Atlantic has now been In spring, again, there is a total reduction of 11°, and in made out with considerable accuracy; and it corresponds autumn of 13°; and in both cases the reduction during closely with what has been already stated as the course of the eastward flow under the parallel of 35° N. is greater the surface currents. There is, of course, a seasonal than the reduction in the northward flow from 25° N. to change, alike in its northern and in its southern division, 35° N. The explanation of this is plainly to be found in this change being more and more marked as we recede the fact that in the early part of the course of the Gulf from the equator. Following the course of the mean annual | Stream its superheated stratum is a thick one, so that when isotherms, however, we find that they cross the South its superficial film is cooled down by a superincumbent Atlantic at nearly regular intervals, in an east and west atmosphere of lower temperature, it is replaced by the direction, the principal departure from that direction being uprising of a deeper stratum having nearly its original shown at their western end in the bend they take towards temperature. But as the stream spreads out superficially, the south under the influence of the warm Brazil Current, its superheated stratum becomes proportionally thinner, and at their eastern in the still stronger bend they take and will consequently be more and more rapidly cooled towards the north under the influence of the cold South down by the superincumbent atmosphere. Even supposing, African Current, which reduces to about 75° the temperature therefore, that it were not subjected to any special cooling of the southern equatorial that flows alongside the Guinea | influence, it appears certain that, as the rate of the current Current, whose temperature is 82°. In the North Atlantic, slackens and its depth dininishes, the cooling process must however, the influence of the movement of oceanic water on continue at an increased rate, so as to bring down the the surface-temperature is very much more marked. The surface-temperature of the stream to the normal isotherm annual isotherms, which cross the Sargasso Sea with nearly of the locality, long before it could reach the shores of regular parallelism, and on the African side tend somewhat Europe. But it has been shown that when it passes to the south, where they meet the colder water of the Newfoundland the Gulf Stream is subjected to a special North African Current, show a strong northward bend on cooling influence that of the Labrador Current with its the American side, along the early course of the Gulf fleet of icebergs, which melt away when borne into it; and Stream; but as its excess of temperature above that of the this produces such an immediate reduction of its surfaceAtlantic generally diminishes as we trace it towards the temperature, that it thenceforth shows very Banks of Newfoundland, this northward deflection progres- although its sub-surface stratum still appears to be warmer sively becomes less. The marked contrast in temperature than that of the ocean through which it flows. which is often there exhibited between two contiguous bands But, further, the Gulf Stream, where it is last recogof water,—a thermometer hanging from a ship's bow show- nisable as a current, is flowing due east, and its southern ing a temperature of 70°, whilst another hanging from the portion turns first south-east and then south, whilst, on the stern shows only 40°,—is due not so much to the elevation other hand, the course of the isothermal lines (see Plate) produced by the Gulf Stream as to the depression produced clearly shows that the flow of warm water which carries by the Arctic Current. This depression manifests itself them northward spreads across the whole breadth of the in the southward bend given, on the American side, alike | Atlantic, from the British Isles to Labrador, even extending to the summer and the winter isotherms (see Plate), beyond up to the west of north into Baffin's Bay. When we the summer isotherm of 70° and the winter isotherm of contrast this immense body of north-moving water with 60°, which may be considered as having nearly their normal the thinned-out film of what is by comparison a mere position; whilst the northward tendency of these same rivulet, it becomes obvious (1) that its northward flow isotherms on the European side not less conspicuously cannot be attributable to the vis a tergo of the Florida indicates a flow of warm water towards the western coasts Current, whilst (2) its convection of heat to the Arctic Sea of the British Isles, Norway, and even Iceland and cannot be accounted for by any amount of excess of Spitzbergen. It has been customary to regard this flow as temperature that is limited to a small depth, since the an extension of the Gulf Stream ; but if that term be temperature of such a stratum, moving north-east at a rate limited (as it ought) to the current that issues from the of (at most) 4 or 5 miles per day, must soon be brought Gulf of Mexico through the Florida Channel, the hypothesis down to that of the atmosphere above it. is found to be untenable so soon as the thermal phenomena Influenced by these considerations, several eminent of that current are carefully examined. For, in the first hydrographers, both British and American, have been place, the popular idea that the Gulf Stream retains its disposed to deny, not only that the temperature of the high temperature with little diminution during its passage North Atlantic is modified in any considerable degree by first northwards and then eastwards is clearly disproved the true Gulf Stream, but that any other agency than that by observation, as is shown by the following table of of warm S.W. winds is concerned in producing the climatic average temperatures taken at different seasons in the amelioration popularly attributed to it

. They maintained, warmest of its bands :

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 comFlorida Channel........

25 N.

77 78 83 82 parisons recently instituted between marine and atmospheric Off Charlestown.

33 N.



temperatures along the western coasts of Scotland, the Off Cape Hatteras

72 73 80 76

Orkney, Shetland, and Faroe Islands, and especially with S.E. of Nantucket Shoals 35 N.

72 S. of Nova Scotia . 35 N.

those obtained along the western coast of Norway. For it

is found that during the winter months there is a constant From this it appears that, while the high surface-tempera- 1 excess of sea-temperature above that of the air, averaging




SO Spring.

3 Summer

35 N.



80 78


« EelmineJätka »