Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/32

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