949 



ATLANTIC OCEAN. 



ATLANTIC OCEAN. 



650 



to a comparatively small distance. But besides the tides two kinds 

 of motion are to be distinguished in the sea, which we shall name 

 with Major Rennell the drift-currents and the stream-currents. 



The drift-currents owe their origin to the effects produced on the 

 surface of the sea by the perpetual or prevailing winds ; the former 

 even where they do not blow with great force, by their uninterrupted 

 continuance displace and push forward the upper strata of the water, 

 and thus produce a motion towards the region to which they blow. 

 These drift-currents are constant, and run always in the same direction 

 and commonly with pretty equal velocity. The drift-currents produced 

 by the prevalent wiuds are not so constant and do not always run in 

 the same direction nor with the same velocity. In the Atlantic 

 Ocean the former kind of drift-current is found only between the 

 tropics, where it is produced by the trade-winds ; and the latter to 

 the north and south of 30 where they are ascribed to the effects of 

 the prevalent winds. 



Tin drift-current is in some measure observable all over that 

 of the Atlantic Ocean which is under the influence of the 

 trade-winds ; but as these winds are not very constant to the north of 

 the 23rd parallel, and rarely extend to the south of the 9th, the 

 current is constant only between these two boundaries. In the region 

 of the calms it is very weak, and often entirely ceases. But in those 

 regions in which the southern trade-winds blow, it is again perceptible 

 and constant, except along the coasts of Africa, where it has rather a 

 northerly than a westerly motion, the latter however becomes by 

 degrees more prevalent in proportion as the wind takes that direction 

 in advancing to the west. The mean velocity of this current is from 

 nine to ten miles per day, or according to the computation of Humboldt 

 only one-fourth of the velocity with which those rivers in Europe 

 commonly flow on which observations have been made. 



The drift-current which in the northern portion of the Atlantic is 

 produced by the prevalent westerly winds flows in a westerly direction, 

 but it is not perpetual, and is so slow that when a ship keeps clear of 

 the gulf stream it only manifests itself generally on the whole course 

 of a voyage from Europe to America and vice vend, retarding the 

 former and forwarding the latter. 



It is easy to conceive that the drift-currents, especially the permanent, 

 are very favourable to navigation, by rendering the voyages to some 

 countries more easy, more certain, and less dangerous. But the 

 stream-currents have commonly proved adverse, causing great loss of 

 life and property, and forcing vessels out of their course. Many 

 navigators running from Madeira to Teneriffe, and expecting to arrive 

 at the latter island, have unexpectedly found their vessels cast upon 

 the shores of Africa, nearly 300 sea-miles out of their course. But 

 the recent full and elaborate charts of the currents, issued by the 

 American government, give such varied and precise information 

 respecting these Atlantic currents that the danger is now greatly 

 lessened. 



We cannot compare the stream-currents of the ocean as Humboldt 

 has done with the rivers of the continents. The stream-currents 

 cover such a portion of the surface of the sea, that were they 

 transferred to the continents they would no longer be considered as 

 rivers, but as large branches of the sea. The opinions which have 

 been formed respecting the causes of these currents have been very 

 various and are still unsettled ; they may be seen under the article 

 CURRENTS in the division of ARTS AND SCIENCES. We shall here only 

 notice the largest of the current-streams which belong to the 

 Atlantic Ocean, and indicate their extent, velocity, and temperature. 



Two large stream-currents traverse the Atlantic Ocean ; the 

 Equatorial Current, running from the coast of Africa to that of South 

 America, and the Gulf Stream, flowing from North America to the 

 shores of Europe. 



The Equatorial Current, so called from its course lying under or 

 near the Line, may be supposed to be formed between the islands of 

 St. Thomas and Anno Bom, in the bight or bay of Benin. Hence it 

 proceeds to the west on both sides of the equator as far as 22 W. 

 long., where it sends off a branch to the north-west. Soon afterwards 

 it declines somewhat to the south, and runs in this direction towards 

 the two capes of St. Augustin and St. Roque on the Brazilian coast. 

 At the distance of about 300 sea-miles from these capes it divides 

 into two currents; the western, or main stream, running to the 

 west-north-west and north-west, along the shores of Guyana, and 

 hence deriving the name of Guyana Current, enters the Caribbean Sea 

 by the straits which separate the Leeward Inlands lying to the south 

 of Martinique from each other and from tho continent of South 

 America ; and in this sea it may in some measure be supposed to 

 terminate its course. The Brazil Current, or the other branch of the 

 equatorial current, runs to the south-west along tho shores of Brazil, 

 to the mouth of the Plata River, and may even be traced to the 

 traits of Magalhaens and of Le Maire. The whole length of this 

 current, from St. Thomas to Cape St. Roque, amounts to upwards of 

 2500 nautical miles ; and if we add the Guyana Current, from the 

 point of division opposite that cape to the strait dividing the island 

 of Triiiidud from that of Grenada, its course is increased by 1500 

 nautical miles more. The breadth of the current is different in 

 di Hi-rent parts. Near tho islands of St. Thomas and Anno Bom it 

 |uit," over three degrees of latitude, occupying about 100 

 miles, But in proportion as it advances to the west it increases in 



breadth, which is probably, as Sabine suggests, owing to its being 

 augmented by the drift-current impelled by the south-east trade- winds. 

 Opposite Cape Palnias it extends from 1 45' N. lat. to near 5 S. lat., 

 thus occupying in breadth more than six degrees, or upwards of 360 

 nautical miles. Farther to the west it enlarges still more, and attains 

 its greatest breadth, extending over 7 or 8 degrees of lat., from 4 or 

 5 south of the equator to 2^" or 3 north of it. Here therefore the 

 breadth of the current occupies 450 geographical miles, or not much 

 less than the whole length of Great Britain, from the Lizard to Cape 

 Wrath. But having soon afterwards, between 22 and 23 W. long., 

 sent off a branch to the north-west, it narrows to about 300 nautical 

 miles ; and this breadth it probably preserves to the point where it 

 divides opposite the capes of St. Augustin and St. Roque. The 

 velocity of the current is different in different parts, aud increases or 

 decreases according to the seasons, it being much greater in summer 

 than in winter. From Anno Bom to 10" W. long, it runs at an average 

 from 25 to 30 miles per day ; but from 10 to 16 W. long, it is much 

 more rapid making, in the same time, from 44 to 79 miles at the 

 end of June and the beginning of July. This seems to be the 

 strongest part of the whole stream. But it is only in the months of 

 May, June, July, and August that it runs with great force ; from 

 October to March it is moderate and sometimes very weak. Between 

 16 and 23 W. long, lies the common track of the vessels ; and here tho 

 rapidity of the current rises often to 45, 50, and even 60 miles per 

 day, but its mean velocity may be estimated at about 28 nautical 

 miles ; it is strongest near the equator, and stronger to the north than 

 to the south of it. From 23 to the coasts of Brazil the current 

 becomes rather stronger, and seems to be less affected by the seasons ; 

 but its velocity in these parts is not exactly ascertained ; it seems 

 however to run 30 miles and upwards per day. The temperature of 

 the water in the current varies also according to the seasons and the 

 different parts of its course, but it is always some degrees lower than 

 that of the ocean. The water of the ocean to the north of the 

 current is 80 or 81 Fahrenheit, and to the south 78 or 79' in 

 summer ; but in the current the thermometer shows, near Anno Bom 

 and St. Thomas, only 75, and not more to a great distance westward, 

 where the temperature falls even to 73, and at this temperature it 

 remains for more than 12 degrees of longitude. Afterwards it rises again 

 to 74, and by degrees to 76 Fahr. In summer the temperature of 

 the current may be estimated as being at an average 5 or 6 degrees 

 under that of the water of the ocean ; but in winter it is much less. 

 This current greatly affects the course of vessels which arc obliged to 

 cross it, and creates great delays to those who, passing from the north 

 to the south, traverse the equator west of the 23 of long., carrying 

 them forcibly to the west beyond Cape St.Hoque, where they are 

 driven towards the northern shores of Brazil, and are not able to 

 regain their course till after weeks of toilsome labour. 



That branch of the equatorial current which separates from it 

 between 22 and 23 W. long., and at about 24 N. lat., is rather 

 favourable to navigation, by forwarding the course of vessels returning 

 from the southern hemisphere. Its course lying in a generally north- 

 western direction, it may be called the North- West Current, Its 

 breadth is considerable, and may be estimated at the point of separa- 

 tion at 180 or 200 miles ; farther northward even at 300 ; and at a 

 more considerable distance at 240 nautical miles ; but its velocity is 

 not so great as that of the main equatorial current. Up to 10 N. lat. 

 however, and even a little farther, it may run from 20 to 24 miles per 

 day ; tut it then slackens and becomes less perceptible, though it may 

 at all times be traced to 18 N. lat., and commonly even to 25. In 

 the latter part it bends more to the northward, and at last is lost in 

 the drift current, to which it seems to give a north-westerlv direction, 

 which is observable all the way from 35 W. long, to Trinidad. Tho 

 accelerated motion of the drift-current in these seas may also in some 

 measure be the effect of this north-west current. 



The Guyana Current is properly speaking the continuation of the 

 equatorial current, and runs from Cape St. Roque in Brazil to the 

 Island of Trinidad, along the low coast of Guyana, and at no great 

 distance from it. It is of considerable breadth and of great velocity, 

 running in some places 2 miles per hour. Here too it is observed 

 that its velocity is much greater in summer than in winter and spring; 

 and it is besides much increased by the waters rushing from the mouth 

 of the Amazonas River into the sea ; for after this has taken place the 

 current runs 3 miles per hour. It is however to be observed, that 

 the waters of the" Amazonas River do not mingle with those of tho 

 current. From the less speciBc gravity of the water, the river-current 

 passes over the ocean current at right angles, and running with great 

 violence causes eddies and whirlpools ; but at last the river passes the 

 current, and is observable at a distance of 500 miles from ito mouth. 

 At a distance of 300 miles from the mouth of the river it was observed 

 by Colonel Sabine to still preserve a velocity of 3 miles an hour ; its 

 original direction being but little altered, and the fresh-water but 

 partially mixed with that of the sea. It is remarkable however that 

 the Amazonas causes no change in the direction of the ocean-current. 

 Farther to the north tho Orinoco enters tho current. This river, 

 which pours a prodigious mass of water into the ocean on both sides 

 of the Island of Trinidad, enters the current at a very acute angle ; 

 and thus soon mixes its water with it, and considerably accelerates its 

 course. Soon afterwan's the current enters the Caribbean Sea by thi- 



