Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/31

Rh current thus directly impelled by wind is termed a &quot; drift- current,&quot; whilst a current whose onward movement is sustained by the vis a tergo of a drift-current is called a &quot;stream-current.&quot; 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 &quot; indraught &quot; or &quot; supply currents,&quot; complete the &quot; horizontal circulation &quot; 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 &quot; equatorial calms,&quot; 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 &quot; back-water &quot; 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 &quot; head- water &quot; of the Equatorial Current. From the recent observations of Capt. ISTares in the &quot;Challenger,&quot; 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 &quot; cold wall. &quot; 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. 