Currents-their Causes and Functions.

134. We now proceed to what are termed the CURRENTS of the ocean-movements which, like great rivers, are ever transferring the waters from one region to another. They form, in fact, the circulatory medium by which the ocean is maintained in a state of equilibrium, and depend primarily upon the unequal temperatures and densities of different zones of the ocean and the unequal evaporation sustained by these zones; and secondarily upon the rotation of the earth, which modifies the directions imparted by these primary causes. Wherever we have waters of different temperatures, or, what is the same thing, of different densities, there the lighter will ascend and the heavier descend; and wherever a deficiency takes place through evaporation, there the waters will flow in from the adjacent parts to make up the deficiency. But difference of density may also arise from different degrees of saltness, and wherever the salter water subsides and flows off as an under-current to some fresher region, there at the same time will the fresher and lighter flow in from above to restore the equilibrium. Understanding these facts, and bearing in mind that the continuity of the ocean is interrupted by continents and islands, reefs and shoals, and further disturbed by winds and tides, it will readily be seen why its currents should assume different characters and courses. In fact, the currents and counter-currents of the ocean are extremely complicated; and though the courses and causes of some of the main streams are intelligible enough, there remains very much to be done in this department of Hydrography.

[On the equilibrium or equalisation of the waters of the ocean in temperature, density, saltness, &c., it has been appropriately remarked by Captain Maury, that "water, while its capacities for heat are scarcely exceeded by those of any other substance, is one of the most complete of nonconductors. Heat does not permeate water as it does iron, for instance, or other good conductors. Heat the top of an iron plate, and the bottom becomes warm; but heat the top of a sheet of water, as in a pool or basin, and that at the bottom remains cool. The heat passes through iron by conduction, but to get through water it requires to be conveyed by a motion, which in fluids we call currents. Therefore the study of the climates of the sea involves a knowledge of its currents, both cold and warm. They are the channels through which the harmonies of old ocean are preserved. Hence, in studying the system of oceanic circulation, we set out with the very simple assumption-that from whatever part of the ocean a current is found to run, to the same part a current of equal volume is bound to return; for upon this principle is based the whole system of currents and counter-currents of the air as well as of the water."]

135. It is usual to arrange the ascertained currents into con

stant, periodical, and variable-the constant being those arising from the combined influences of unequal temperatures and densities in the waters of the ocean, the rotation of the earth, and the trade-winds; the periodical by the tides, the monsoons, and the sea and land breezes in tropical countries; and the variable, such as may be produced by local peculiarities in the tides and winds, the melting of ice in polar regions, and other similar causes. It is also customary to speak of drift currents and deep-sea currents -the former due to the long-continued agency of the wind, and only affecting the waters to a trifling depth, the latter arising from the great primary causes of temperature and density above alluded to, and extending their influence hundreds of fathoms beneath the surface. In like manner, it may be useful to note the distinction between a marine, or upper current, and a submarine, or under-current; between a current flowing one way, and a counter-current coming from an opposite direction; and between the mode of naming winds and water-currents-the former being named after the direction from which they blow, as a "west wind" (that is, one blowing from the west), and the latter after the direction to which they are flowing, as an "easterly current," that is, one flowing towards the east.

136. The constant and deep-sea currents being the more important, it is to these that we would mainly, and in the first place, direct attention. As already mentioned, the heat of the torrid zone, by warming the equatorial waters, renders them lighter, and occasions a greater evaporation there than in any other region; and as a consequence, the waters of the polar regions, being heavier, set in as an under-current towards the equator to restore the equilibrium. It must be noted, however, that the Atlantic and Pacific being land-locked, as it were, to the north, the principal exchange of cold and warm water takes place towards the Antarctic Ocean, into which both merge widely and without interruption. Here then we have two great primary currents setting in from north and south respectively; but as they proceed towards the equator they come stage by stage into latitudes where the earth's circumference rotates with greater velocity, and as they cannot at once partake of this increased momentum, they fall behind, as it were, and gradually assume a westerly course, in which their velocity is augmented by the influence of the trade-winds. They become, in fact, a combined equatorial current, where, growing warmer and warmer, they ascend to the surface, and are partly evaporated and partly flow over in warmer and lighter surface-currents northwards and southwards to either pole, again to become colder and again to find their way to the equator in incessant circulation.

From these four primary flows-the two from the poles towards the equator, and the two from the equator towards the poles— arises the great circulatory system of the ocean, which is modified and broken up into a number of minor currents by configuration of coast, form of bottom, unequal reception of heat by different areas, the influx of rivers, and other kindred causes. These various currents, having different directions, volumes, velocities, and temperatures, will be better understood, perhaps, by being arranged under the three great oceans-Atlantic, Indian, and Pacific-in which they respectively occur.

137. The principal and better-known currents of the Atlantic are the Equatorial, the Guiana, the Brazil, the Gulf Stream, the Guinea, and the Arctic. Besides these there are some minor drifts and branches, the courses of which will be better understood by reference to the Map (p. 140) than by any amount of description. The Equatorial, as the name implies, manifests itself chiefly in the region of the equator, and flows across the ocean from the African towards the American continent. When more than half-way across it shows a tendency to bifurcate into a north-west branch and a south-west branch, and this tendency increases till within 300 or 400 miles of Cape St Roque, when it fairly divides-sending one main stream northwards by the coasts of Guiana into the Caribbean Sea; and another, somewhat feebler, southwards along the shores of Brazil. The length of the Equatorial Current, from the coasts of Africa to the Caribbean Sea, is about 4000 miles; its breadth at its commencement 160, and where it divides 450 miles; its velocity is from 20 to 60 miles a-day; and its average temperature about 75° Fahr., or from 4° to 6° under that of the ocean through which it flows. Its southern, or Brazil branch, flows at a distance of about 250 miles from the coast (the intermediate space being occupied by variable currents), and at the rate of 25 miles per day; a rate, however, that is sensibly diminished by the cross stream from the Plata, and which gradually declines till the current ultimately dies away in the Strait of Magellan. The north-west branch of the Equatorial, spreading out as it proceeds and gradually diminishing in speed, ultimately merges into the drift of the north-east trade-winds; while the Guiana section proceeds unimpeded to make the circuit of the Caribbean Sea and the Gulf of Mexico.

138. As they make the circuit of the Caribbean Sea and the Gulf of Mexico, these western branches of the Equatorial Current acquire more heat, a greater degree of saltness, and that intense blue colour so characteristic of briny waters. With these new ac

quisitions they leave the Mexican Gulf, and, pressing through the narrow channel of Florida, become the celebrated Gulf Stream— of all the Atlantic currents the most wonderful in its character and the most important in its results. Doubling the Cape of Florida, this Gulf Stream (that is, stream from the Gulf of Mexico) flows north-east in a line almost parallel to the American coast; touches the southern borders of the banks of Newfoundland; and thence, with increasing width and diffusion, proceeds across the Atlantic, till, in the region of the Azores, it spreads out into two great branches-one curving southwards towards the equator, and the other flowing northwards, impinging in its course against the western coasts of Europe, and ultimately losing itself in the waters of the Arctic Ocean. The length of this great ocean-river from its commencement to the Azores is 3000 miles, and its greatest breadth about 120 miles. When it leaves the Strait of Florida its velocity is about 4 miles an hour; off Cape Hatteras, in North Carolina, it is reduced to 3 miles; on the Newfoundland banks it is further reduced to 1 miles; and this gradual abatement of force continues with its diffusion across the Atlantic. A similar decrease takes place in its temperature, the maximum of which in the Strait of Florida is 86°, or 9° above that of the ocean in the same latitude. Off Newfoundland, in winter, it is said to be from 25° to 30° above the water through which it flows; in mid-ocean, from 8° to 10°; nor is the heat wholly lost when it impinges against the shores of western Europe. The Gulf Stream is thus, in reality, a great thermal ocean-river, incessantly flowing from warmer to colder regions, diffusing warmth and moisture along its course, and tempering the climates of countries that lie within its influence. Compressed, as it were, at its commencement between two areas of colder water, its deep-blue warm current rises in convexity above the surrounding ocean; but as it proceeds it cools, becomes diffused, assumes the ordinary level, and partakes of the greener hue of northern waters. It sets a limit to the southward flow and chilling influences of the Arctic iceberg, that melts away in its warm stream, and becomes at the same time the great natural barrier between the Life of the Northern and Southern Atlantic.

139. The Equatorial Current, flowing westward from Africa to the Gulf of Mexico, and the Gulf Stream, flowing from that gulf eastward to the Azores, and thence curving southwards, produce a great whirl, as it were, in the Atlantic, in the centre of which there is the still water of the Sargasso, or Grassy Sea. This area, so called from the vast accumulation of the Sargassum bacciferum and other floating sea-weeds, swarms with a Life, vegetable and animal, peculiarly its own, and presents one of the