OCEAN CURRENTS. The consensus of scientific opinion at the present day is to the effect that there are two independent circula tions involved in the movement of the waters of the sea, the first, the horizontal, having its source in the energy supplied by the wind; the second, the vertical, sustained by differences of temperature. The phenomena ordinarily de scribed as ocean currents, consisting of the hori zontal motion of the layer of water immediately at and near the surface, belong wholly to the former. The vertical circulation applies to the mudh more compreben,ive creeping move ment of the warm equatorial waters of the ocean toward the poles, a movement which is confined to the upper strata. and of the cold polar water toward the equator, confined to the depths of the ocean.
SCIWAcE: CFREENTS. As our knowledge of the movement of the surface waters of the ocean has increased, it has become more and more apparent that these so-called eurrents are very unstable both in velocity and direction. The source—in deed. the only source—from which information concerning them is to be derived is found in the log-books of ships at sea. in which the difference between the ob,erved and the computed position at noon of each day is entered as the current ex perienced by the ship during the preceding twen ty-four hours. Upon assembling a number of suet observations, extending over any period of time and covering a limited portion of the sur face, a one-de!Yree or five-degree square. for in stanee, it will he seen that these exhibit the ut most lack of apTeement. the only indieation of being. indeed, that eurrents in a cer tain given direction appear with somewlmt great er frequency than those in any other. These ir regularitie, stand in close relation to the agency by which the elITedt, themselves are produced, viz. the winds, the movements of the water being in response to the impulse communi cated to it by the moving air. To explain, how ever, the fact so frequently noted, that the re• corded set of the waters is in direct opposition to that which the prevailing wind would lead us to expect, some little consideration i. necessary.
If through any cause a thin layer of liquid is set in motion in its own plane, the layer imme diate]) below it and with which it I. in contact does not remain stationary, but likewise receives an impulse. This second layer i'xcret,es a like influence over the third. the third over the fourth, and so on. the velocity ultimately attained by each successive layer being proportional to its distance from the bottom layer. which is supposed to be at rest. In the ease of sea-water the rapidity with which the surface velocity is propagated downward is exeeedingly slight. It has been calculated, for instance, that a period of 2:39 years would elapse before a layer at a depth of fifty would attain a velocity equal to half that at the surface, the current at the latter being supposed to flow steadily all the time. In a similar manner, a sub-surface currm.t. once established, a like reInctance to modify its direction. Immediately at the surface the set of the waters will thus be in close ae cordance with the direction of the wind; at some little distance below the surface. however, the variation, will be by no means so closely fol lowed, owing to the slu:tgishness with which the impulse is communicated downward; and at a moderate depth it may be assumed that the minor fluctuations are eliminated, and that the mean direction and strength of the current become ap parent, being those due to the resultant of the winds.
The system of winds covering each of the great oceans. the North Atlantic. the South Atlantic, the North Pacifies the south Pacifie, and the Indian, is practically identical, consisting as it does of a general westward motion of the air on the equatorial side of the tropical belts of high pressure. and of a like 'tuition on the polar side—the former constituting the trade winds, northeast in the Northern lfemisphere, southeast in the Southern; the hitter, the pre vailing, westerly winds of higher In each of the oceans there is a general movement of the surface waters in response to these winds; in the tropical latitudes of either hemisphere toward the west—the north and the south equatorial drift—in extra-tropical lati tudes toward the east. The north equatorial and the south equatorial drift carry the waters of the Atlantie toward the shores of America. and the waters of the Pacific toward the shores Id Asia and Australia, at a rate varying from 12 to 24 miles per day. The central line of either drift is well defined; along its marginal limits, how ever, north and south, eompensation currents manifest themselves, due to the disturbed equi librium. whieh, spreading out at first in sheaf like form, ultinmtely reverse their direction and flow to the those on the equatorial side of 'the main uniting to the eastward flowing counter-equatorial current. most appar ent during July, August, and September, when it is by the southwest monsoon winds of the African and the Central American coasts; those on the polar side becoming merged in the general easterly drift of higher latitudes.
The immense mass of water carried by the equatorial drifts in the great oceans causes an ac cumulation upon the eastern continental shores and a consopient disturbance of equilibrium, which is in each case partially adjusted by a str«un current, or current due to gravity alone, which in every case follows the shore line and is directed away from the equator. In the North Atlantic Ocean this current is known as the Gulf Stream; in the South Atlantic as the Bra zilian Current ; in the North Pacific as the liuro Sivo; in the South Pacific as the Australian Current in the Indian Ocean as the .Mozambique Current. These arc more constant in direction and force than the ordinary drift currents, but likewise are subject to stoppage or even to re versal. Upon attaining, middle latitudes the combined effect of the deflective force of the earth's diurnal rotation and of the prevailing westerly winds serves to turn them off shore, and their identity is henceforth lost in the gen eral easterly drift of the tenqierate zone. The easterly- drift of extra-tropical latitudes exhibits none of the persistency of the westerly drift of the tropical, the reported tieing appar ently quite as variable as the winds themselves.
In the case of the North Atlantic Ocean. the easterly 'hilt divides to the northwestward of the Azores. one portion carrying to the southward along the Peninsular and African shores.' and finally reuniting with the north equatorial cur rent, the whole system forming a vast gentle eddy in close agreement with the prevailing winds; the remaining portion carries eastward and northward toward the shores of Great Britain, Seandinavia, and Iceland. Analogy would sug gest a like division in the ease of each of the other oceans, with the possible exeirption of the North Pacific. The information as to these oceans thus far accumulated is. however, not suf ficient to establish the hypothesis.