OCEAN. r25 OCEAN CURRENTS. don, 1873) ; Thomson and Murray, Report of the tScienti/ic ICestdts of the uyngv of the Vlial- li-iujrr Dutiiuj IdtJ-Ttj (ib., 187U-10UO) ; Wild, Thulamsa (ib., 1S77) ; Sigsbee, Uevp-^vti Hound- iiKJ mill Drcdi/iiiy (Washington, 1880); Wil- liani^. (Itoi/rujiliy of the Oceaiin (London, 1881) ; Mitlicdet, Lu mcr (7th ed., Paris, 1882); Attlniayr and others, Uaiulbiich (lev Ozeano- yiapliie und maritimcn Melcorologic (Vienna, 1883) ; Maury, The Physical Oeogiaphy of the Sea, and Its Mcteoroloyy (l!Hh cd.. New Vork, 1883); Simmonds, C'ommereial ProiJiiels of the •Sea (London, 1883) ; Bogushnvski and Kriimmel, Haiiflbarh der Ozcanographie (Stuttgart, 1884- 87 ) ; Woodward, "On the Form and Position of the Sea Level," in Vnitid States Oeoiot/ical Sur- rey, bulletin .'iS (Washington, 1888) ; id., .IH- ycmeiiie ileercslcunde (Leipzig, 1893); Daubrec, "Deep-Sea Deposits," in Smithsonian Institution Report for 1S93 (Washington, 1804) ; Thoulet, Oceanoyraphie (2d ed., Paris, 180.5); Buelian, Report on Oceanic Circulation (London, 1805); Wharton. "The Physical Condition of the Ocean." in Smitlisonian Institution Report for ISS.'t (Washington, 1806) ; Snpan. Physische fjrdlcunde (2d ed., Leipzig. ISOfi) ; iil.. "Die Bodcnformen des Weltmeer>." in Petermann's Mitteilunyen ( Uotha, 1890) ; Oceanic Depths and Serial Temperatures from Indian Marine Survey and British Submarine Teleyraph Com panics {hondou, 1800) ; Kirchoti', "The Sea in the Life of Nations," in Smitlisonian Institution Report for 1901 (W'ash- ington. 1902) ; Kriimmel, Der Ocean (Leipzig, 1902). OCEAN (in Law). See High Seas. OCEAN CURRENTS. The consensus of .scicnlitie opinion at the present day is to the efl'cct that there are two independent circula- tions involved in the movement of the waters of the sea, the first, the liorizontal, 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 wliolly to the former. The vertical circulation applies to the much more comprehensive creeping move- ment of the warm equatorial waters of the ocean toward the poles, a movement which is confined to tile ujiper strata, and of the cold polar water toward the equator, confined to the depths of the ocean. Slrf.ce Cukrekts. As our knowledge of the movement of the surface waters of the ocean has increased, it lias become more and more apparent tliat these so-called currents 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 shi])S at sea, in which the dilTorence Iietween the oliserved 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. L'pon assembling a number of
- uch observations, extending over any period of
time and covering a limited portion of the sur- face, a one-degi-ee or five-degree square, for in- stance, it will be seen that these exhiliit the ut- most lack of agreement, the only indication of consonance beiuL'. ind 1. that current- in a cer- tain given direction appear with somewhat great- er frequency than those in any other, lliese ir- regularities stand in close rehition to the agency by which the currents themselves are jiroiluced, viz. the winds, the movements of the surface water being in response to the impulse coninuini- 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 eonsiileration is necessary. If through any cause a thin layer of liquid is set in motion in its own phme, the layer iiuiue- diately below it and with which it is in contact does not remain stationary, but likewise receives an impul.se. This second layer e.xerci.ses a like inlhicnce over the third, the third over the fourth, and so on, the velocity ultimately attained by each successive layer being ]iroportioiial to its distance from the bottom layer, which is supposed to be at rest. In the case of sea-water the rapidity with which the surface velocity Is propagated downward is exceedingly slight. It lias been calculated, for instance, that a period of 239 j-ears would elapse before a layer at a depth of fifty fathoms 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-surfiice current, once established, exhibits a like reluctance to modify its direction. Immciliately at the surface the set of the waters will thus be in close ac- cordance with the direction of the wind; at some little distance below the surface, however, the variations will be by no means so closely fol- lowed, owing to the sluggishness 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 tho.se 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 Pacific, the South Pacific, 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 easterly motion on the polar side — the former constituting the trade winds, northeast in the Northern Hemisphere, southeast in the Southern: the latter, the pre- vailing westerly winds of higher latitudes. 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 equatorial 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 Atlantic toward the shores of .merica. and the waters of the Pacific toward the shores of .sia and .ustralia. 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, compensation currents manifest themselves, due to the disturbed equi- librium, whifh. spreading out at first in sheaf- like form, ultimately reverse their direction and flow to the eastward, those on the equatorial side of the main drifts uniting to form the eastward flowing counter-equatorial current, most appar- ent during .Inly. -August, and September, when it is reenforccd by the southwest monsoon winds
