OCEAN CURRENTS 171 



the bottom deposit down to a depth of about 200 fathoms. 



3. Seiches are oscillations in a body of water in an enclosed 

 basin or bay, or even in the open ocean, where the water is 

 caused to swing to and fro round one or more pivots or 

 " nodes." Temperature seiches and density seiches may 

 also occur beneath the surface in a body of water where 

 there is a " discontinuity layer " causing an abrupt change 

 in temperature or density of the water above and below. 

 The lower layer may then swing backwards and forwards 

 without causing movements at the surface. 



4. Currents. True ocean currents are bodies of water 

 of definite constitution, often differing markedly from the 

 surrounding water, through which they flow like a river 

 without much mixing and retaining a clearly defined border 

 (as in the case of the Gulf Stream). Ocean currents are all 

 in the long run due to the energy derived from the sun, but 

 the more immediate causes may be stated as — 



(1) The sun's heat causing differences of temperature, 



(2) Differences in amount of evaporation and of rainfall, 

 and hence of density of the water. 



(3) Prevalent winds. The direct frictional action of the 

 wind is a prime factor in oceanic circulation. 



As these causes have much the same action in each of 

 the three great oceanic areas —the Atlantic, the Pacific, and 

 the Indian Ocean — they give rise to comparable systems of 

 currents, modified in each case by local factors, such as the 

 shape of the land. In the Atlantic, for example, the chief 

 oceanic currents describe a figure of eight (8) moving, as a 

 result of the rotation of the earth, clockwise to the north — • 

 east — south in the North Atlantic, and counter-clockwise to 

 the south — east — north in the ^outh Atlantic, the central 

 crossing being in the interval between the great North and 

 South Equatorial currents which flow westwards before the 

 trade- winds. In this interval lies the Counter-Equatorial 

 current flowing eastwards to the African coast, where it 

 becomes the Guinea current. (See Fig. 9.) 



