56 Currents of the Ocean, 



An equatorial surface-current will remain such so long as its 

 temperature is sufficiently high to render it lighter than the sur- 

 rounding waters, but as during its progress towards higher 

 latitudes its temperature decreases, it will, on account of its 

 greater saltness, sink below the fresher waters of these latitudes, 

 and continue its course as a warm under-current towards the 

 polar regions. On the other hand, a polar surface-current, 

 although composed of fresher water, will, on arriving at a 

 certain latitude, sink below the tropical waters on account of its 

 low temperature and consequent greater specific gravity, and 

 continue its course towards the Equator as a cold under-current. 



But the temperature of the ocean decreases not only from 

 the Equator towards the Poles, but also from the surface towards 

 the bottom. Hence, in the tropical regions, the warm but salt 

 surface-water will sink on becoming cooled by its contact with 

 the strata beneath, and impart some of its heat to the latter ; 

 while in the polar regions, the fresh but cold surface-water 

 produced by the melting of the ice will, on becoming more salt 

 by its admixture with the surrounding water, sink in its turn 

 and lower the temperature of the strata with which it comes in 

 contact. 



The final result of these exchanges of temperature, which 

 constitute what may be called the vertical circulation of the 

 oceanic waters, appears in the oblique position, and the con- 

 sequent spreading out of the isotherms as we recede from the 

 Equator (Plates 9 and 19). The isotherm of 5 C, for example, 

 which near the Equator is found at a depth of 300 fathoms, is 

 met with at 600 fathoms in lat. 50 S. in the Southern Ocean, 

 and at 800 fathoms in lat. 50 N. in the North Atlantic. 



The Southern Ocean is the main feeder of its three gigantic 

 offshoots — the Atlantic, the Pacific, and the Indian Oceans, which 

 it supplies through the medium of both surface and under- 

 currents. The former, driven by the westerly winds against 



