40 THE OCEAN. [Book II. 



rising to tlie surface ir[ tlie' equatorial regions under 

 the action of the force of vertical circulation ; which 

 latter necessitates a circulation from the equator at 

 the surface and towards the equator at the bottom of 

 the ocean in the central parts of the district s w n E, 

 in Plate IX., just as the horizontal force necessi- 

 tates a chculation from the equator on the west and 

 towards the equator on the east side of the district. 



Thus a constant circulation between the equa- 

 torial and polar regions is caused, for as the polar 

 stream e c d is carried southwards, displacing the 

 stream a b c in the lower strata, an equal volume of 

 the stream A b c must be thrown into the course of 

 the stream c d e in the upper strata. 



Section IX. — Subdivision of Districts. 



The course of the currents which we have 

 described in connection with Plate IX. we have 

 termed the natural course of the currents within the 

 district; but in fact it is clear that instead of the 

 horizontal force of circulation being contained in one 

 stream and meeting the vertical force, also in one 

 stream, as at x, there might, by the horizontal force 

 being partially deflected by obstructions in its course 

 before reaching the west side of the ocean, be many 

 such meeting-points formed within a district, all 

 being nevertheless of a similar nature : but, since the 

 number of these meeting-points in any district must 

 be determined by the configuration of the coast-lines 



