60 THE OCEAN 



BOOK II.— CURRENTS. 



CHAPTER VII. 



GKEAT MOVEMENTS OF THE SEA.— GENERAL CAUSES OF CURRENTS. — 

 THE FIVE OCEANIC RIVERS. 



Currents, that is to say, the real movements of the sea, much less 

 visible to the eye than the apparent displacements which constitute 

 the waves, are notwithstanding o^f much greater importance in the 

 economy of our planet. By their action enormous volumes of water, 

 thousands of miles wide, and hundreds of fathoms deep_, move across 

 the oceanic basins ; the water of the polar seas is carried to equatorial 

 regions, while these on their side send their waves in the direction 

 of the poles. The liquid mass circulates incessantly, as if in a 

 vast whirlpool, in every ocean of the globe, and we can follow in 

 thought its gigantic circuit from the fields of ice to the warm atmo- 

 sphere of the tropics. Currents are indeed only the ocean itself in 

 motion, and by their action the waters of the sea are successively 

 distributed over all parts of the globe. They are the windings of 

 the great *' salt river " of Homer, which rolls around the earth in one 

 immense circuit. Every drop that has not already been raised in 

 vapour to commence its long journey through clouds, mists, glaciers, 

 and rivers, continually changes its place in the abysses of the sea ; 

 it descends to tlie bottom, or mounts to the surface ; it moves from the 

 equator to the pole, or from the pole to the equator ; and thus traverses 

 all parts of the ocean. It is to this continual displacement of its 

 innumerable particles that the sea owes its uniformity in such a 

 surprising manner, under all latitudes, as regards the appearance, 

 composition, and saltness of its waters. 



Every difierence of level which is produced on the liquid surface in 

 consequence of prolonged winds, heavy rains, or very active evapor- 

 ation, causes, as a necessary result, the formation of a current ; for 



