Masterpieces of Science 



form a deep layer, which creeps along the ocean- 

 floor towards the equatorial regions; while the 

 warmer and relatively lighter water floats on the 

 surface in a contrary direction, or from equatorial 

 towards polar seas. By such means, a complete 

 circulation might be established; and it has con- 

 sequently been said that every drop of water in 

 the open ocean may, in course of time, be brought 

 up from the greatest depths to the surface. 

 Other meteorological conditions, however, may 

 exert an influence of the same kind, as great as, 

 or even greater than that produced by difference 

 of temperature. Sir Wyville Thomson regards 

 the influx of cold water into the Pacific and 

 Atlantic Oceans from the south as an indraught 

 due to "the excess of evaporation over precipita- 

 tion in the northern portion of the land hemis- 

 phere, and the excess of precipitation over evapo- 

 ration in the middle and southern part of the 

 water hemisphere. " 



It seems probable that ocean currents are of 

 no great importance as agents of denudation or 

 of transport. A slow circulation of the entire 

 mass of the ocean, brought about by such com- 

 paratively slight differences of density in the 

 water of different parts of the ocean, as are here 

 under consideration, might perhaps facilitate 

 the dispersion of the finest sedimentary matter. 

 Again, where the surface currents strike upon 

 the shore they must do something in the work 

 of denudation, though as a rule this will be ex- 

 tremely slight; the effect of currents, indeed, 

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