364 THE DEPTHS OF THE SEJ. [chap. 7111. 



This remarkable diversion of the isothermal lines 

 from their normal direction is admittedly caused 

 by ocean currents affecting the temperature of the 

 surface Avhile conveying the warm tropical water 

 towards the polar regions, whence there is a con- 

 stant counterflow of cold water beneath to supply 

 its place. 



We thus arrive at the well-known result that the 

 temperature of the sea bathing the north-eastern 

 shores of the North Atlantic is raised greatly above 

 its normal point by currents involving an inter- 

 change of tropical and polar water; and that the 

 lands bordering on the North Atlantic participate 

 in this amelioration of climate by the heat imparted 

 by the water to their prevailing winds. 



This phenomenon is not confined to the North 

 Atlantic, although from its peculiar configuration 

 and relation to the land that ocean presents the 

 most marked example. A corresponding series of 

 loops, not so well defined, passes southwards along 

 the east coast of South America, and a very marked 

 series occupies the north-eastern angle of the Pacific 

 off the Aleutian Islands and the coast of California. 



Two principal views have been held as to the 

 causes of the currents in the North Atlantic. One of 

 these, which appears to have been first advanced in 

 a definite form by Captain Maury, and which has 

 received some vague support from Professor Buff, is 

 that the great currents and counter-currents of warm 

 and cold water are due to a circulation in the watery 

 shell of the globe, comparable to the circulation of 

 the atmosphere, — that is to say, caused by tropical 

 heat and evaporation, and arctic cold. 



