564 THE DEPTHS OF THE SEA. [cHAP. VI, 
This remarkable diversion of the isothermal lines 
from their normal direction is admittedly caused 
by ocean currents affecting the temperature of the 
surface while 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 infer- 
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, 
