II THE PROBLEMS OF THE DEEP SEA 63 



be doubted that the cold of the regions towards 

 the poles must tend to cause the suj)erficial water 

 of those regions to contract and become specifically 

 heavier. Under these circumstances, it would 

 have no alternative but to descend and spread 

 over the sea bottom, while its place would be 

 taken by warmer water drawn from the adjacent 

 regions. Thus, deep, cold, polar-equatorial currents, 

 and superficial, warmer, equatorial-polar currents, 

 would be set up ; and as the former would have 

 a less velocity of rotation from west to east than 

 the regions towards which they travel, they would 

 not be due southerly or northerly currents, but 

 south-westerly in the northern hemisphere, and 

 north-westerly in the southern ; while, by a parity 

 of reasoning, the equatorial-polar warm currents 

 would be north-easterly in the northern hemi- 

 sphere, and south-easterly in the southern. Hence, 

 as a north-easterly cuiTent has the same direction 

 as a south-westerly wind, the direction of the 

 northern equatorial-polar current in the extra- 

 tropical part of its course would pretty nearly 

 coincide with that of the anti-trade winds. The 

 freezing of the surface of the polar sea would not 

 interfere with the movement thus set up. For, 

 however bad a conductor of heat ice may be, the 

 unfrozen sea-water immediately in contact with 

 the undersurface of the ice must needs be colder 

 than that further off ; and hence will constantly tend 

 to descend through the subjacent warmer water. 



