OCEANIC CIRCULATION 119 



to an extension of the areas with increase of 

 depth. It follows that the great mass of the 

 ocean intermediate between the upper layers 

 and the bottom largely exhibits the effect of 

 vertical movements. On the other hand, on 

 the eastern sides of the oceans, whence the 

 trade-winds start on their course, there is an 

 upwelling of the colder water of the greater 

 depths towards the surface. These cold areas 

 of a lower surface temperature and salinity 

 are also continued down to the bottom, with 

 a tendency to an expansion of the areas with 

 descent. The ice-cold and nearly ice-cold 

 water which occupies the bottom of the ocean 

 in all latitudes necessitates a constant supply 

 of water of a very low temperature from the 

 surface of the Southern and Antarctic Oceans, 

 and in a less degree from the sub-Arctic 

 Ocean. This slow-moving descent of cold 

 water, and its slow creep in the deeper layers 

 and along the bottom of all parts of the ocean, 

 are effected on the one hand by the reduction 

 in intertropical regions of the surface waters 

 by evaporation, and by the extratropical 

 prevailing winds blowing the surface waters 

 polewards, and on the other hand by the 

 greater densities of the ocean in high latitudes 

 and the " head " of water accumulated there 

 by the prevailing south-westerly winds of 

 the northern hemisphere and the prevailing 



