SPECIFIC GRAVITIES AND OCEANIC CIRCULATION. 323 



From lat. 30° to 63° S., as already explained, pressure diminishes uninterruptedly 

 from 30'150 inches to 29*000 inches over a broad ring going round the whole globe 

 during all seasons of the year. Now, over the whole of this vast space, westerly and 

 north-westerly winds sweep as strong winds, subject to little, if any, variation with 

 season. From the enormous quantities of warm water they impel before them into the 

 Southern Ocean they may well be considered as playing the most conspicuous part of 

 all the prevailing winds in the circulation of the waters of the ocean. 



As regards the salinity of the surface of the ocean, it is seen (Challenger Report, Map 

 1) that the broad result is a high salinity in tropical and sub-tropical regions, where 

 temperature and evaporation are high, and where the rainfall is small ; and a low salinity, 

 where temperature or evaporation are low and the rainfall large. 



In the anti-cyclonic regions of the ocean salinity is large, since out of these regions 

 winds blow in all directions, and the drain thereby caused being compensated by vast 

 descending currents of very dry air, evaporation is necessarily large, thus occasioning a 

 high salinity. Of this the anti-cyclonic region of the North Atlantic, which embraces the 

 Sargasso Sea, is a good illustration. 



As already explained, the prevailing winds of the North Atlantic do not blow home 

 to the eastern coasts of the United States, the prevailing winds there being south- 

 westerly, and, consequently, the higher salinity is at some distance seaward from the 

 coast. But, to the south of lat. 40° N., the prevailing winds become more westerly as 

 they advance in their easterly course, and as they near the north-western coast of Africa 

 become north-westerly and then northerly ; consequently, they blow home to the coast 

 of the north-west of Africa, and there we find the higher salinities close on the 

 coast. 



In the South Atlantic the south-east trades blow home to the coast from Cape St. 

 Roque to the estuary of the La Plata in a manner more direct and unimpeded than any 

 other part of the globe can show ; and, consequently, it is off this coast where the highest 

 salinities are anywhere found. For a considerable portion of the year the winds of 

 Western Australia are from the west, blowing out of the anti-cyclonic region of the 

 Indian Ocean. On this coast also a higher salinity prevails close inshore. From- 

 Australia to South America the salinitv is also high. 



The volume and strength of the oceanic current, generated and maintained by the 

 prevailing winds of the North Atlantic, is impressively manifested by the high latitude, 

 about lat 74° N., reached by the surface salinity of the ocean, 1*0260 and upwards. In 

 the North Pacific the highest latitude reached by this degree of salinity is lat. 36° N. 

 In other words, this salinity is pushed 38° of latitude farther northward in the Atlantic 

 than in the Pacific. 



On the other hand, in the tropical parts of the Pacific is an extensive region, 

 stretching in length from Panama to long. 170° E., and in breadth from 15° to 20° 

 of latitude, over which the degree of salinity is a little less than 1*0260. This 

 is a region characterised by light trade winds and extensive upwelling of the water 



