324 BR ALEXANDER BUCHAN ON 



from lower depths. The salinity is also under 1'0260 in the Gulf of Guinea and 

 East Africa, from about lat. 1° N. to lat. 10° S., these comparatively low salinities 

 being in all probability occasioned by the heavy rainfall characteristic of both these 

 coasts. 



But by far the most remarkable region of low salinity, both as regards extent and 

 the very low degree to which it is reduced, is that which extends from India, through the 

 East India Islands, to long. 143° E. Further, it extends across the equator to lat. 9° S., 

 and over no inconsiderable breadth the salinity is less than 1"0250. It is to this exten- 

 sive region of comparatively brackish water which the prevailing winds drive first north- 

 wards and then eastwards across the Pacific that we must look for an explanation of 

 the extraordinarily low salinity of the North Pacific, taken as a whole and at all depths, 

 brought about by this surface current. Indeed, the salinity of this ocean may be 

 regarded as abnormally low compared with the other oceans, but more particularly with 

 the North Atlantic. 



Another consideration which has the most vital bearings on the question of oceanic 

 circulation is the position of the line of lowest mean atmospheric pressure in inter-tropical 

 regions, seeing that it is towards this line that the prevailing winds and their attending 

 ocean currents flow. 



As regards the Atlantic, through all its breadth and in all seasons, this critical line of 

 lowest pressure is situated to the north of the equator. It follows, therefore, that since 

 the surface currents of the South Atlantic, generated and maintained by the south-east 

 trades, cross the equator, they convey a high temperature and a high salinity into a 

 hemisphere other than that in which they have their origin. The remarkable salinity of 

 the North Atlantic, which is markedly higher than that of any other ocean, has its 

 explanation in the enormous overflowings into it by the surface currents of the South 

 Atlantic, together with the equally remarkable contributions to the salinity at the 

 greater depths from the Mediterranean Sea to be afterwards referred to. 



Quite different is it with regard to the Pacific Ocean, where, in its western division, 

 the line of lowest atmospheric pressure is for eight months of the year to the south of 

 the equator, where, accordingly, northerly winds, with their accompanying ocean 

 currents, cross the equator to lat. 15° S., as shown by the current charts in course of 

 preparation by the Meteorological Council. 



The result is that the temperature and salinity conditions of these two great oceans 

 are reversed. In the Atlantic the highest temperature and salinity are north of the 

 equator, but in the Pacific to the south of it ; and the lowest temperature and salinity 

 are in the Atlantic, south of the equator, but in the Pacific to the north of it. 



Another important result of the geographical distribution of atmospheric pressure in 

 the Atlantic and Pacific Oceans respectively is that in the Atlantic the north and the 

 south trades are stronger and more persistent than those of the Pacific, and the inter- 

 space between them characterised by calm and light variable winds is therefore much 

 narrower in the Atlantic than in the Pacific. From this it follows that the region of 



