336 DR ALEXANDER BUCHAN ON 



when they pass the West Indies into the great south-westerly aerial and north-easterly 

 ocean currents of the Atlantic. The effect of this is a drain of no inconsiderable body 

 of the water of the Atlantic from intertropical into extratropical regions. 



Another important consideration is that, in the Atlantic to the north of lat. 30° N. 

 and to the south of lat. 30° S., the prevailing winds and the powerful ocean currents 

 accompanying them are directed into higher latitudes. The intertropical portion of the 

 Atlantic has therefore its surface more or less lowered by the north-easterly currents of 

 the North and the south-easterly currents of the South Atlantic. Further, the currents 

 propelled polewards b} T these great wind systems must more or less raise the level of 

 the ocean in the higher latitudes towards which they blow. 



But evaporation and the rainfall tend still further to accentuate these differences of 

 level. If the intertropical part of this ocean be considered by itself, the evaporation is 

 very greatly in excess of the precipitation, and, consequently, the level of this part of 

 the ocean is still further lowered. Over by far the larger portion of the regions swept by 

 the trade winds the loss from the excess of evaporation is about 7 feet per annum. On 

 the other hand, on advancing from the tropics to higher latitudes the evaporation and 

 precipitation tend to approximate, and ultimately the rainfall exceeds the evaporation 

 and very greatly so in high latitudes. Thus, the level of the ocean in these regions is 

 still further increased. If the powerful ocean current poured southward out of the 

 South Atlantic, the large snow and rainfall, and the widespread melting of ice be 

 considered, it is evident that the Southern Ocean to the south of the Atlantic w T ill have 

 its level raised more than any other ocean from these causes. The result will be first 

 a more decided downward movement of its w T aters, and, as regards the greater depths, 

 an approximately horizontal movement of the waters from higher towards lower 

 latitudes. 



The part of the ocean signalised by high specific gravities, from the surface down at 

 every stage to the bottom, is beyond all comparison that region of the North Atlantic 

 tabulated above. This becomes the more pronounced as the greater depths are reached, 

 viz., depths under 400 fathoms, Maps 6 and 7, a result doubtless to be traced to the 

 outflowing under-current of high specific gravity from the Mediterranean. But, un- 

 fortunately, we must wait for observations from the regions to the north of lat. 40° N. 

 and from Brazil to the north-east towards the routes followed by the "Challenger," 

 ere the far-reaching influence of this powerful under-current can be properly repre- 

 sented. 



The region of high specific gravity next in order is that of the South Atlantic, 

 which is less at the surface and at great depths, but nearly the same from 100 to 400 

 fathoms, Maps 2 to 5. At depths exceeding 800 fathoms it is markedly less than in 

 the North Atlantic, being, however, at these depths all but equal to the specific gravities 

 found in the deeper waters of the Pacific to the east of Australia, Map 7. A most 

 regrettable blank in the South Atlantic observations of specific gravity and temperature 

 at depths exceeding 800 fathoms is from the coast to, say, 30° of west longitude seaward 



