320 DR ALEXANDER BUCHAN ON 



fathoms at least; but that at 1500, 2000, and greater depths, it increases to 10258. 

 With reference to the increased salinities of the ocean at depth of 1500 fathoms and 

 greater depths, it is suggested in the Challenger Report under the heading " The Southern 

 Ocean," pp. 36-38, that it probably has its origin in the relatively high salinity of the 

 surface water of the south portions of the Atlantic, Indian, and Pacific Oceans. These 

 surface waters are driven southwards by the strong west-north-westerly winds till, 

 reaching the region characterised by heavy rainfall and extensive ice-melting, they sink 

 to great depths, bearing their high salinity with them. Future observations, which may 

 more adequately represent the temperature and salinity of the great Southern Ocean 

 through its depths can alone teach us the true state of the circulation of the waters of 

 this part of the ocean. 



On the other hand, the actual specific gravities at the observed temperatures of the 

 different depths, which alone determine movement, are essentially different from the 

 above. Here there is a steady increase of specific gravity, with depth from 1*0252 at 

 the surface to 1*0280 at 2000 fathoms and greater depths. It follows from what has 

 been said that this increase of specific gravity with depth is wholly occasioned by the 

 decrease of temperature down to at least 800 fathoms, as shown by the maps of sea 

 temperature given with the Challenger Report on Oceanic Circulation. But, at depths of 

 1500 and under, the increasing specific gravity is due both to the slowly diminishing 

 temperature and also to the actual increase of the salinity of the ocean at these great 

 depths. It cannot be doubted that the increased salinity at great depths, where the 

 differences of temperature with depth are very small, is an important factor concerned in 

 the distribution over the bed of the ocean of low temperatures, chiefly from the 

 Antarctic and sub-Antarctic regions, and in a less degree, the Arctic and sub-Arctic 

 regions of the globe. 



The distribution of the different degrees of salinity of the ocean over its surface is 

 determined by the prevailing winds taken in connection with their relative dryness, the 

 upwelling from lower depths which occurs chiefly along those coasts where the prevail- 

 ing winds blow from the land seawards, and the amount of the rainfall. 



The prevailing winds over the ocean may be best studied in detail by referring to the 

 Challenger Report on Atmospheric Circulation, pp. 48-69, and the Maps 27-52 of that 

 Report, which show the isobaric lines and prevailing winds of the globe for the months 

 and the year. In these maps the general movement of the atmosphere over the different 

 oceans through the months of the year is clearly shown. The outstanding features of 

 the circulation of the atmosphere bearing on this discussion will be conveniently shown 

 by a somewhat detailed examination of the prevailing winds in January and July. 



Prevailing Winds in January. — In the North Atlantic, north of lat. 35° N, atmos- 

 pheric circulation is ruled by the low pressure in the neighbourhood of Iceland taken in 

 connection with the systems of high pressure over Eurasia on the one hand and North 

 America on the other. From this distribution of the mass of the earth's atmosphere it 



