SPECIFIC GRAVITIES AND OCEANIC CIRCULATION. 335 



The striking feature as regards these salinities of the higher latitudes is the much 

 higher salinity of the northern part of the Atlantic and its continuation northward and 

 then eastward through the Arctic Ocean. The high salinity of the surface has been 

 already referred to the prevailing south-westerly winds of the North Atlantic and the 

 strong ocean current which accompanies them. In this case, again, the high salinity 

 prevails not only at the surface, but at all depths down to the bottom of the ocean. 



Emphasis is laid on the fact that in each of these areas the relatively high or low 

 saliuity of the surface waters is continued uninterruptedly downwards through all depths 

 to the bottom of the ocean. # In this connection, Map 8, showing the salinities at 

 100 fathoms, may be compared with Map 7, which shows the specific gravities at the 

 bottom of the ocean, the differences being virtually the same as the salinities at these 

 depths, and observations are greatly more numerous at these than at tire other depths. 

 This points to the conclusion that by means of the vertical movements of the water 

 there is a vital connection maintained between the surface and the bottom of the ocean, 

 it being plain that, if there be no such connection, the water occupying the deeper 

 parts of the ocean would, through the diffusion of the salt, show a salinity virtually the 

 same everywhere. 



Viewed broadly it is seen that salinity is low in the high latitudes and high in inter- 

 tropical regions. But the geographical distribution of the specific gravities is the reverse 

 of this (Map l), the difference being the simple result of differences of temperature. 

 So far as the water of the sea itself is concerned, or viewed apart from the winds, it is 

 not the salinity but the specific gravity at the temperature of observation that originate 

 and maintain movement in the ocean. 



In Map 1, giving the specific gravity of the surface, the red colouring shows those 

 regions where the specific gravity is above the general average of the ocean. It being 

 there where the specific gravity is large, the downward movement of the water is also 

 large, and the deeper the red the downward tendency is the greater. The absolutely 

 greatest is in the Arctic Ocean, between Iceland and the north of Norway. On the other 

 hand, the shadings of blue indicate those regions where the specific gravity is low, 

 and the deeper the tint the lower is the specific gravity. The absolutely lowest is in 

 the Bay of Bengal and the offshoots of the Chinese Sea to the west. 



In the Atlantic it is only in the Gulf of Guinea where the specific gravity falls so low 

 as 0'0020 below the general mean of the ocean, the low specific gravity here being 

 caused by heavy rains. In the interval between the north and south trade winds it 

 does not elsewhere fall to 0'0020 below the mean of the ocean, thus forming a marked 

 contrast to the much lower and widespread low specific gravity of the Pacific. This 

 great difference is due to the less breadth occupied by the trade winds with their 

 attendant currents in the Atlantic as compared with the Pacific, the greater strength of 

 the Atlantic trades and currents, and the diversion of this system of winds and currents 



* See Mr J. Y. Buchanan's Papers, Proc. Roy. Soc, vol. xxiii. p. 123, and Proc. Roy. Geog. Soc, December 1886, 

 where this peculiarity is shown to hold good from the surface to 200 fathoms. 



