SPECIFIC GRAVITIES AND OCEANIC CIRCULATION. 325 



higher surface temperature and of lower salinity in the Atlantic is comparatively much 

 contracted in width ; whereas in the Pacific the breadths occupied are several times 

 greater. It is interesting to note that the lower salinity in the central Atlantic, between 

 the high salinity to the north and south, marks with great distinctness the region between 

 the two trade winds ; and special attention is drawn to the circumstance that in this 

 region, from long. 30° W., the salinity constantly diminishes eastward to the head of the 

 Gulf of Guinea, where it falls below 1*0260, being here lower than anywhere else on the 

 west coasts of Europe and Africa, from the English Channel southward to the Cape of 

 Good Hope. 



So far as observed, the region of lowest salinity is the Sea of Okhotsk, where the mean 

 is only 1*0240 ; and the next lowest between Greenland and Spitsbergen, where Mohn 

 gives the low mean specific gravity of 1*0245, and the same is the mean of the six observa- 

 tions made by the " Challenger " at the most southern stations. The salinity is also low 

 on the west and east coast of the southern division of South America, owing to the heavy 

 rainfall in the west and the upwelling along the eastern coast. 



But when we come to deal with specific gravities at the observed temperatures of 

 the surface, a widely different result is at once seen (Map 1). The broad result is that 

 the distribution of the specific gravity is the reverse of the salinity, the lowest being 

 now inter-tropical and the highest extra-tropical. The absolutely highest, 1*0277, or 

 0*0025 above the general mean of all the oceans, occurs in the North Atlantic from 

 about Faro, immediately to the north of the Wyville Thomson Eidge, and thence extends, 

 first, in a north-easterly direction, and then an easterly direction mid-way between the 

 north of Norway and Spitzbergen. The great oceanic current which has this region as 

 its final destination may be considered as starting on its course somewhere between the 

 Sargasso Sea and the north-west coast of Africa, and thence successively over the 

 Atlantic accompanying the north-east trade wind ; through the West India Islands, where 

 it gradually assumes a northerly direction ; then a north-easterly direction, from Florida 

 towards Spitzbergen ; and, finally, an easterly course, in the direction of Nova Zfembla. 

 It is thus the oceanic current which runs uninterruptedly by far the longest course of 

 any oceanic current, and, consequently, it is in the latter part of this long course where 

 the specific gravity of the surface waters of the ocean attains a higher degree than 

 observations show to occur anywhere else over the ocean. It is of the utmost im- 

 portance to note that this high specific gravity is not occasioned, as generally is 

 the case where it occurs, by an abnormally low temperature of the water of the sea. 

 On the contrary, the temperature of the sea is here very greatly higher than is 

 observed anywhere else in such high latitudes. The high specific gravity is mainly 

 the result of the higher salinity of the water. This affords the strongest proof that can 

 be adduced, that the great surface currents of the ocean are caused b}" the prevailing 

 winds. 



The absolutely lowest specific gravity, 1*0222, or 0*0030 under the general mean of 



VOL. XXXVIII. PART II. (NO. 9). 2 X 



