SPECIFIC GRAVITIES AND OCEANIC CIRCULATION. 341 



surface water is blown to the leeward shores of the lochs, and colder water, by upwelling, 

 rises to the surface on the wind-shore of the lochs. These cold areas of a lower surface 

 temperature and salinity are also continued down to the bottom, with a tendency to an 

 expansion of the areas with" descent. 



The ice-cold water which occupies the bottom of the ocean in all latitudes necessitates 

 a constant supply of water of a very low temperature from the deep water of the Southern 

 and the Antarctic Oceans, and in a less degree the Arctic Ocean. This slow moving 

 current of cold water along the bottom of all parts of the ocean is effected, on the one 

 hand, by the reduction, in the intertropical regions, of the surface waters by evaporation 

 and by the extratropical prevailing winds blowing polewards, and on the other by the 

 greater specific gravities of the ocean in high latitudes and the " head " of water accumu- 

 lated there by the prevailing south-westerly winds of the northern and the prevailing 

 north-westerly winds of the southern hemisphere. 



The increase of the temperature which may be considered as setting in from 1500 

 fathoms upwards to the surface, implies that this excess of temperature has its origin 

 wholly in the surface temperature. The restricted extent and continuity through all 

 depths of these two contrasted areas may be regarded as the balance struck by the 

 forces which produce the upward movement, in lifting the cold water of great depths 

 towards the surface, and the downward movement in transferring the surface warmth to 

 greater depths. The result would have been materially different if the depth of the ocean 

 with respect to its extent had not been so insignificant as it is. But additional experi- 

 ments on a large scale and observations are needed before these upward and downward 

 movements in the ocean which are conducted on so vast a scale can be explained, or 

 even adequately described, as to the way in which they are effected. 



There are subsidiary causes powerfully influencing oceanic circulation, the chief of 

 which are abnormally heavy rainfall, such as occurs in the west of the Pacific ; under- 

 currents of a high temperature and specific gravity from the Mediterranean and Red 

 Seas ; the causes leading to the extensive upwelling seen in the Pacific to the south-east 

 of the Sandwich Isles, and analogous positions in the Atlantic and Indian Oceans, which 

 are closely connected with the supply of a portion of the water of the great surface 

 currents of these oceans ; and the intertropical position of the line of lowest mean 

 barometric pressure, resulting in a temperature much higher in the North than in the 

 South Atlantic, and much higher in the South than in the North Pacific Ocean. 



[Appendix. 



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



