280 THE ATLANTIC. [chap. v. 



mass of water gradually and uniformly rises in temperature 

 toward the head of the gulf. 



3, That water at any given temperature (below 4° C.) can 

 only occur in the Atlantic where there is a direct communi- 

 cation with the belt of water at the same temperature in the 

 Southern Sea without the intervention of any continuous bar- 

 rier. (The actual result of the present arrangement of such 

 barriers is, that, however great the depth may be, no water at 

 a temi^erature lower than l°-9 C. is found in the eastern basin ; 

 none at a temperature lower than l°-6 C. in the north-western ; 

 and none beneath the freezing-point anywhere in the Atlantic, 

 except in the dejjression between the coast of South America 

 and the central ridge, to the south of the equator.) 



■i. That the temperature of the Atlantic is not sensibly af- 

 fected by any cold indraught from the Arctic Sea. (I purposely 

 neglect the Labrador Current and the small branch of the Spitz- 

 bergen Current, for these certainly do not sensibly affect the 

 general temperature of the North Atlantic.) 



5. That although there is a considerable flow of surface-wa- 

 ter through the influence of wind-currents from the Atlantic 

 into the Southern Sea, that flow is not sufficient to balance the 

 influx into the basin of the Atlantic (the constant influx being 

 proved by the maintenance of a general uniformity in the course 

 of the isothermobathic lines, and by the maintenance in all the 

 secondary basins of the minimum temperature due to the height 

 of their respective barriers); that, for several reasons (the lower 

 barometric pressure, and the supposed greater amount of rain-fall 

 in the Southern Sea ; the higher specific gravity at the surface 

 than at greater depths in the Atlantic ; the higher specific grav- 

 ity of the surface-water in the Atlantic to the north than to the 

 south of the equator), it is probable that the general circulation 

 is kept up chiefly by an excess of evaporation in the region of 

 the Xorth Atlantic, balancing a corresjionding excess of precip- 

 itation over evaporation in the water liemi sphere. 



