Surface and Under-Current s. 55 



in places where great rivers pour their masses of fresh water 

 into the sea. 



According to the extensive series of specific gravity observa- 

 tions on sea-water made on board H.M.S. " Challenger," by Mr. 

 T. Y. Buchanan, M.A., chemist to the expedition, it appears that 

 the specific gravity of salt wat;er under the influence of tempera- 

 ture varies between a minimum of 1.02 1 and a maximum of 1.028 

 (to use round numbers), and the specific gravity, as affected by 

 the percentage of salt contained in the water, between 1.024 and 

 1.027. If we select the sections surveyed by the " Challenger" 

 between St. Paul Rocks at the Equator, the Cape of Good 

 Hope, Kerguelen Land, and the Antarctic Circle, we find that 

 the specific gravity of the surface water, according to the percent- 

 age of salt, commences with 1.027 near the Equator, falls to 

 1.026 towards lat. 40 S., to 1.025 between lat. 40 and 50 S., 

 remains at that figure as far as lat. 6o° S., and finally sinks to 

 1.024 in the immediate vicinity of the ice-barrier. On the 

 other hand, the specific gravity of the surface-stratum, under the 

 influence of temperature, commences with 1.023 near the 

 Equator, rises to 1.026 towards lat. 40 S., attains 1.027 near 

 lat. 50 S., and continues the same down to the ice-barrier. 

 The temperature of the bottom-stratum in the different oceanic 

 basins remains uniformly within a few degrees of o° C, hence 

 its specific gravity is generally about 1.028. The percentage 

 of salt in the bottom-water was found to decrease from the 

 Equator towards the polar regions, the specific gravity falling 

 from 1.027 to 1.025. 



It will thus be seen that the difference in specific gravity 

 due to temperature is more than double the difference arising 

 from the varying percentage of salt ; whence we conclude that the 

 order of the oceanic strata depends, in the first instance, upon 

 temperature, in the second, upon the amount of salt held in 

 solution. 



