THE OCEAN. 



may be considered to represent shoals ; and partly on the 

 sea, whose surface forms a liquid base or floor in immediate 

 contact with the lower and denser strata of humid air. 



Proceeding both upwards and downwards from the 

 common limit or plane of contact of the atmosphere and the 

 ocean, we find the strata of air and of water subject to 

 definite laws of decreasing temperature. The decrease is 

 much more rapid in the ocean than in the atmosphere ; the 

 sea has, in all zones, a tendency to keep up the temperature 

 of its superficial strata by the sinking of the cooled particles 

 which are the heaviest. It has been found, by an extensive 

 series of careful observations, that the surface temperature 

 of the ocean, throughout a band extending forty-eight degrees 

 on either side the equator, is, in its usual and mean condition, 

 somewhat warmer than the adjacent strata of the air. ( 362 ) By 

 reason of the diminishing temperature at increasing depths, 

 fish and other inhabitants of the sea, whose organs are fitted 

 for deep water, may find even under the tropics the low 

 temperature of cooler latitudes. This circumstance, which 

 , is analogous to that of the temperate and even cold Alpine 

 climate of the elevated plains of the torrid zone, has an im- 

 portant influence on the migration and on the geographical 

 distribution of many marine animals. The depths also at 

 which fishes live, exercise, by reason of the increased pressure, 

 a modifying influence on their cutaneous respiration, and on 

 the quantity of oxygen and nitrogen contained in their 

 swimming bladders. 



The saline particles of sea water causing it to attain its 

 maximum of density at a lower temperature than is the case 

 with fresh water, we comprehend why water brought up from 

 oceanic depths, in the voyages of Kotzebue and Dupctit 



