314 COSMOS. 



Payta, which inclines furthest westward, the current is sud- 

 denly deflected in the same direction from the shore, turning 

 so sharply to the west, that a ship sailing northward passes 

 suddenly from cold into warm water. 



It is not known to what depth cold and warm oceanic cur- 

 rents propagate their motion; but the deflection experienced 

 by the south African current, from the Lagullas bank, which 

 is fully from 70 to 80 fathoms deep, would seem to imply the 

 existence of a far extending propagation. Sand banks and 

 shoals lying beyond the line of these currents may, as was first 

 discovered by the admirable Benjamin Franklin, be recognized 

 by the coldness of the water over them. This depression of 

 the temperature appears to me to depend upon the fact that 

 by the propagation of the motion of the sea, deep waters rise 

 to the margin of the banks and mix with the upper strata. 

 My lamented friend, Sir Humphrey Davy, ascribed this pheno- 

 menon (the knowledge of which is often of great practical 

 utility in securing the safety of the navigator) to the descent 

 of the particles of water that had been cooled by nocturnal 

 radiation, and which remain nearer to the surface owing to 

 the hinderance placed in the way of their greater descent by 

 the intervention of sandbanks. By his observations Franklin 

 may be said to have converted the thermometer into a sound- 

 ing line. Mists are frequently found to rest over these depths, 

 owing to the condensation of the vapour of the atmosphere by 

 the cooled waters. I have seen such mists in the south of 

 Jamaica, and also in the Pacific, defining with sharpness and 

 clearness the form of the shoals below them, appearing to 

 the eye as the aerial reflection of the bottom of the sea. A 

 still more striking effect of the cooling produced by shoals 

 is manifested in the higher strata of air, in a somewhat analo- 

 gous manner to that observed in the case of flat coral reefs, 

 or sand islands. In the open sea far from the land, and when 

 the air is calm, clouds are often observed to rest over the spots 

 where shoals are situated; and their bearing may then be 

 taken by the compass in the same manner as that of a high 

 mountain or isolated peak. 



Although the surface of the ocean is less rich in living forms 

 than that of continents, it is not improbable that on a further 

 investigation of its depths, its interior may be found to possess 

 a greater richness of organic life than any other portion of our 





