180 PHYSICAL GEOGRAPHY OV THE SEA, AND ITS METEOltOLOGY. 



dependent for their well-being upon temperature, as are the 

 fauna and the flora of the dry land. Were it not so we should 

 find the fish and the algae, the marine insect and the coral, dis- 

 tributed equally and alike in all parts of the ocean. The arctic 

 whale would delight in the torrid zone, and the habitat of the 

 pearl oyster would be also under the iceberg, or in the frigid 

 waters of polar seas. 



372. Those of southern unlike those of northern seas. — Never- 

 theless, though the constituents of sea water be the same in kind, 

 we must not infer that they are the same in degree throughout 

 all parts of the ocean, for there is a peculiarity, perhaps of 

 temperature, perhaps of transparency, which marks the inhabit- 

 ants of trans-equatorial seas. MM. Peron and Le Sueur, who 

 have turned their attention to the subject, assert that out of 

 many thousand examples they did not find a single one in which 

 the inhabitants of trans-equatorial were not distinguishable from, 

 those of their species in cis-equatorial seas. 



373. Tlie capacity of water to convey heat. — Water, while its 

 capacities for heat are scarcely exceeded by those of any other 

 substance, is one of the most complete of non-conductors. Heat 

 does not permeate water as it does iron, for instance, or other 

 good conductors. Heat the top of an iron plate, and the bottom 

 becomes warm ; but heat the top of a sheet of water, as in a pool 

 or basin, and that at the bottom remains cool. The heat passes 

 through iron by conduction, but to get through water it requi7*es 

 to be conveyed by a motion, which in fluids we cali currents. 

 Therefore the study of the climates of the sea involves a know- 

 ledge of its currents, both cold and warm. They are the channels 

 through which the waters circulate, and by means of which the 

 harmonies of old ocean are preserved. 



374. Currents of the sea to he considered in pairs. — Hence, in 

 studying the system of oceanic circulation, we set out with the 

 very simple assumption, viz., that from Avhatever part of the 

 ocean a cun-ent is found to nm, to the same part a current of 

 equal volume is bound to return ; for upon this principle is based 

 the whole system of currents and counter- currents of the air as 

 well as of the water. Hence, the advantage of considering them 

 as the anatomist does the nerves of the human system — in pairs. 

 Currents of water, like currents of air, meeting from various 

 directions, create gyrations, which in some parts of the sea, as on 

 the coast of Norway, assume the appearance of whirlpools, as 



