176 THE PHYSICAL GEOGRAPHY OF THE SEA. 



is a peculiarit}^, perhaps of temperature, perhaps of transparency, 

 which marks the inhabitants of trans-equatorial seas. MM. Pe- 

 ron and Le Sueur, who have turned their attention to the subject, 

 assert that out of many thousand cases they did not find a single 

 one in which the inhabitants of trans-equatorial were not distin- 

 guishable from those of their species in cis-equatorial seas. 



878. Water, while its capacities for heat are scarcely exceeded 

 The capacity of wa- ^J tliosc of any othcr substancc, is one of the most 

 ter to convey heat, complctc of nou-couductors. Hcat does not per- 

 meate water as it does iron, for instance, or other good conduct- 

 ors. 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 requires to be 

 conveyed by a motion, which in fluids we call currents. There- 

 fore the study of the climates of the sea involves a knowledge 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. 



37-i. Hence, in studying the system of oceanic circulation, we 

 Currents of the sea sct out with the vcry simple assumption, viz., that 



to be considered in , n ji . • p t 



pairs. from whatever part oi the ocean a current is lound 



to run, to the same part a current of equal volume is bound to re- 

 turn ; for upon this principle is based the whole system of cur- 

 rents 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 gyra- 

 tions, which in some parts of the sea, as on the coast of Norway, 

 assume the appearance of whirlpools, as though the water were 

 drawn into a chasm below. The celebrated Maelstrom is caused 

 by such a conflict of tidal or other streams. The late Admiral 

 Beechey, E.IST.,^ gave diagrams illustrative of many " rotatory 

 streams in the English Channel, a number of which occur between 

 the outer extremities of the channel tide and the stream of the 

 oceanic or parent wave." "They are clearly to be accounted 

 for," says he, "by the streams acting obliquely upon each other." 



* See an interesting paper by him on Tidal Streams of the North Sea and English 

 Channel, pp. 703 ; Phil. Transactions, Part ii., 1851. 



