OCEANIC CURRENTS. 117 



may be of much higher importance than we are 

 at present aware. Even if we restrict our view of 

 their beneficial results to those which refer to 

 navigation, we may perceive how considerable 

 they are. There are numerous rivers on our 

 coasts, the bars at the entrance of which would be 

 impassable unless during a rise of tide, and many 

 places now used as harbours could not be so 

 employed were the water always to remain at 

 a low level. The currents, moreover, produced by 

 the flow and recess of the tides, are of no small 

 importance to the navigation of estuaries, giving 

 motion to ships when there is no wind. 



In speaking of the saltness of the sea-water 

 some reference was incidentally made to the cur- 

 rents of the ocean. A few further remarks on the 

 subject are, however, desirable in this place. 



The currents of the ocean differ entirely from 

 the tides, not only in being permanent phenomena, 

 but as arising from causes altogether different. 

 The oceanic currents are very numerous, and it is 

 impossible to contemplate them without perceiv- 

 ing that they constitute a system of circulation 

 carried on in obedience to physical laws, and indi- 

 cating in a very striking manner the design of 

 the All-Wise. 



There are, as already stated, currents which 

 run from the Atlantic into the Mediterranean, 

 and from the Indian Ocean into the Eed Sea, 

 and on the principles laid down with great inge- 

 nuity by Lieutenant Maury, it may be considered 

 as fully demonstrated that under-currents proceed 

 i 3 



