INTRODUCTIOX. 71 



gation itself, and is no less interesting in a physical 

 point of view. These currents are either local 

 and partial, either constant, or periodical and irre- 

 gular. The first de})end either on local circum- 

 stances, contractions of the coast, or the ebb and 

 flood, sometimes on unknown natural causes ; the 

 latter are rather parts of the economy of nature 

 in general, and are partly consequences of prevail- 

 ing winds, perhaps also of the rotation of the 

 earth, of the alternations of cold and heat, of eva- 

 poration, and other causes still unknown. Acci- 

 dental currents may also be caused by storms in 

 the vicinity. The currents in the sea, like those 

 in the atmosphere, seem sometimes to run above 

 each other, for the most part, in opposite directions. 

 On shore they may be easily perceived by their 

 accelerated motion on the coast. It is more diffi- 

 cult to investigate them on board the ship, which 

 is itself carried away by them. There is no fixed 

 point from which to measure this motion, and the 

 usual means by which the seaman determines 

 his change of place, the log, is uninfluenced by 

 the currents in the sea. It is only by combin- 

 ing the astronomical determination of the ship's 

 place with the ordinary ship's reckoning, that 

 it can be discovered how far the ship has been 

 driven from the shore, how far it has been carried 

 by the current. A daily comparison of the results 

 of both methods is, therefore, a necessary occupa- 

 tion for every mariner, who is provided with the 



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