XI.] THE SEA AND ITS WORK. 173 



it is probably very feeble at depths much less than this ; 

 and, in most cases, on our own shores, it is not very marked 

 below the limit of the lowest tide. 



Winds not only agitate the sea and produce irregular 

 waves, but where they are constantly blowing over the ocean 

 in a definite direction they cause the surface-water to ' take 

 a similar course, and thus produce steady drifts or currents. 

 Dr. Croll has shown that the direction of the great ocean 

 currents agrees very closely with that of the prevailing 

 winds. Bottles thrown overboard from ships in the open 

 ocean may be carried by these currents for hundreds of 

 miles, and ultimately cast upon distant shores. Pieces of 

 wood, and nuts and seeds, known to be native to the West 

 Indies and tropical America, are occasionally drifted across 

 the Atlantic, and are washed on to the western shores of 

 England, Scotland and Ireland, and even across to Norway. 

 In like manner, the little Portuguese man-of-war {Physalia), 

 and those oceanic snails with violet shells called lanthince, 

 are now and then brought as visitors to our coasts, though 

 usually confined to warmer seas far to the south and 

 west. 



Perhaps the best known of these oceanic currents is the 

 Gulf Stream, which is a broad body of warm water sweep- 

 ing out of the Gulf of Mexico, through the Strait of 

 Florida. After running northwards, nearly parallel to the 

 coast of the United States, it strikes across the Atlantic 

 Ocean in a north-easterly direction. Warm currents, which 

 continue the direction of the Gulf Stream, set on to the 

 western shores of Britain and even extend to the coast of 

 Norway ; while, other currents, parting with these in mid- 

 ocean, turn to the south and sweep round the coasts of Spain 

 and Northern Africa. The cause of the Gulf Stream is un- 

 doubtedly to be sought in the so-called "Trade Winds," which, 



