286 FIRST YEAR SCIENCE 



dangering all vessels which are not securely moored. 

 Such tidal surfs are called bores and are found in the 

 Seine, Amazon and other rivers. 



The tidal current as it sweeps between islands often 

 forms eddies and whirlpools which make navigation very 





AN ATOLL IN THE MID-PACIFIC. 



dangerous. An example of this is found at Hell Gate, 

 N. Y., and at the famous Maelstrom off the coast of Nor- 

 way. On the other hand in flat countries where the rivers 

 are shallow, ports which could not otherwise be reached 

 are made accessible to ships of considerable burden at the 

 time of high tide. At these places the time of leaving or 

 making port changes each day with the time of high tide. 

 A striking example of this is the port of Antwerp. 



