18 THE WEALTH OF THE SEA 



the sea; others, such as the jellyfishes, are quite 

 transparent. 



Movements of Ocean Waters 



Those who visit the sea-shore for the first time 

 are greatly impressed with the height and regularity 

 of the tides. The tidal wave rises and falls every 

 twelve hours, twenty-five and a half minutes. In the 

 Antarctic Ocean it moves from east to west in a 

 free and uninterrupted course around the earth. In 

 the other oceans the tidal wave is more or less modi- 

 fied because of the continents, which interrupt its 

 passage around the earth. When the tidal wave 

 strikes the shore it is reflected and moves back into 

 the ocean, meeting and modifying the incoming tidal 

 wave. Irregular coasts have tides very different from 

 those of regular coasts. Some places, such as the 

 eastern coasts of Scotland and England, receive their 

 tides by two separate waves that reach them by 

 different routes from the north and south; at some 

 places along these coasts a tide of double height 

 occurs; other places are almost without tides; while 

 at still other points four separate and distinct 

 periods of high water occur in every twenty-four 

 hours and fifty-one minutes. 



The tides are caused by the attraction of the sun 

 and moon. Since the moon is so much closer than the 

 sun, the tides produced by it are about two and a 

 quarter times greater than those produced by the 

 sun. When the sun and moon act simultaneously, the 

 tidal wave is higher than usual. Such tides are called 

 spring tides. They occur twice during each revolu- 



