16 THE STORY OF LTFE IN THE SEAS. 



four hours the water of the sea rises and falls. 

 This movement is due to the attracting influences 

 of the sun and moon, and is, as is well known, 

 greater when the moon is full and when it is new 

 than at the intermediate times. If the distribu- 

 tion of land and water on the surface of our globe 

 were different, and a free waterway occurred 

 round the world, right in the Equatorial band we 

 should probably find a double tidal-wave rush- 

 ing round the earth every twenty-four hours. 

 As it is, however, the great tidal- wave is checked 

 by the continents, and as it approaches the coasts 

 is retarded and diminished in force. In Archi- 

 pelagoes and along broken coast lines the tidal- 

 waves produce true surface currents, which fre- 

 quently run with great rapidity and exert con- 

 siderable corroding action upon the rocks. In 

 many estuaries and bays the tide rushes in with 

 such force that the water is heaped up to a great 

 height against the land. At the entrance to the 

 Bay of Fundy, for example, the rise at spring- 

 tides is no less than 70 feet, and at the Cardiff 

 docks the difference of level between high and 

 low spring-tides is 42 feet. The tumultuous ebb 

 and flow of such masses of water along the coast 

 is fatal to some forms of animal life and favour- 

 able to others, and so to some extent it modifies 

 the character of the Fauna. 



In addition to the surface currents of the 

 coast, produced by the tidal-waves, there are 

 also the true ocean-currents, which must be 

 briefly considered. They are caused by the 

 winds which blow constantly in a definite direc- 

 tion across the oceans. The prevailing winds 



