166 THE WATERS OF THE EARTH 



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 leav- 

 ing or making port changes each day with the time of high 

 tide. A striking example of this is the port of Antwerp. 



The tidal currents are also continually changing the water 

 in bays and harbors and thus keeping them from becoming 

 stagnant and foul. They also bring food to many forms of 

 inshore life which have but little or no power of movement, 

 such, as clams and other shellfish. The ebb of the tide 

 exposes some of these and gives man a chance to acquire 

 them readily for food. 



Man and the Ocean. At first thought it would seem 

 better for the life of the world if the proportion of land and 

 water were reversed. Yet when we consider that almost 

 barren wastes constitute many continental interiors and that 

 plenty of rainfall is necessary to make land habitable, 

 the utility of the great water surfaces becomes apparent. 

 From the evaporation of the ocean surface comes nearly 

 all the water which supplies man, land animals, and plants. 



It is not only true that all streams eventually run to the 

 sea but it is also true that all their water comes from the 

 sea.- Other things being equal, the smaller the surface for 

 evaporation the less the water supplied to the land. Be- 

 sides supplying the land with water, the ocean has a great 

 effect on its climate. 



The animals of the sea also furnish food for thousands. 

 The value of the world's fishery products is nearly one half 

 billion dollars a year. A large part of the earth's population 

 is now, and always has been, located not far from the shore 

 of the ocean. 



