162 THE WATERS OF THE EARTH 



was once an island detached from the coast of Spain. Shift- 

 ing sand bars, especially if covered with water, are exceed- 

 ingly dangerous to vessels, and coasts where these are abun- 

 dant need especial protection by lighthouses and life-saving 

 stations. The greatest Mediterranean port of France during 

 the thirteenth century, Aigues-Mortes, has been closed in 

 by sand bars so that there is no longer access to the sea and 

 only the relics of the former great city now exist. Thus 

 have the moving sea-sands overthrown the plans of men. 



Ocean Currents. The ocean is a region of never-ceasing 

 motion. At considerable depths its motion is very slow, 

 but near the surface, where the prevailing winds can affect 

 it, the movement is considerable. Circulating around each 

 ocean there is a continuous drift of surface water extending 

 to a depth of from 300 to 600 feet and varying in rate from 

 a few miles up to fifty or more miles a day. In fact these 

 rotating currents are the chief natural basis for the divi- 

 sion of the oceanic area into six oceans, as our geographies 

 generally divide them. 



These currents circulate in the northern hemisphere in 

 the direction in which the hands of a watch move and in 

 the .southern hemisphere in the opposite direction. In 

 the centers of these rotating areas the water is nearly motion- 

 less and here are often found great masses of floating sea- 

 weed filled with a great variety of small animals. These 

 accumulations of seaweed are called sargasso seas. 



The temperature of winds blowing from the sea is modi- 

 fied by these currents and greatly affects the habitability 

 of the earth for man. The editor of the National Geographic 

 Magazine makes the striking statement that " the Gulf 

 Stream carries enough heat toward Europe every twenty- 



