THE OCEAN 333 



down. Since the causes producing differences of level are always 

 in operation, movements due to these differences are always taking 

 place. 



Movements due to the wind. Winds produce temporary differ- 

 ences of level, as just noted, but they also affect the water in other 

 ways. They produce waves, and they drag along the surface of 

 the water over which they blow. In some cases, this movement is 

 fast enough so that it may be seen or felt. This movement is 

 sometimes called drift. 



Since winds are always blowing, the movements which they 

 cause are always taking place. When winds have a constant direc- 

 tion, as in the zone of trades, there is always movement of the sur- 

 face water in the same direction. A steady movement in one di- 

 rection will always cause a return movement, thus producing a 

 circulation of the sea-water. 



Movements due to the sun and moon. Bodies attract each 

 other in proportion to their masses, and inversely as the squares 

 of their distances. That is, a body which weighs twice as much as 

 another has twice the attractive force at the same distance. If one 

 of two bodies of the same mass (or weight) is twice as far from a 

 third body as the other is, their attractive forces on the third are 

 to each other as 1: 4. 



The side of the earth towards the moon is nearer the moon than 

 the center of the earth is, and so is attracted by the moon 

 more strongly than the center. The opposite side is attracted less 

 strongly than the center, and these differences of attraction disturb 

 the waters of the earth. The attraction of the sun produces similar 

 effects. The resulting movements of the sea are the tides. 



Movements due to occasional causes. The occasional causes 

 of movement (p. 332) sometimes make violent waves which last 

 but a short time. Illustrations of their nature and effects have 

 already been given in connection with earthquakes and volcanoes. 

 Faulting and landslides along shore, as well as earthquakes and 

 volcanoes, may cause movements of the water. 



