THE TIDES. 147 



arrow in the diagram (Fig. 19). The return current 

 in the ocean, however, is not always an under 

 current, such as I have indicated in the diagram, 

 but may sometimes be a lateral current. Thus a 

 gale of wind blowing over ten degrees of latitude 

 will cause a drag of water at the surface, but the 

 return current may be not an under current but a 

 current on one side or the other of the area affected 

 by the wind. Suppose, for instance, in the 

 Mediterranean there is a strong east wind blowing 

 along the African coast, the result will be a current 

 from east to wes f along that coast, and a return 

 current along the northern coasts of the 

 Mediterranean. 



The rise and fall of the water due to these 

 motions are almost inextricably mixed up with 

 the true tidal rise and fall. 



There is another rise and fall, also connected 

 with the heating effect of the sun, that I do not 

 call a true tide, and that is a rise and fall due to 

 change of atmospheric pressure. When the 

 barometer is high over a large area of ocean, then, 

 there and in neighbouring places, the tendency to 



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