I 12 



A TEXTBOOK OF OCEANOGRAPHY 



The Equilibrium Theory. — Let us consider the influence 

 of the Sun and moon on the earth's surface. 



For simplicity, take the sun alone in the first place. Let 

 S be the sun, and E the earth. If the attraction of the sun 

 were not présent the earth would move in a straight line from 

 E to A'; but as a matter of fact the earth actually moves 



FiG. 19A. — Equilibrium Theory of the Tides. 



from E to X'. The attraction of the sun at any point is 

 inversely proportional to the square of its distance from the 

 point, so that particles on the side of the earth nearest the sun 

 are more attracted than those on the far side. Assume now 

 that the surface, of the earth is entirely covered with water, 

 then it foUows that this universal océan would take an 

 ellipsoïdal form. The waters would be heaped up most at the 



FiG. 19B.— Equilibrium Theory of the Tides. 



points X and Y, and would be flattest at PP' (AX' being the 

 earth's axis). 



Every point on this imaginary universal océan would rotate 

 once in twenty-four hours round the axis AX', and since the 

 sun's gravitational attraction on the line XY remains constant, 

 it follows that once in every twenty-four hours every point in 

 the océan comes under the influence of the nearer protubérance 



