1 16 A TEXTBOOK OF OCEANOGRAPHY 



last quarter. In this case the tide-ellipsoid produced by 

 the moon has its maximum protuberance coincident with 

 the maximum depression of the tide-ellipsoid produced by the 

 sun. Consequently, the resulting height of the observed tide 

 is a minimum and we get the period of neap-tide. 



The Strength of the Tide-Producing Forces. 



Let the mass of the earth = i and that of the moon = M. 

 Then M = 1/81-45. The attractive force of the moon at the 

 earth's centre is proportional to M/r 2 , where r is the distance 

 between the centres of the earth and moon. At the point on 

 the earth where the line joining the earth and moon's centres 

 cuts the surface the attraction of the moon is somewhat greater, 

 being proportional to Af /(r-r 7 ) 2 , where r' is the radius of the 

 earth. 



The difference between these two attractive forces is that 

 which gives the ** pull" on the water particles at the earth's 

 surface and produces the protuberance at this point. This 

 difference is 



- i 



which is for all practical purposes 



r* 



The force with which the earth attracts a particle at its surface 

 is __ ? and the force which produces the tides is the fraction of 



this 



2Mr' i 



r 3 ' r' 2 r 3 8945000 



that is, the tide-producing force of the moon is about one- 

 nine-millionth of the force of gravity. 



