96 



-o 



Let F p he the polar axis of the earth, and A l> n B the ocean, 

 or water spheroid. 



Let us suppose that the moon M is in the plane of the equator, 

 and that O is a fixed point on the earth. 



When the moon is on the meridian of Q, as in the figure^ the 

 depth of the water is Q F, and it is " high water." 



Owing to the revolution ot the earth, Q is carried eastward, and 

 passes under thinner portions of the water spheroid, till six hours 

 after high water, when it reaches the thinnest part of the shell ; it is 

 then " low water ;" the depth then again begins to increase, till twelve 

 hours after high water, when B is at q, and the depth qf is equal to 

 Q F, and it is again high water. The changes during the next twelve 

 hours are precisely the same as those we have just traced. Thus, in 

 the course of a day of twenty-four hours, there are two high waters 

 and two low waters ; and it is further easy to see that the state of the 

 tide is exactly the same at intervals of 12 hours. These are tenned 

 the '■'■ sevii diurnal tides V 



/L^ 



Next, suppose that the moon is not in the plane of the equator, so 

 that the earth's axis, F/>, is inclined to the axis, A E a, of the water 



1 



