THE TIDES 



causes (lie waters of the earth to assume an ellipsoidal form. 

 The water rises and falls according to the attraction of these 

 l\vo bodies, and since the sun and moon appear perpendicular 

 above a given point on the earth's surface at different times, 

 it follows that each tide ellipsoid is at different places at 

 different times. Each point on' the ocean is acted upon by two 

 forces, the sun and the moon, each tending to produce its own 

 tide-ellipsoid. But since both the influence of the sun and the 

 moon is felt simultaneously, it follows that the actual tide- 

 ellipsoid present at any point is a combination of the tide- 

 ellipsoid formed by the action of the sun added to that formed 



FIG. IQE. DISTRIBUTION OF TIDE-PRODUCING FORCE ON 

 THE EARTH'S SURFACE. 



by the action of the moon. Since the positions of the sun and 

 the moon only alter slowly, it follows that the resulting tide- 

 ellipsoid only alters slowly. There are two extreme positions. 

 One is when the earth, sun, and moon are all in the same 

 straight line (full moon and new moon). Here the axes of the 

 tide-ellipsoids produced by both sun and moon are in the same 

 straight line and can be added together. This produces the 

 high-tides known as springs, and the sun and moon are either 

 in opposition or conjunction (Syzygy). The second extreme 

 case occurs when the sun and moon are in quadrature i.e., at 

 right angles to one another, the moon now being in the first or 



