THE TIDES 115 



causes the waters of the earth to assume an ellipsoïdal form. 

 The water rises and falls according to the attraction of thèse 

 two bodies, and since the sun and moon appear perpendicular 

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

 it follows that each tide ellipsoid is at différent places at 

 différent times. Each point on' the océan is acted upon by two 

 forces, the sun and thè 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 présent at any point is a combination of the tide- 

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



FiG. 19E.— Distribution of Tide-producixg 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 extrême positions. 

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

 straight line (full moon and new moon). Hère 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 extrême 

 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 



