340 PHYSIOGRAPHY 



their revolutions. The result is that neither falls toward the other. 

 But on the side of the earth nearest the moon the attraction is 

 stronger than at the center of the earth, and is greater than the 

 centrifugal tendency. The attraction of the moon therefore tends to 

 make the earth bulge up on the side nearest the moon. On the oppo- 

 site side of the earth the attraction is weaker than at the center, 

 and is less than the centrifugal force. Here, too, the earth tends to 

 bulge out. The solid part of the earth is so rigid that it does not 

 rise enough to be felt or seen. But the waters of the ocean move 







Fig. 266. Diagram to show the tendency of the moon to raise the water on 

 the side of the earth next to the moon, and on the opposite side at the 

 same time, forming two high tides. M = moon, and E = earth. 



easily, and rise a little, and the rise takes place on opposite sides 

 of the earth at the same time. This makes the high tides. Be- 

 tween the high tides the water sinks a little, making the low tides. 



If a weight is attached to a string and whirled, the weight 

 tends to fly away in a straight line. It is prevented from 

 doing so by the string which holds it in its circular path. The 

 tendency to fly away is what is called centrifugal force. 



If all the earth were covered with ocean water, its surface would 

 have two great tidal bulges, or waves (Fig. 266), at the same time. 

 The highest part of one would be a point directly under the moon, 

 and the highest point of the other would be opposite the first. 

 Each wave would cover half the earth, and the borders of the two 

 would meet in a great circle, where the surface of the water 

 would be lowest. The rotation of the earth makes the tides appear 

 to move about the earth. 



If the moon were not revolving about the earth, high tide at 

 any place should come every 12 hours. But the moon moves for- 

 ward in its orbit about the earth, so that it takes 24 hours and 52 

 minutes for a given place to have the same relation to the moon 



