THE TIDAL WAVE. 297 



rotation becomes zero. This happens when both hemispheres 

 are arranged in a certain manner symmetrically, or when no 

 parts of the earth can change their relative position ; in the 

 latter case a sort of symmetry is produced by the rotation. 



The form of the earth deviates from a perfectly symmet- 

 rical sphere on account of the three following causes : (1) 

 the flattening of the poles, (2) the mountains on the surface, 

 and (3) the tidal wave. The first two causes do not change 

 the velocity of the earth's axial rotation. In order to com- 

 prehend clearly the effect of the tidal wave, we shall imagine 

 the earth to be a perfectly symmetrical sphere uniformly sur- 

 rounded by water. The attraction of the sun and the moon 

 disturbs the equilibrium of this mass, and two flat mountains 

 of water are formed. The top of one of these is directed 

 towards the moon, and the summit of the other is turned 

 away from it. A straight line passing through the tops of 

 these two mountains is called the major axis of this earth- 

 spheroid. 



In this state the earth may be imagined to be divided 

 into three parts a smaller sphere, and two spherical seg- 

 ments attached to the opposite sides of the latter, and repre- 

 senting the elevations of the tidal wave. The attraction of 

 the moon on the small central sphere does not change the ro- 

 tation, and we have therefore only to consider the influence 

 of this attraction on the two tidal elevations. The upper ele- 

 vation or mountain, the one nearest the moon, is attracted 

 towards the west because its mass is principally situated to the 

 east of the moon, and the opposite mountain, which is to the 

 west of the moon, is attracted towards the east. The upper 

 tidal elevation is not only more powerfully attracted because 

 it is' nearer to the moon, but also because the angle under 

 which it is pulled aside is more favourable for lateral deflec- 

 tion than in the case of the opposite protuberance. The pres- 

 sure from east to west of the upper elevation preponderates 

 therefore over the pressure from west to east of the opposite 

 13* 



