Dr. J. E. Mayer on Celestial Dijnamics. 407 



spite of this, the motion of the ocean is on the whole in an op* 

 posite direction. 



Theory and experience thus agree in the result that the in- 

 fluence of the moon on the rotating earth causes a motion of 

 translation from east to west in both atmosphere and ocean. 

 This motion must continually dimmish the rotatory effect of the 

 earth, for want of an opposite and compensating influence. 



The continual pressure of the tidal wave against the axial 

 rotation of the earth may also be deduced from statical laws. 



The gravitation of the moon affects without exception all 

 parts of the globe. Let the earth be divided by the plane of 

 the meridian in which the moon happens to be into two hemi- 

 spheres, one to the east, the other to the west of this meridian. 

 It is clear that the moon, by its attraction of the eastern hemi- 

 sphere, tends to retard the motion of the earth, and by its 

 attraction of the western hemisphere, to accelerate the same 

 rotation. 



Under certain conditions both these tendencies compensate 

 each other, and then the action of the moon on the earth's 

 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 symmetrical 

 sphere on account of the three following causes : — (1) the flat- 

 tening 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 comprehend 

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

 to be a perfectly symmetrical sphere uniformly surrounded 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 segments at- 

 tached to the opposite sides of the latter, and representing the 

 elevations of the tidal wave. The attraction of the moon on the 

 small central sphere does not change the rotation, and we have 

 therefore only to consider the influence of this attraction on the 

 two tidal elevations. The upper elevation 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 at- 



2E2 



