THE AUTHOR 's THEORY OF THE TIDES 175 



What causes this phenomenon! is naturally the next 

 question. I attribute it to the same causes that Newtou 

 erroneously employed to explain precession; only in- 

 stead of referring the solar and lunar attractions spe- 

 cifically to the equatorial ring, I prefer to refer them to 

 the more general entity of the earth's center of gravity. 

 I am logically privileged to make use of these causes, be- 

 cause I have freed them by employing others to explain 

 the phenomenon of precession. 



The earth having been described already in these 

 pages as a body seeking its lowest center of gravity with 

 reference to the sum of the stellar attractions, it follows 

 that that center is eccentrically located with respect to 

 her center of figure; that is to say, the two do not coin- 

 cide, but the c. g. is slightly north of the c. f . In this 

 condition we have a repetition of the basis upon which I 

 predicated the explanation of the moon's longitudinal li- 

 bration. (See, also, Chap. XIII.) The distinction be- 

 tween that case and this, however, lies partly in the 

 fact that the earth is supplied with mobile oceans, where- 

 as the moon is solid clear through ; and parti} 7 in the dis- 

 tances, sizes, configurations, and relative motions of the 

 attracting bodies. The principle of purchase, or lever- 

 age, accordingly again comes into play, necessitating ap- 

 plication of the rule of inverse cubes in passing upon the 

 relative perturbations of the sun and moon our old ratio 

 of 1 to 2.25. That such a complicated combination of 

 dynamical factors as this should produce diurnal oscil- 

 lations of the earth seems to me self-evident, and I shall 

 therefore not labor the point. Nor do I think it neces- 

 sary to enlarge on the presence of the annual term found 

 by Dr. Chandler's analysis. The 430-days term, how- 

 ever, demands elucidation: 



W-ere the sun and moon always in alignment with the 

 earth, and did they not move from their places, the si- 

 dereal day would be the same as the solar day and the 

 " lunar day" (if we may permit ourselves this last ex- 

 pression for the sake of simplicity). In that case, there 

 would be no fluctuation of latitude at all. But the moon 



