GRAVITATIONAL STABILITY OF THE EAKTH. 241 



partly on the ratio of the density of the ocean to the mean density of the planet, and 

 partly on the angular velocity. Owing to tidal friction, the angular velocity of the 

 Earth's rotation is heing gradually diminished. The effect of this is that lth the 

 ellipticity of the lithosphere and that of the geoid are being diminished, and the 

 difference of these ellipticities is also being diminished. If, therefore, the shape of 

 the lithosphere were continually adjusted to the instantaneous angular velocity, the 

 value of the coefficient e of 57 would diminish continually, and the adjustment 

 would involve a continually increasing deformation. Eventually the deformation 

 would be so great that the strength of the material would be too small to withstand 

 it, and local fractures would take place.* There is, therefore, a constant tendency 

 for the sea-level to rise in the polar regions and to fall in the equatorial regions, the 

 separation l>etween the regions of rising and falling sea-level l>eing marked by the 

 zero-lines of the zonal harmonic of the second degree, that is, by the parallels of 

 latitude, alxnit 35 N. and 35 S. This rise and fall would l>e checked at intervals by 

 subsidences, accompanied by series of earthquakes, in equatorial regions. 



The effects produced by diminution of the displacement of the centre of gravity, 

 and by changes in the ellipticity of the equator and in the obliquity of the principal 

 planes, appear to be of a different character from the effect of diminishing angular 

 velocity. The former would seem to l)e spasmodic and occasional, but always in the 

 same sense at the same place ; the latter would appear to consist of continuous 

 movements in the same sense, extending over long periods, which are followed by 

 comparatively short periods of spasmodic change in the opposite sense. 



These remarks are frankly speculative, and I am well aware that many causes 

 which have contributed to geological changes have l>een left out of sight. They are 

 put forward as tentative suggestions which, it is hoped, may prove to be of some 

 assistance in the solution of some of the still unsolved problems of geology. 



My best thanks are due to Professor W. J. SOLLAS and Dr. H. N. OICKSON for 

 much kind help in regard to geological and geographical questiona 



* According to a "Note" in 'Nature,' vol. 39 (1889), p. 613, this effect of diminishing speed of rotation 

 noted hy M. A. Bi.YTT. I have not seen the paper referred to in the " Note." 



VOL. CCVII. A. 2 I 



