TIDES AND THE RIGIDITY OF THE EARTH 3 



always presents the same face to the Earth. Tidal friction in 

 the Earth-Moon system can also cause the Moon to recede 

 from the Earth and it is possible that the Moon was once much 

 nearer to the Earth than it is now, even possible that it was 

 once part of the Earth. The theory of the effects which tidal 

 friction in such a system as that of the Earth and Moon— a 

 moderate-sized planet accompanied by an exceptionally large 

 satellite and revolving around the Sun — were traced in a 

 masterly manner by Sir G. H. Darwin in a series of memoirs 

 published some thirty years ago. The theory was necessarily 

 coloured to some extent by the then prevalent scientific ideas 

 concerning cosmogony, ideas derived mainly from Laplace's 

 nebular hypothesis. A perfectly natural chain of reasoning 

 leads directly from the discussion of the theory of the tides, 

 through tidal friction, to the most speculative regions of thought 

 as to the origin and evolution of planetary and stellar systems. 



Evolution of Planetary Systems 



Until recently Laplace's hypothesis held the field ; though 

 the authors of some modern theories might demur to such a 

 description, it still seems fair to describe all the more recent 

 hypotheses as modifications of that propounded by Laplace. 

 Darwin himself broke away somewhat from the Laplacian 

 doctrine when he suggested that the Moon became detached 

 from the Earth as a single satellite and not as a ring. J. H. 

 Jeans broke away still more when he suggested that gravita- 

 tional instability or the tendency of gravitating matter to 

 concentrate about local nuclei, rather than increased speed of 

 rotation due to cooling, might have been the cause of the 

 disintegration of the primitive nebula into detached masses. 

 But the modern revival of interest in the nebular hypothesis is 

 largely due to the criticisms levelled against it by T. C. 

 Chamberlin and F. R. Moulton and the propounding by them of 

 a view put forward as alternative and named the " planetesimal" 

 hypothesis. In this view the solar system is supposed to have 

 been developed from a spiral nebula, a type of celestial object 

 with which modern telescopes have made us familiar, consisting 

 of a central condensation from opposite parts of which there 

 emanate a pair of spiral arms. Such an object is supposed 

 to have originated from a single star through enormous tidal 



