DOUBLE STARS 143 



are in such admirable harmony with the leading features 

 of the Solar System, has caused it to be generally regarded 

 as representing, at any rate approximately, the manner in 

 which the planets came into existence. 



The phenomena of double stars suggest, however, that 

 another mode of development is possible from a nebula, 

 and in its attack upon this problem it seems possible that 

 mathematical science has been more successful. It is true 

 that the solution of the problem is still far from complete', 

 and that it cannot be regarded as having passed beyond 

 the limits of speculation. It rests, however, upon two 

 independent researches, that, whatever application they 

 may have to double stars, are in themselves of very great 

 interest. One of these, starting from a study of the 

 mutual relations of the movements of the Earth and 

 Moon, is due to Professor George Darwin ; the other, 

 which is a masterly study of the changes experienced by 

 certain rotating masses of fluid, has been developed by 

 M. Poincare". We shall first sketch very briefly the 

 leading points of Darwin's investigation of the past 

 history of the Moon. 



From the fact that the intensity of the Moon's attrac- 

 tion is greater upon the parts of the Earth that are 

 nearer to it than upon the parts that are more remote, 

 there arises a tendency for the Earth to become stretched 

 along the diameter that is at any particular instant directed 

 towards the Moon 1 . If the Earth were fluid, it would 

 yield to this tendency, but, as it is in the main solid, it is 

 unable to do so. The waters upon its surface are how- 

 ever free, and they consequently flow, continually tending 

 to accumulate in two high tides, one immediately under 



1 The stretching tendency is accentuated by another cause, the con- 

 vergence of the attraction exercised upon different parts of the Earth 

 by the Moon towards the Moon's centre. 



