148 



THE EVOLUTION OF 



It is interesting to notice that under these conditions 

 neither could have been spherical. Their rapid rotation 

 would of itself cause each to tend to a spheroidal form, 

 while a further modification would result from the 

 tendency of each to become elongated by tidal action 

 along the diameter directed to the other. Darwin has 

 determined the forms assumed by two equal masses of 

 fluid of uniform density throughout when rotating as a 

 whole and almost in contact, and a section of the pair 

 through their common axis of rotation is given in fig. 5. 



Axis of rotation. 



FIG. 5. 



The forms of the bodies are not described by any simple 

 geometrical figure. 



At a time in the remote past, the Earth and the Moon 

 were almost in contact and they were then revolving round 

 their centre of mass precisely as if they were connected 

 together by a rigid bar. From this it seems an almost 

 unavoidable inference that they were formed by the 

 division of a single mass, and that this mass was either 

 fluid or gaseous. It may appear to those who are unin- 

 itiated in the mysteries of the science of hydrodynamics 

 that it should be a simple matter to decide whether or 

 not such a division of a rotating fluid mass is in accordance 



