DOUBLE STARS 147 



three and five hours in length. In this first, as in the 

 last condition, we see the Earth and Moon rotating as a 

 whole about their common centre of mass, each con- 

 tinually presenting the same face to the other. While, 

 however, in the first condition they are nearly in contact 

 and represent a passing phase, in the last they are far 

 apart and their condition is permanent 1 . It is possible to 

 show that the first stage could not have occurred less 

 than 50,000,000 years ago. 



By the application of the laws of mechanics to the phe- 

 nomena of the tides, it is thus possible to trace the history 

 of the Moon backward to a time when it revolved in 

 close proximity to the Earth's surface. At this point the 

 investigation indicates the beginning of tidal friction 

 between the two bodies, and beyond it the mathematical 

 expressions have no physical meaning. Speculation, 

 however, cannot rest satisfied without an attempt to 

 imagine a still earlier condition of the system, and there 

 is an obvious direction for it to take. It is certain that, 

 when rotating in a period of between three and five 

 hours, the Earth could not have been far removed from 

 a condition of instability, roughly resembling a grindstone 

 on the point of breaking into fragments through the 

 violence of its rotation. It is an obvious suggestion that 

 the Moon is a part of the Earth detached from its parent 

 by the rapidity of its rotation, and there can be little 

 doubt that it actually came into existence in this way. 



It is probable that, at the time of their proximity and 

 their probable separation, the Earth and the Moon were 

 both in a liquid, or even in a partially gaseous, condition. 



1 In this study of the Earth- Moon system, a lesser tidal effect caused 

 by the Sun has been ignored. The consideration of the solar tidal effect 

 would slightly affect the forms of some of the above statements, but 

 would not affect the general conclusions, so far as they are applicable to 

 the study of this lecture. 



L 2 



