12 TIDAL EVOLUTION. 



Prof. G. H. Darwin of Oxford has calculated that this 

 state of affairs of the earth-moon system could not have 

 existed less than 50,000,000 years ago. 



Studying the theory of tidal evolution of the past, we 

 therefore find that it insensibly leads us to a belief that the 

 moon was once a part of this earth, and that from a prepon- 

 derance of the centrifugal force it became in some way or 

 other fractured off and left behind* This, of course, must 

 have taken place when the matter of the system was still in a 

 plastic or molten, or perhaps even in a gaseous state. 



But plainly this is onlyan illustration of L,a Place's cel- 

 ebrated nebular hypothesis that all the ponderable material 

 now constituting the various bodies of the solar system once 

 have existed as a rarefied mass of gaseous matter a nebula 

 and that as this nebula cooled by radiating its heat into space, it 

 contracted and was gradually shaped into its present form. 



In the future the moon will continue to retard the earth 

 in her diurnal motion and to increase and separate herselt 

 more and more from her primary. In the same way that the 

 earth has caused our satellite to complete one revolution on 

 her axis in the same time as is required for her lunation 

 (thereby forcing her to always exhibit the same face toward 

 the earth) , in exactly the same way the moon will compel our 

 earth to slow down the time of the diurnal motion until it 

 corresponds exactly to the moon's periodic time. It has been 

 computed that the length of the month as well as the length 

 of the day at that important epoch will be about 54 of our 

 days or about 1400 hours. 



We will then have what is known as a stable dynamical 

 equilibrium, with the earth and the moon always presenting 

 the same faces toward each other, and this state of affairs 

 would last forever, providing that no extraneous forces were 

 brought to bear on the system. 



But as every particle of matter attracts every other par- 

 ticle in the universe, it follows that the sun also, must, 

 in the same way as the moon, raise tides on the earth. So 

 the sun does, too, but as the tide generating power of attraction, 

 as far as distance goes, acts on the inverse square, we have so 



