14 TIDAL EVOLUTION. 



indeed corresponds to the letter with what tidal evolution tells 

 us was the past conditions of our own earth. 



Then again observe the comprehensive fact that both 

 Mercury and Venus, the two planets nearest to the sun, 

 always, according to the most reliable observations, turn the 

 same face toward the sun. Tidal friction has, as it were, 

 arrested the axial revolutions of both of these planets, and 

 forced them to forever pay obeisance to their primary. 



Leaving the so'ar system and passing outward to the 

 siderial we soon find further corroboration of our theory. 

 We have seen this evening that one of the effects of tidal 

 friction is that the two attracting bodies keep pushing further 

 and further away from each other. In the case of the earth- 

 moon system the pushing is pretty well one-sided on account 

 of the great difference in size of the two bodies. But suppose 

 that both bodies were of about the same mass. What would 

 be the result? This is what the consequence would be. The 

 earth would, push the moon and the moon the earth about an 

 equal amount, and this would in its turn give rise to exceed- 

 ingly elongated elliptical orbits. Now one of the most 

 remarkable facts connected with nearly all the binary systems 

 of double stars far away in the bosom of unmeasurable space 

 is their exceedingly eccentric elliptical orbits, caused undoubt- 

 edly by mutual tidal action of the components of the systems. 

 My authority for this statement is Herr T. J. J. Lee of Berlin. 



There is another collateral consequence which must follow 

 from a combination of tidal action and gravitation on a primi- 

 tive plastic planet. During the course of evolution the earth's 

 mass must have suffered a screwing motion so that the polar 

 regions have traveled a little from west to east, relative to the 

 equator. Prof. G. A. Darwin of Oxford thinks that this fur- 

 nishes a possible explanation of the north and south trend of 

 our great continents. 



Tidal evolution has indeed furnished explanations for 

 many important questions about the remote past and dim, 

 distant future of the bodies of our solar system. It has per- 

 haps helped us along one more step in our painstaking search 

 for truth and groping for exact knowledge. This is, however, 



