THE MOON. "79 



as unimportant, showed that the Moon has a 

 minute increase of speed for which the theory 

 of gravitation will not account, a conclusion 

 opposed by Plana, Hansen, and Pontecoulant, 

 but fully confirmed by Delaunay. Delaunay 

 suggested in 1865 that the minute apparent 

 increase was due to the retardation of the 

 Earth's rotation by tidal friction. This brings 

 us to the subject of celestial evolution, which 

 is discussed in another chapter. 



