2 7 o POPULAR LECTURES AND ADDRESSES. 



and may have gradually shifted through 10, 20, 30, 

 40, or more degrees without at any time any per- 

 ceptible sudden disturbance of either land or water. 

 Lastly, as to variations in the earth's rotational 

 period. You all no doubt know ho w, in 1853, Adams 

 discovered a correction to be needed in the theor- 

 etical calculation with which Laplace followed up 

 his brilliant discovery of the dynamical explanation 

 of an apparent acceleration of the moon's mean 

 motion shown by records of ancient eclipses, and 

 how he found that when his correction was applied 

 the dynamical theory of the moon's motion ac- 

 counted for only about half of the observed apparent 

 acceleration, and how Delaunay in 1866 verified 

 Adams's result and suggested that the explanation 

 may be a retardation of the earth's rotation by 

 tidal friction. The conclusion is that, since the 

 1 9th of March, 721 B.C., a day on which an eclipse 

 of the moon was seen in Babylon, commencing 

 " when one hour after her rising was fully passed," 

 the earth has lost rather more than n.oirV.Truir f ner 

 rotational velocity, or, as a timekeeper, is going 

 slower by i i ! seconds per annum now than then. 



