38 POPULAR LECTURES AND ADDRESSES. 



that the moon's mean angular motion has been 

 accelerated somewhat relatively to the earth as time- 

 keeper ; and he estimated the amount of this accel- 

 eration to be 20 seconds of angular velocity per 

 century gained per century. 1 Laplace, accept- 

 ing this conclusion, attempted to explain it by 

 showing that the planets cause indirectly an ac- 

 celeration of the moon's angular velocity through 

 their influence in producing a secular diminution 

 of the eccentricity of the earth's orbit. The 

 principle is admitted, and to Laplace is attributed, 

 and must always be attributed, the very great 

 discovery of the cause of an apparent secular 

 acceleration of the moon's mean motion. He 

 calculated out the results of this discovery, and 

 they seemed to tally precisely with the supposition 

 that the earth's velocity of rotation had been con- 

 .! since 721 B.C. ;- but in 1853 our great 

 Kni;-lish physical astronomer, Adams, pointed out 



Physical Astronomy i p. 60. 



'-'-I..iplnix''s theory ^nvr him 2l"'7 pi-r century of nnpilnr VeJocit) 

 'Miry during tin- lust 2,500 yt-:n>, \vhii h 

 'tion by 8 per cent. 

 Grant's History of Physical Astronomy, p. 63. 



