ADDRESS-SECTION A, B.A. 1876. 271 



According to this rate of retardation, if uniform, the 

 earth at the end of a century would, as a timekeeper, 

 be found 22 seconds behind a perfect clock, rated 

 and set to agree with her at the beginning of the 

 century. Newcomb's subsequent investigations in 

 the lunar theory have on the whole tended to confirm 

 this result ; but they have also brought to light some 

 remarkable apparent irregularities in the moon's 

 motion, which, if real, refuse to be accounted for 

 by the gravitational theory without the influence of 

 some unseen body or bodies passing near enough 

 to the moon to influence her mean motion. This 

 hypothesis Newcomb considers not so probable as 

 that the apparent irregularities of the moon are 

 not real, and are to be accounted for by irregu- 

 larities in the earth's rotational velocity. If this 

 is the true explanation, it seems that the earth was 

 going slow from 1850 to 1862, so much as to 

 have got behind by 7 seconds in these 12 years, 

 and then to have begun going faster again so as 

 to gain 8 seconds from 1862 to 1872. So great an 

 irregularity as this would require somewhat greater 

 changes of sea-level, but not many times greater 



