256 W. B. Taylor — Crumpling of the Earth's Crust. 



about 11/ per century for the amount of acceleration of the 

 moon's mean motion arising from a secular change of the 

 eccentricit} 7 of the earth's orbit was supposed to be correct ; 

 but since that time it is known that Adams and Delaunay, and 

 also quite recently, Mr. Cayley, have all obtained less than 6" 

 for the amount of secular acceleration per century. This 

 determination, compared with the most reliable determination 

 of the acceleration deduced from the discussion of the recorded 

 observations of ancient eclipses, leaves about 6" to be 

 accounted for by tidal action or some other unknown cause." 

 He then proceeds to state that Mayer's hypothesis of a one- 

 and-a-quarter hour retardation, and his own of a two-hour 

 retardation of the tidal wave, " are both much too great to be 

 reasonable hypotheses, or to be necessary to account for the 

 balance of the moon's secular acceleration over and above the 

 late determination.'" And he concludes " that tidal action is 

 adequate to account for 6 7/ of secular acceleration upon a very 

 reasonable and probable hypothesis with regard to the magni- 

 tude and retardation of the tidal wave by friction after making 

 due allowance for the effect of a probable contraction of the 

 earth's volume."* 



In 1867, Prof. William Thomson, discussing the same subject 

 says: "The tides must tend to diminish the angular velocity 

 of the earth's rotation, and this tendency is not counterbalanced 

 to more than a very minute degree by the tendency to accele- 

 ration which results from secular cooling and shrinkage."! 



This brief historic summary has been introduced to empha- 

 size the strengthening 3onviction among physicists of the 

 necessary variability of that standard unit of time, which to 

 the astronomers had seemed the most permanent constant of 

 our solar system. And these references serve to establish 

 beyond a reasonable doubt the remarkable fact that our present 

 day is considerably longer than the day of early geologic 

 times. For nearly two centuries has the moon been seen in 

 advance of its periodic position — to the small amount (as esti- 

 mated from ancient eclipses), of about ten or eleven seconds of 

 angle per century. Of this relative displacement about one- 

 half is now known to be due to the actual falling behind of 

 the terrestrial observer, and the other half to Laplace's great 

 discovery — the influence of the diminishing eccentricity of the 

 telluric orbit.;}: 



Mr. George H. Darwin (as is well known) has in recent years 

 been engaged in elaborate investigations of this subject, and he 

 has drawn some important consequences from his discussion. 



* Proceedings Am. Acad. Arts and Sci., Dec. 13, 1864, vol. vi, pp. 379-383. 

 f Thomson and Tait's Natural Philosophy, 1867, vol. i, Sec. 830, p. 686. 

 \ An influence which must reach its inferior limit in about one precession 

 period. 



