1877.] The Rev. S. H aught on on Physical Geology, 



513 



This retardation (according to Adams and Delaunay) amounts to one 



second in the length of the day during 100000 years ; hence we obtain 



-p 



the following equation to determine the value of — : — 



F 1-53x100000x365-25x86400 w 1 



X = nnnnnn^rv, aaaaaa X ~L=: 



O 7,000000x21,000000 2~86400 



Hence, finally, 



F 2 7,000000X21,000000 

 ; = -X 



C~tt 1-53 x 100000 x 365-25 x (86400) 2 ' 

 which gives 



F 1 



C 4457' 



"We have already defined 



(17) 



irFdu 



y=—r- 



But du is the daily change of angle in the position of the equatorial 

 axis of rotation, p, and is equal to 



du = 



2ir 



304-75' 



F 



1 





: 4457' 



c 



307 



A" 



= 306' 



Hence 



which gives 



4457x306x304-75' 

 1 



68,588 



"We are now in a condition to apply equation (12) to determine how 

 long it would take to destroy a wabble of 69 miles, if suddenly produced. 



Our astronomical instruments are now so perfect that an annual dis- 

 placement of 10 ft. in the pole could be detected. This would correspond 

 with a radius of 5 ft., instead of 69 miles, for our wabble. 



A , = ^ = 69x5280 = 72;864t 

 P» 5 



Hence 



^ = 1 ^i^=log e (72,864) x 68,588, 



y 



= 767,940 wabbles, 



=SS x767 > 94 °y ears > 



= 640,730 years. 



