Variation Phenomenon of Terrestrial Magnetism. 503 



tial motion are equivalent to 16 years. As, according to 

 Wilde, it would take one complete differential revolution of 

 the inner sphere to produce a complete cycle of secular change, 



the period of the latter would be 360 X -77- or 960 years and the 



6 



annual rate of retardation of the inner sphere — — or 22*5 



960 

 minutes of arc. That is, in the time that the terrestrial or 

 electro-magnetic shell has made 960x365J rotations, the 

 vaprous sphere has made one less. Wilde carefully points out 





1! ! 







■ 







« 



o- io' if' 1 



0' IS° 







»' \ 



f 4 





/ 10 



f" 



f 



f , 



f 















S T 

















wJ 







_ 4 







*r 



1ST 









SO 















































' 





so 



1300 



















































So 

 1900 





















( 













I 



s 



y 















SO 



















































so 



2/00 



















































so 



270,1 













ON 



nor- 











CA 



>E 



F G 



;00 



HOPE 







'- 



ST 



HE 



Lf| 



i* 



















CB5 



£R 



ATION 











1-:* 



ONE 



rAO'Dr- 



















here that the " westerly march of the needle on the magnetic 

 globes as shown in Table II (represented by the dotted curve 

 on diagram A), exhibits similar relations in the period and rate 

 as those in Table I," (represented by the full curve) ! He 

 evidently forgot that he had adjusted his mechanism in the 

 first place, so that the maximum declination of 24° 38' at 

 London would be obtained, hence he only got out wha the put 

 in. London, then, we shall have to put aside and count 

 neither for nor against him. 



Before grinding out the next stations, the mechanism under- 

 goes a modification. As a first attempt to approximate to 

 the complex magnetic condition of the earth, Wilde lines the 

 interior of the outer globe under the land areas with sheet-iron 

 to render them more magnetic than the ocean areas. This 

 proves a dead failure. He next reverses his conditions and 

 puts sheet iron below the ocean areas, when, behold, a most 

 remarkable change takes place and most interesting correspond- 

 ences with facts, as exhibited in the present distribution of 

 terrestrial magnetism, are obtained ! But as these properly 

 refer to the distribution of terrestrial magnetism and not to 



Am. Jour Sol— -Thikd Series, Vol. XLIII, No 258.— June, 1892. 

 33 



