Variation Phenomenon of Terrestrial Magnetism. 507 



I shall not be able to point out all the interesting features 

 brought out by this diagram. Only such as will be a direct 

 refutation of Wilde's theory will be touched upon. 



(1.) The period. — The diagram at once shows not only how 

 far the periods fall short of 960 years, but also, which is the 

 important point, how greatly they differ among themselves. 



In the following table I have given some of the periods and 

 ranges/ This table alone will overthrow Wilde's theory. 

 From it will be seen that the secular variation period is not a 

 universal one, but is different for every station. So that if 



Period, 

 Station. years. Range. 



Paris*. 470 32° 



Christiania 420 27 



Cape Town 590 30 



St, Helena 582 31 



Ft. Albany 300 14 



St. Johns 347 18 



Eastport .. 288 8 



New York City 277 6 



Charleston 257 5'5 



New Orleans 257 6 



Rio de Janeiro 450 (?) 20 (?) 



the cause of the secular variation is the differential rotation of 

 an electro-dynamic substance within the earth's crust, then to 

 satisfy every point on the globe it must have every possible 

 rate of rotation, or every station must have its own electro- 

 dynamic sphere roll around in the inscrutable region below us ! 

 Is this not tantamount to Barlow's idea that the secular varia- 

 tion at every station could be explained by supposing that each 

 had its own magnetic pole describing an orbit about the rota- 

 tion pole ? But, says Sir John Herschell, if that is so, haven't 

 you renounced the idea of polarity altogether ? 



(2.) The diagram will show that there are stations where the 

 declination does not cross the zero line, but remains west 

 or east. At New York City, for example, in about 1933 we 

 shall have a full period and there is no question but what 

 the declination remains west. Charleston is mostly east, 

 but crosses the zero line. New Orleans, doubtless, remains 

 east during its whole period. Probably better illustrations of 

 places where the declination remains east will be found with 

 the lapse of time along the Pacific coast. And how unequal 

 the amounts of easterly and westerly declination are about the 

 zero line is abundantly shown by Paris and Christiania. 



* The period at London is estimated to be between 500 and 550 years, range 35°. 



