EDMUND HALLEY. 447 



years. But who actually discovered the secular variation 

 is not certainly known. Bond attributes the honor to 

 John Mair other contemporary authority to Gellibrand, 

 who at least has the preponderance of assent in his favor. 1 



The whole subject of compass variation, however, was 

 thoroughly studied by Dr. Edmund Halley, 2 a mathema- 

 tician and astronomer of great ability, who proposed the 

 odd theory to account for it, that the earth has four mag- 

 netical poles, two near each geographical pole, and that 

 the needle is governed by the pole to which it happens to 

 be nearest. Unfortunately, however, the observed changes ' 

 in the variation itself over certain periods of time inter- 

 fered so greatly with this doctrine that it became evident 

 to Halley that the notion of four fixed poles would not 

 meet the observed conditions. Thereupon he evolved a/- 

 still more striking supposition, to the effect that the earth 

 really consists of two concentric magnetic shells, each hav- 

 ing poles differently placed and not coincident with the 

 geographical poles. Then as the poles on the inner shell 

 "by a gradual and slow motion change their place in re- 

 spect to the external, we may give a reasonable account 

 of the four magnetic poles, as also of the changes in the 

 needle's variations." 



It is hard to believe that the imagination could exercise 

 such control in . the days of Newton. Yet the theory at- 

 tracted considerable attention and had even great vitality, 

 for in 1698, thirteen years after he had proposed it, Halley) 

 induced William III. to appoint him a captain in the Navy I 

 and give him command of a ship, in order to make long 

 voyages for the express purpose of establishing the truth 

 of his supposition. He made two voyages to various parts 



T Dr. Wallis (Phil. Trans., 1702, No. 278, 1106), says that "at about the 

 beginning of the reign of Charles I., Gellibrand caused the great concave 

 dial in the Privy Garden at Whitehall, which is still remaining, to be 

 erected in order to fix a true meridian line. 



2 Phil. Trans., No. 28, p. 525, 1667; No. 148, p. 208, 1683; No. 195, p. 

 563, 1692. 



