REPORT ON THE MAGNETISM OF THE EARTH. Ill 



sidered that the earth acts upon a magnetized bai% and upon 

 iron, like a magnet, the directive power of the needle being due 

 to the action of magnetism of a contrary kind to that at the 

 end of the needle directed towards the pole of the earth. He 

 applied the term "pole" to the ends of the needle directed 

 towards the poles of the earth, according to the view he had 

 taken of terrestrial magnetism, designating the end pointing 

 towards the north, as the south pole of the needle, and that point- 

 ing towards the south, as its north pole*. It is to be regretted 

 that some English philosophers, guided by less correct views, 

 have since his time applied these terms in the reverse sense, 

 which occasionally introduces some ambiguity, though now they 

 are used in this country, as on the Continent, in the sense ori- 

 ginally given to them by Gilbert. 



In 1668 Bond published a Table of computed variations in 

 London, for every year, from that time to the year 1716 f. The 

 variations in this Table agree nearly with those afterwards ob- 

 served for about twenty- five years, beyond which time they 

 differ very widely ; and I only notice this Table as the first em- 

 pirical attempt at the solution of a problem which is, as yet, 

 unsolved. Bond afterwards proposed to account for the change 

 in the variation and dip of the needle by the motion of two 

 magnetic poles about the poles of the earth. He professed not 

 only to give the period of this motion, but to be able to point 

 out its cause, and even proposed to determine the longitude 

 by means of the dip J. He, however, did not make public either 

 his methods or his views ; but with regard to the longitude, it 

 is probable they were the same as those afterwards adopted by 

 Churchman. 



Halley considered that the direction of the needle at different 

 places on the earth's surface might be explained on the suppo- 

 sition that the earth had four magnetic poles §, and that the 

 change in the direction at the same place was due to the motion 

 of two of these poles about the axis of the earth, the other two 

 being fixed. He does not enter into any calculations to show 

 the accordance of the phaenomena with such an hypothesis, but 

 conjectures that the period of revolution of these poles is about 

 700 years li. 



Since this time, calculations have been made by various au- 

 thors, both on the hypothesis of two magnetic poles and on 

 that of four, with the view of comparing the results of these 



, * Gilbert, De Magnete, Sfc, lib. i. cap. iv. 



t Philosophical Transaction)!, 1668, vol. iii. p. 789. 



X Ibid. 1673, vol. viii. p. 6065. § Ibid. 1683, vol. xiii. p. 208. 



II /6irf. 1692, vol. xvii. p. 563. 



