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been governed ckiefly by those of a strong maguetic pole revolving round the pole 

 of the earth in about 500 years. I may bere state that in order to satisfactorily 

 explain ali magnetic phenomena four maguetic poles appear to be necessary. 



« Let MN be a compass-needle at London directed to the true north 0. In the 

 year 1660 that was its direction; in 1580 it was 11° 17' E; in 1816 it was 24° 17' W; 



«Along the are WAE mark offAE = ll° 17' 

 to the east of A, and AW = 24° 17' to the west 

 of A ; and let A be at the point which marks 

 true north direction. 



« Let lines ME, MA, MW be drawn and 

 produced to meet, in D, 0, and C, a concentric 

 are drawn through the north pole 0. 



« Describe a circle CGrD through the points 

 C and D ; MD and MC being tangents. 



« Erom E to W mark off ali the different 

 positions of the needle in its westward movement 

 from 1580 to 1816, and let the lines be con- 

 tinued to meet the circle along DNC ('). 



« Along the are PE mark off the positions 

 of the needle moving to the east since 1816, 

 and let the lines be continued to meet the circle 

 in CH. 



« Along ST mark off the position of the 

 needle moving east before 1580, and let the line be produced to meet the circle in K. 



« Now it has been observed that the needle moved very slowly about D and C, 

 the points of eastern and western declination, and not only so, but the movement 

 gradually decreased as it approached and gradually increased as it receded from the 

 points D and C. Such would be its movement if the needle be supposed to point 

 constantly to something moving along the circumference DNCH. 



« It follows that when that which direets the needle carne to the point N on the 

 circumference, the needle would point due north, and there would be a line of no 

 declination passing through London. This actually happened in the year 1660. 



« It follows that the greatest movement of the needle would be when it pointed 

 to V, the centre of the circle, and also before and after. Now during the thirty 

 years from 1670 to 1700 (the needle pointing to V about the year 1685), the ave- 

 rage rate of movement was 15' a year; a rate considerably greater than that of any 

 other period since observations have been made; indeed it was nearly doublé the ave- 

 rase rate since 1580, which is about 8'. 



« It follows that places to the east of London, as Kònigsberg and Copenhagen, 

 would have the line of no declination, which passed through London in 1660, pass 

 there about 1600 and 1620; and such was the case. 



(') Only the chief positions are given in the figure. 



and in 1877 it was 18° 57' W. 



