— 106 — 



the point D, where it was in 1580, having nitide a complete doublé oscillation and 

 the pole a complete revolution in 492 years. 



« We know that in 1541 the needle had about 7° east longitude and was at K. 



prds 



Now from K round through N to H is about — of a circle. The magnetic pole 



o 



l rd 



has therefore to pass over HGK (about -— of the circle) before completing a re- 



o 



volution. 



« KNH has been traversed in 336 years; at the same rate HGK will be traversed 

 in about 168 years. Therefore the time of performing one revolution will be about 504 

 years, which confìrms what has been already shewn, that the magnetic pole makes one 

 revolution in about 500 years. 



No. 2. The dip of the magnetic needle of London since 1576. 



« Having shewn that the declination-needle in its secular movement, as observed 

 for the last 300 years, is always directed to a magnetic pole which' appears to re- 

 volve round the pole of the earth in about 500 years, I purpose strengthening this 

 conclusion by adducing the evidence afforded by the dip. This evidence will besides 

 shew in the clearest manner that the north end of the needle is (contrary to the 

 belief and teaching of the last 300 years), a north pole, not a south pole. 



« Let a piane cutting the earth be supposed to pass through V (fig. 1) the centre 

 of the magnetic polar curve and through M, or London, where the needle is sup- 

 posed to be, and on zY project the positions, u, x, s, of the magnetic pole in the 

 years 1576, 1833, and 1866, as indicated by the position of the needle. 



« Let the section resulting from this 

 k piane cutting the earth be represented 



by zM (fig. 2). Let e be the centre of the 

 earth, and er a radius at right angles 

 to the semi-axis, Ne. The positions u, 

 y, oo, s, z (fig. 1) of the needle will be 

 represented in fig. 2 by the same let- 

 tere. M is the position of London, and 

 ho is a dipping-needle there in a hori- 

 zontal position. Now, if the needle dips 

 to the magnetic pole (even supposing it 

 to be in the same position in which Sir 

 James Koss found it in the year 1833), 

 it ought to dip to the position x (figs. 1 

 and 2); but even then the angle of dip 

 would be only about 20°, whereas the 

 dip at that time was about 70°. Now 

 this angle 20° is the complement of the dip, and this is an . important clue to the 

 solution of the matter. 



