— 105 — 



« It follows that when the magnetic pole was at G, a line of no declinata passed 

 throngh London, travelling west, abont the end of the 15* century. There is of 

 conrse no record of such for London; bnt we do know that in 1492 Columbus found 

 it east ofthe Azores. What has become of this line of no declination I shall shew 



when I treat fully of such lines. 



«It follows from the figure that the needle at S 4 Petersburg would be nearly 

 stationary from 1722 to 1774, as the magnetic pole from that place would appear to 

 be moving in almost a straight line. Why there is such a small amountof westerly 

 declination at ^Petersburg I shall shew when I treat of the declination at that place 



« It follows, if this theory is correct, that the magnetic pole must have been not 

 far from 90° west longitude when the needle had its greatest western declination at 

 London in 1816. Now Sir James Ross in 1830 found the magnetic pole in 96" 43 

 west lono-itude. The needle in London in 1830 pointed to about 96» west longi-ude; 

 the pokwas discovered to be in the same longitude. Supposing the pole to have 

 travelled at the same rate since, it ought to be now in about 115» west longitude 

 In confirmationofthis I may say that a needle at London pointing to 114 45 west 

 longitude would have 18° 50' westerly declination. This is the declination of the 

 needle at London at the present time. , „ , 



« It may then be reasonably concluded that the magnetic pole will now be found 

 in latitude 72° 15' N and longitude 114° 45' W, or in the middle of Pnnce Al- 



bert Land. „ , n ., . 



« It will be seen from the figure that while the needle passes tornato G it is 



retracing values it had when it moved from N to C If therefore we find the po- 

 sitions of the needle the same number of years before and after 1816, they ought 

 not to differ much. 



« Take 38 years before and after 1816: 



1816 24- 17' 1816 24° 17' 



1778 22° 11' 185 4 22° 0° 



~38 2 T_ 6' ~~38 2° 17' 



These differences 2°6 r and 2° 17' agree very well. Or suppose we take a much lon- 



ger period, say 60 years: 



1816 24° 17' 1816 24° 17' 



1756 18° 50' 1876 19° 8' 



~~ 60 5^27T 60 5° 9' 



These differences 5° 27' and 5° 9' are very dose for such a long period as 120 years, 

 60 before and 60 after 1816 ('). 



« From an eiamination of the movement of the needle along NO, it would appear 

 that about 1892 the western declination will be 16° 10', that it will decrease slowly 

 till about 1905, will increase slightly for the next 20 years, and after 1925 wiH 

 begin to move along rapidly, reaching the point G and the declination becoming 0 

 about 1990. Continuing its conrse eastward, about the year 2072 it will arnve at 



(i) Compare this with the difference 9° 50' between 1700 and 1 



