It must be borne in mind that the magnetic meridians in the 

 Plates only represent the present, and that these directions are 

 perpetually varying to either side of the true meridian, within an 

 arc of about 15° at the equator, and about 60° near the polar 

 circle. 



The direction of the magnetic needle in England about 250 

 years ago was 11° east of north, it is now about 25° west of 

 north ; it is constantly undergoing a slow oscillation. 



The following Table shows the change which has taken place 

 in London from the years 1576 to 1831*: — 



Years. 



Variations. 



Years. 



Variations. 





/ // 





o / // 



1576 



11 15 Easterly. 



1760 



19 30 O.Vesterlv. 



1580 



11 17 „ 



1774 



22 20 „ ' 



1622 



6 12 „ 



1778 



22 11 „ 



1634 



4 5 „ 



1790 



23 39 „ 



1657 \ 

 1662 J 



No Variation. 



1800 

 1806 



24 36 „ 

 24 8 „ 



1666 



34 Westerly. 



1813 



24 20 17 „ 



1670 



2 6 0,, 



1815 



27 27 18 ,, 



1672 



2 30 „ 



1816 



24 17 9 ,, 



1700 



9 40 „ 



1820 



24 11 7 „ 



1720 



13 „ 



1823 



24 9 40 „ 



1740 



16 10 „ 



1831 



24 „ 



Besides the progressive changes in the direction of the needles, 

 there are annual or periodical movements and daily oscillations 

 constantly taking place in all parts where magnetic observations 

 have been made. The dip of the needle, like the horizontal 

 variation, has different values in different parts of the globe ; on 

 an average, being nothing or horizontal near the equator, and 

 perpendicular to the horizon at the poles. Hence, if a magnetic 

 needle were to be balanced at the equator, it would be found to 

 decline from its horizon as we proceed from thence towards the 

 poles ; its average position forms a curve, called the s Magnetic 

 Curve/ from pole to pole, as shown in Plate IV. 



On looking at the general direction of the magnetic curves 

 and the magnetic meridians, we plainly observe that they tend 



* See Brewster's Treatise on Magnetism, 



