MAGNETISM. 59 



dian were constant for the same place, or even if it varied according 

 to any discoverable law : since it would afford a ready mode of 

 determining the longitude of a place by a comparison of an astro* 

 nomical observation of its latitude with another of the magnitude of 

 the declination. And in some cases it may even now be applied to 

 this purpose, where we have a collection of late and numerous obser- 

 vations. Such observations have from time to time been arranged 

 in charts, furnished with lines indicating the magnitude of the 

 declination or variation at the places through which they pass, 

 beginning from the line of no variation, and proceeding on the 

 opposite sides of this line to show the magnitude of the variation 

 east or west. It is obvious that the intersection of a given parallel 

 of latitude, with the line showing the magnitude of the variation, 

 will indicate the precise situation of the place at which the observa- 

 tions have been made. 



The line of no variation passed in 167 5 through London, and in 

 1666 through Paris : its northern extremity appears to have moved 

 continually eastwards, and its southern parts westwards; and it now 

 passes through the middle of Asia. The opposite portion seems to 

 have moved more uniformly westwards ; it now runs from North 

 America to the middle of the South Atlantic. On the European 

 side of these lines, the declination is westerly ; on the South 

 American side, it is easterly. The variation in London has 

 been for several years a little more than 24. In the West In- 

 dies it changes but slowly ; for instance it was 5 near the island of 

 Barbadoes, from 1700 to 1756. 



The dip of the north pole of the needle in the neighbourhood of 

 London is 72. Hence the lower end of a bar standing upright, as 

 a poker, or a lamp-iron, becomes always a north pole, and a tem- 

 porary south pole of a piece of soft iron being uppermost, it is 

 $ omewhat more strongly attracted by the north pole of a magnet 

 placed over it, than by its south pole ; the distribution of the fluid 

 in the magnet itself being also a little more favourable to the attrac. 

 tion, while its north pole is downwards. It is obvious that the 

 magnetism of the northern magnetic pole of the earth must resemble 

 that of the south pole of a magnet, since it attracts the north pole ; 

 so that if we considered the nature of the distribution of the fluid 

 rather than its situation in the earth, we should call it a south pole. 

 Although it is impossible to find any places for two, or even for a 



