MAGNETISM. 403 



The declination from the meridian, and the vari- 

 ation of this, in different parts of the world, is 

 very uncertain, and actual trial is the only method 

 of ascertaining it. This circumstance forms a 

 great impediment to the improvement of naviga- 

 tion. Great pains have been taken by navigators, 

 to ascertain the declination in various parts of the 

 world, and such declinations have been marked in 

 maps, charts, books, &c. ; but still, on account of 

 the constant change to which this variation is 

 liable, these can only serve for a few years ; nor 

 has the law of this variation been yet discovered. 

 When the variation was first observed, the north 

 pole of the magnetic needle declined eastward of 

 the meridian of London; but it has since that 

 time been changing continually towards the west ; 

 so that in the year 1657, the magnetic needle 

 pointed due north and south. At present, it de- 

 clines about 24-§-° westward, and it seems to be still 

 advancing towards the west. 



Before volcanic eruptions and earthquakes, the 

 magnetic needle is often subject to very extraordi- 

 nary movements. 



It is also agitated before and after the appear- 

 ance of the aurora borealis. 



The variation of the compass was first observed 

 by Columbus, in his voyage in which he discovered 

 America : and his sailors were very much alarmed 

 in consequence of it ; thinking the compass would 

 fail in directing their route. 



Of the Magnetic Inclination, or Dip of the Needle. 



If a needle which is accurately balanced, and 

 suspended so as to turn freely in a vertical plane, 



d d 2 



