NA VI G A TION. 



the last 20 years, and no doubt it has been 

 decreasing during the 217 years which have 

 elapsed since the needle pointed due north. 



21. The fact brought out is, that the whole 

 system of terrestrial magnetism, with its poles 

 and lines of force, is travelling round the earth's 

 axis at the rate of once round, relatively to the 

 earth, in 960 years, backwards or the way of 

 the sun ; or, which amounts to the same, the 

 system of terrestrial magnetism lags behind the 

 earth's rotation at the rate of one turn less per 

 960 years. The north magnetic pole is about 

 20 from the true north pole. In 1659, the north 

 magnetic pole was between London and the true 

 north pole, and since that time it has travelled 

 82 westwards in a circle round the true pole, so 

 that it is now in about 82 of west longitude, 

 and still 20 from the true north pole. 



In the year 2139, it may be expected to be 

 again due north of London, but on the far side of 

 the true north pole in longitude 180, and so on. 



22. ORDINARY MARINER'S COMPASS, AND 

 AZIMUTH COMPASS. The mariner's compass is 



