lO 



PHYSIOGRAPHY. 



[chap. 



then no longer exhibits this indifference to direction, but 

 ■when left free always takes up a definite position, one end 

 pointing in a northerly and the other in a southerly direc- 

 tion. This property is taken advantage of in constructing 

 the mariner's compass. About two hundred and twenty 

 years ago the compass pointed exactly north and south in 

 London ; but from the year 1660, or thereabouts, the end 

 which tends northwards, and is therefore commonly called 

 the north pole o{ the needle, began to turn a little to the 

 west ; this variation from true north continued until the 



MAGNETIC 

 NORTH 



TRUE NORTH 



Fig. 2. — Maenetic Declinalion. 



year 181 8, when it reached its greatest divergence, and 

 since that time it has been steadily creeping back. The 

 divergence of the position of the magnetic needle from 

 the true north-and south line is called its declination, or 

 by nautical men, its variation. In 1818 the declination 

 amounted to nearly 25°, and in the present year (1877) it 

 is 19° 3' in London ; that is to say, ,the end or pole of 

 the needle which turns in a northerly direction, instead of 

 pointing due north, points 19° 3' to the west of true 



