QUADRANTAL EFFECT OF INDUCED MAGNETISM. 137 



be found that the quality of the magnetism of the bar 

 is always the same as that of the dipping-needle. 



Now vary the experiment by holding the bar so that 

 its length is contained in the plane to which the dip- 

 direction is normal. Its disturbing power ceases en- 

 tirely : it has no sensible charge of magnetism. 



All these phenomena are exactly similar to those 

 described in Article 57, conceiving the earth's action to 

 be similar to that of a steel magnet : and the explana- 

 tion is the same as that in Article 58, that the attraction 

 of the earth's red magnetism draws towards itself the 

 blue magnetism which is in the particles of the iron : 

 and similarly for the attraction of the blue on the red. 



61. Effect of the terrestrially-induced magnetism in 

 a mass of soft iron which is carried round a compass, at 

 the same level as the compass, and with the same part of 

 the mass always directed to the compass-center. 



This case is one which theoretically deserves atten- 

 tion, and which in practical application is very impor- 

 tant, inasmuch as in iron-built and other ships it re- 

 presents the state of things where, partly from the iron 

 of the ship-construction and partly from the iron intro- 

 duced for corrective purposes, there is much iron ad- 

 mitting of induction from terrestrial magnetism, at 

 nearly the same elevation as the compass, and revolv- 

 ing round it as the ship swings round, always presenting 

 the same part to the compass. 



