TERRESTRIAL MAGNETISM, ETC. 317 



gradually 1 diminishes to zero, and then increases 

 gradually in the contrary direction. The object to 

 be attained in applying it to the binnacle is that 

 with this gradual change of its magnetism, it shall 

 always as exactly as possible counterbalance the 

 changing part of the force on the compass, due to 

 the part of the ship's magnetization which changes 

 with the gradual change of the vertical component 

 of the terrestrial magnetic force. If this changing 

 part of the ship's disturbing force on the compass 

 is a pull aft in the northern magnetic hemisphere, 

 and a pull forward in the southern magnetic 

 hemisphere, the Flinders bar must be on the 

 forward side of the binnacle. On the other hand, 

 if the regularly changing part of the ship's force be 

 a pull forward in the northern hemisphere, and aft 

 in the southern hemisphere, the Flinders bar must 

 be on the after side of the binnacle. The former 

 is the most frequent case for the chief navigating 

 standard compass and for the steering compass of 



1 The change of polarity in vertical bars in the ship, which takes 

 place in crossing the magnetic equator, has sometimes been falsely 

 supposed to be abrupt, and mistakes in respect to compass courses 

 have been made in consequence. 



