PARTIAL FAILURE OF COMPASS-CORRECTIONS. 181 



the factor cos a is negative for iron which is astern 

 of the compass, it will be seen that ; though N may be 

 large in a merchant-ship, W"here the compass is .placed 

 very near the stern ; yet it will be small in a ship of war 

 where the compass is much nearer the center of the 

 ship. 



There is another serious cause of error. The ver- 

 tical stern-post and rudder-post become in fact, by 

 induction, vertical magnets : but the upper end which 

 acts on the compass is charged with blue magnetism in 

 the northern hemisphere, and red magnetism in the 

 southern hemisphere. This change, however, may be 

 neutralized by adopting a suggestion of Mr. Rundell, 

 that another vertical bar be fixed in the ship on the 

 opposite side of the compass. In ships of the Royal 

 Navy, the compass is too distant from the stern-post to 

 be sensibly affected. 



When due attention is given to these considerations, 

 and the ship is not very new, it has been shewn by the 

 writer, in the Phil. Trans., 1855, from examination of 

 the disturbances in several ships, that, with insignificant 

 alteration in the position of the magnets, a ship's 

 compass will be perfectly correct in all parts of the 

 world. 



81. Continuation of the history. Investigation of 

 the effects of the ship's heeling. 



In the Admiralty Manual of the Compass, and 

 in other papers, Mr. Archibald Smith has elaborately 

 discussed the forces acting on the compass. Those 



