176 ON MAGNETISM. 



had been found, ascertaining the effective total hori- 

 zontal force acting on the compass needle; this total 

 force, which is really in the deviated direction of the 

 compass, was resolved into a~N. direction and an E. 

 direction, and gave the whole force to the N. and that 

 to the E.; subtracting the terrestrial force from the 

 former, the ship's disturbing forces to the N. and to 

 the E. were found. Similar operations were performed 

 with the ship's head E., S., and W.; and in each, the 

 amount of disturbing force in the direction of ship's 

 head and ship's starboard-side were found. On ex- 

 amining these it was at once seen that the four values 

 for force directed to the ship's head agreed so nearly, 

 and the four values to the ship's side agreed so nearly, 

 as to leave no doubt that nearly the whole disturbing 

 force was a force similar to permanent magnetism 

 making a definite angle with the ship's keel. Then, 

 deductively, using the means of these values, and com- 

 pounding them with terrestrial horizontal force in 

 different positions of the ship's head, on the principles 

 of Article 55, the resulting direction of the needle in 

 33 positions of the ship was found to agree with the 

 observed direction; only a small quadrantal difference 

 remained, which was evidently explained by the quad- 

 rantal term in Article 68, as produced by masses of 

 iron towards the ship's head. 



Similar treatment of observations on three other 

 compasses in different parts of the ship gave nearly 

 the same value for the permanent magnetism trans- 

 versal to the keel, but smaller values for the longi- 



