146 ON MAGNETISM. 



that of the south end of the needle, with equal forces, 

 which balance : and in like manner there is equilibrium 

 between the attraction of the north part of the sphere 

 on the south end of the needle and that of the south 

 part of the sphere on the north end of the needle : and 

 the needle is not disturbed. But in all the other posi- 

 tions, the magnetism with which the nearest part of the 

 sphere is charged is of such a quality that it attracts the 

 nearest pole of the needle : and, when the sphere is in 

 north-east or south-west position, the north end of the 

 needle is made to deviate to the east : and, when the 

 sphere is in north-west or south-east position, the north 

 end of the needle is made to deviate to the west. 



67. In these cases, the magnitude of the deviation 

 produced in the compass is independent of the magnitude 

 of the terrestrial horizontal force. 



In order to judge of the law of compass-deviation in 

 this case and in the case of the last article, as depending 

 on the geographical position of the compass, that is, as 

 depending on the magnitude of the terrestrial horizon- 

 tal force (the only geographical element which affects 

 this problem), it is necessary to observe that, the needle 

 being directed in the magnetical meridian by the terres- 

 trial horizontal force, and being made to deviate by a 

 deviating force, the amount of deviation produced will 

 depend upon the value of the fraction 



deviating force 

 terrestrial horizontal force ' 



But, in a given position of a mass of iron, the deviat- 



