164 ON MAGNETISM. 



In order to eliminate any conceivable excentricity 

 in the location of the permanent magnetism, the opera- 

 tion may be repeated, using a mark near the blue pole 

 of the magnet : and the mean of the two results may be 

 taken. For simplicity of reduction, suppose that all 

 observations are made with the separation between the 

 centers of the magnet and the needle equal to the unit 

 of measure, or (in England) 1 foot. As in Article 29, 

 let A be the magnet-power of the magnet, and E the 

 local horizontal magnet-power of the earth. Also let / 

 be the magnet-power induced by the action of the 

 terrestrial vertical force. In one position of the mag- 

 net, its power is A\-b I, and in the other position it is 

 A L On account of the peculiarity of the magnet's 

 position (which is different from those in Articles 26, &c.) 

 these are not the powers which act in the present ex- 

 periment : the real acting powers (see Article 53) will 

 be found by multiplying those by an unknown constant 

 e: so that the real acting powers in the experiment are 

 e (A + P) and e (A I). Let O t and 2 be the deviations 

 in the two experiments. Then as in the last sentence 

 of Article 26, making c = 1 and neglecting K (which 

 which will have the same proportion in the two experi- 

 ments), 



A 4- / sin l 

 whence ~~ A r^ 1 ~~- T\ 



