229 



suspended magnet being, as before, perpendicular to the 

 magnetic meridian. Hence, if the bar be devoid of perma- 

 nent magnetism (or q =: 0), and if the forces r and x remain 

 unchanged during the experiments, we have 



tan« = a cost//, 

 a being a constant. 



In order to observe whether the deflections of the sus- 

 pended magnet obeyed this law, a small divided circle was 

 attached to the piece upon which the iron bar moved, in such 

 a manner that the axis of the pivot passed through its centre. 

 The circle being fixed, and the bar connected with the 

 moveable arm carrying the vernier, we have the means of 

 determining the angle through which it is moved. The 

 plane of the motion coinciding with the magnetic meridian, 

 the inclination of the bar to the vertical was altered by 5° 

 between the successive observations of the position of the 

 suspended magnet. The following Tables contain the re- 

 sults of two such series of observations. The first column of 

 each gives the inclination of the bar to the vertical ; the se- 

 cond, its inclination {\p) to the direction of the magnetic 

 force, i. e. the former angle increased by the complement of 

 the magnetic inclination (19° 10'). The third column con- 

 tains the observed readings of the scale, corresponding to 

 the positions of the suspended magnet ; the fourth, the dif- 

 ferences between each of these readings and the reading 

 belonging to the vertical position of the bar, converted into 

 angular measure ; the fifth, the actual deflections ; the sixthj 

 the calculated deflections, as deduced by the formula given 

 above; and the seventh, the difi^erences. 



In order to derive the numbers of the fifth column from 

 those of the fourth, it is necessary to know the deflection cor- 

 responding to the vertical position of the bar. This angle 

 is determined by placing the bar vertically, with its acting 

 pole above and below successively, and noting the readings 

 of the horizontal circle, when the same division of the move- 



