ON A NEW MAGNETICAL INSTRUMENT. 227 



observed results may be cleared, appf oximately, of those parts of 

 the changes which are foreign to the question. Still it must be 

 admitted that such a complication of the results tends to weaken 

 their evidence; and it was therefore desirable to obtain further 

 proof, in a manner less exceptionable. 



The object being to alter the inducing action according to a 

 known law, and to observe the changes of the induced force, as 

 shown by the position of the suspended magnet, it is manifest that 

 it may be attained by simply varying the angle which the iron bar 

 makes with the direction of the earth's magnetic force, the distance 

 of its pole from the suspended magnet remaining unchanged. In 

 fact, it will be seen, by pursuing the same reasoning as before, that 

 if R denote the total force of the earth, and ^ the angle which the 

 bar makes with its direction, the equation of equilibrium of the 

 suspended magnet is 



pit, cos i// + q = X tan u, 



the line connecting the pole of the bar with the centre of the sus- 

 pended magnet being, as before, perpendicular to the magnetic 

 meridian. Hence, if the bar be devoid of permanent magnetism 

 (or q = 0), and if the forces R and X remain unchanged during 

 the experiments, we have 



tan u = a cos ^, 

 being a constant. 



In order to observe whether the deflections "of the suspended 

 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 sus- 

 pended magnet. The following Tables contain the results of two 

 such series of observations. The first column of each gives the in- 

 clination of the bar to the vertical; the second, its inclination' (<A) 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 contains the observed readings of the scale, corresponding 

 to the positions of the suspended magnet ; the fourth, the differ- 



Q2 



