THE INTENSITY OF TERRESTRIAL MAGNETISM. 255 



according to the known laws of statics. There are, in this point 

 of view, three cases to be distinguished, according as the two 

 positions of the body, in which it would be in a state of equi- 

 librium arising from either of the two forces acting singly, either 

 coincide, — or are opposite, — or form an angle with each other. 

 It is easily seen that the difference between these three cases 

 rests on the relation of the two angles, which the straight line 

 joining the two lower points of connexion of the thread forms 

 with the magnet bar ; and which the straight line joining the 

 two upper points of suspension forms with the magnetic meri- 

 dian. If we imagine the body in that position of equilibrium 

 which is due solely to the mode of suspension, the magnet bar 

 must be, in the first of our three cases, in the magnetic meridian, 

 and in its natural position («. e. the north pole towards the north); 

 in the second case, it must be in the magnetic meridian, but in 

 the reverse position ; and in the third case, it must form an 

 angle with the magnetic meridian. For the sake of brevity, I 

 will call these three positions of the magnet bar, the direct, the 

 reverse, and the transverse positions. 



In the direct position, the action of terrestrial magnetism on 

 the magnet bar does not change the position of equilibrium cor- 

 responding to the mode of suspension ; but the apparatus is re- 

 tained in the same position by an increased force, which is the 

 sum of the two directive forces. 



In the reverse position, the equilibrium does not cease, but it 

 is only stable when the magnetic directive force is smaller than 

 the directive foi'ce arising from the mode of suspension ; and the 

 body is then only retained in this position by a force which is 

 the ditference between the two directive forces. If the magnetic 

 directive force were the greater, the equilibrium woidd be un- 

 stable, and the body once disturbed from that position would 

 not return to it, but woidd depart further and further from it, 

 and only come to rest in the opposite position, in which the bar 

 is in its natural position in space, but the suspending threads 

 cross one another. 



Finally, in the third case, where the two directive forces form 

 an angle with each other, the conflict of these forces will end 

 in an intermediate position, where, on the one hand, the bar will 

 not be in the meridian, and, on the other, a straight line through 

 the lower points of connexion of the threads will not be parallel 

 to a straight line through the upper points. This intermediate 



