SOME SPECIAL MUTUAL CHANGES 83 



marked, it may be perceived that this end will always point 

 in the same direction. If it is displaced from this position, it 

 will oscillate for some time, and then come to rest. It is also 

 capable of moving in a vertical plane if properly suspended ; 

 it will be inclined in a degree which is found to vary in 

 different places on the earth's surface. 



We have, then, indications of a stress existing between a 

 given magnet and the earth, and also of a stress between one 

 magnet and another. 



In these experiments, as in the experiments on electric 

 stress, the suspension of the bodies used is necessary, in order 

 to overcome the stress of gravitation existing between them 

 and the earth. This stress is so powerful that those less power- 

 ful are prevented from producing their special changes. 



When a suspended magnet is observed to place itself 

 always in the same position with regard to the earth, and to 

 oscillate about that position if displaced, we are unable to 

 detect any change in the earth itself \ just as we are unable 

 to perceive any change of the earth co-existent with the 

 fall of a body, but the mutual nature of the change is very 

 readily seen in the case of the magnetic stress between two 

 magnets. 



A change, exactly corresponding with the oscillation of a 

 magnet which has been displaced from its position of equili- 

 brium, takes place when a suspended body, such as a pendulum, 

 oscillates under the stress of gravitation. 



If that end of a magnet which moves so as to point to 

 the north is brought near the corresponding end of another 

 magnet, the stress will be such that they are mutually re- 

 pelled. If the two ends which point southwards are brought 

 together, they are likewise mutually repelled. A north and a 

 south -pointing end, however, mutually attract. This property 

 is easily shown by suspending the magnets from their centres. 

 Likewise, magnets placed lengthways near each other are 

 attracted or repelled according as their corresponding ends 

 are reversed or together. 



We learn, therefore, that a magnet possesses polarity, or a 

 dual variation in space, similar to an electric field, 



