322 FEAGMENTS OF SCIENCE. 



is pulled towards the north pole of the earth. The 

 needle, however, being suspended from the arm of a fine 

 balance, its weight is found unaltered by its magneti- 

 zation. In like manner, when the needle is permitted 

 to float upon a liquid, and thus to follow the attraction of 

 the north magnetic pole of the earth, there is no motion 

 of the mass towards that pole. The reason is known 

 to be, that although the marked end of the needle is 

 attracted by -the north pole, the unmarked end is 

 repelled by an equal force, the two equal and opposite 

 forces neutralizing each other. 



When the pole of an ordinary magnet is brought 

 to act upon the swimming needle, the latter is at- 

 tracted, the reason being that the attracted end of 

 the needle being nearer to the pole of the magnet 

 than the repelled end, the force of attraction is the 

 more powerful of the two. In the case of the earth, 

 its pole is so distant that the length of the needle is 

 practically zero. In like manner, when a piece of iron 

 is presented to a magnet, the nearer parts are attracted, 

 while the more distant parts are repelled ; and because 

 the attracted portions are nearer to the magnet than the 

 repelled ones, we have a balance in favour of attraction. 

 Here then is the special characteristic of the magnetic 

 force, which distinguishes it from that of gravitation. 

 The latter is a simple un polar force, while the former 

 is duplex or polar. Were gravitation like magnetism, 

 a stone would no more fall to the ground than a piece 

 of iron towards the north magnetic pole : and thus, 

 however rich in consequences the supposition of Kepler 

 and others may have been, it is clear that a force like 

 that of magnetism would not be able to transact the 

 business of the universe. 



The object of this discourse is to enquire whether 

 the force of diamagnetism, which manifests itself as a 



