PARAMAGNETIC AND DIAMAONETJC FORCES ">39 



netization, a dipping needle, when its centre of gravity is 

 supported, stands accurately level; but, after magnetiza- 

 tion, one end of it, in our latitude, is pulled toward the 

 north pole of the earth. The needle, however, being sus- 

 pended from the arm of a fine balance, its weight is found 

 unaltered by its magnetization. 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 toward that pole. 

 The reason is known to be, that although the marked 

 end of the needle is attracted by the north pole, the un- 

 marked 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 attracted 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 re- 

 pelled; and because the attracted portions are nearer to 

 the magnet than the repelled ones, we have a balance in 

 favor of attraction. Here then is the special characteris- 

 tic of the magnetic force, which distinguishes it from that 

 of gravitation. The latter is a simple unpolar 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 toward 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 



