176 ANNUAL OF SCIENTIFIC DISCOVERY. 



ON THE DISPOSITION OF FORCE IN PARAMAGNETIC AND DIAHAG- 



NETIC BODIES. 



The following is an abstract of a lecture on the above subject, recently 

 delivered before the Royal Institution of Great Britain, by Prof. Tyndall, 

 F. R. S.: 



The motion of an attractive force, which draws bodies towards the centre 

 of the earth, was entertained by Anaxagoras and his pupils, by Dcmocritus, 

 Pythagoras, and Epicurus ; and the conjectures of these ancients were 

 renewed by Galileo, Huyghens, and others, who stated that bodies attract 

 each other as a magnet attracts iron. Kepler applied the notion to bodier 

 beyond the surface of the earth, and affirmed the extension of this force tr 

 the most distant stars. Thus it would appear, that in the attraction of irot 

 by a magnet originated the conception of the force of gravitation. Never 

 theless, if we look closely at the matter, it will be seen that the magnetic- 

 force possesses characters strikingly distinct from those of the force which 

 holds the universe together. The theory of gravitation is, that every parti' 

 cle of matter attracts every other particle ; in magnetism also we have the 

 phenomenon of attraction, but we have also, at the same time, the fact of 

 repulsion, and the final effect is always due to the difference of these two 

 forces. A bodv mav be intensely acted on bv a magnet, and still no motion 



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of translation will follow, if the repulsion be equal to the attraction. A dip- 

 ping needle was exhibited ; previous to magnetization, the needle, when its 

 centre of gravity was supported, stood accurately level ; but, after magnet- 

 ization, one end of it was pulled towards the north pole of the earth. The 

 needle, however, being suspended from the arm of a fine balance, it was 

 shown that its weight was unaltered by its magnetization. In like manner, 

 when the needle was permitted to float upon a liquid, and thus to follow the 

 attraction of the north magnetic pole of the earth, there was no motion of 

 the mass towards the pole referred to; and the reason was known to be, that 

 although the marked end of the needle was attracted by the north pole, the 

 unmarked end was repelled by an equal quantity, and these two equal and 

 opposite forces neutralized each other as regards the production of a motion 

 of translation. When the pole of an ordinary magnet was brought to act 

 upon the swimming needle, the latter was attracted, the reason being that 

 the attracted end of the needle being much nearer to the pole of the magnet 

 than the repelled end, the force of attraction was the more powerful of the 

 two ; but in the case of the earth, the pole being so distant, the length of the 

 needle was practically zero. In like manner, when a piece of iron is pre- 

 sented to a magnet, the nearer parts are attracted, while the more distant 

 parts are repelled ; and because the attracted portions are nearer to the mag- 

 net than the repelled ones, we have a balance in favor of attraction. Here, 

 then, is the most wonderful characteristic of the magnetic force, which dis- 

 tinguishes 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 towards the 

 north magnetic pole ; and thus, however rich in consequences the supposi- 



