160 Prof, Tyndall, on the Disposition of Force [Feb. 1, 



Kepler applied the notion to bodies beyond the surface of the 

 earth, and affirmed the extension of this force to the most distant 

 stars. Thus it would appear, that in the attraction of iron by a 

 magnet originated the conception of the force of gravitation. 

 Nevertheless, 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 gravi- 

 tation is, that every particle 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 body may 

 be intensely acted on by a magnet, and still no motion of translation 

 will follow, if the repulsion be equal to the attraction. A dipping 

 needle was exhibited : previous to magnetization, the needle, when 

 its centre of gravity was supported, stood accurately level ; but, 

 after magnetization, 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 pro- 

 duction 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 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 most wonderful characteristic 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 towards the north magnetic pole : 

 and thus, however rich in consequences the supposition of Kepler 

 and others may have been, it was clear that a force like that of mag- 

 netism would not be able to transact the business of the universe. 



The object of the evening's discourse was to inquire whether the 

 force of diamagnetism, which manifested itself as a repulsion of cer- 

 tain bodies by the poles of a magnet, was to be ranged as a polar 

 force, beside that of magnetism ; or as an unpolar force, beside that of 

 gravitation. When a cylinder of soft iron is placed within a helix, 



