91 



that the magnetism in these points is directly as the distance from 

 the magnetic centre ; the reciprocal force on a small trial cylinder 

 being as the squares of the distances from the centre. 



Some striking analogies in the state of a magnetized steel bar and 

 the common Leyden jar are noticed in this communication, from 

 which it would appear that the conditions of electrical and magnetic 

 force are precisely the same, and from which the author concludes 

 that magnetic attraction is reducible, as in electricity, to an action 

 between opposed surfaces ; he thinks that a predisposition to identify 

 these forces with that of gravity and other central forces has led 

 many profound mathematicians and philosophers to question unduly 

 the accuracy of every result not in accordance with such a deduction. 

 He observes that Sir Isaac Newton considered "that the virtue of the 

 magnet is contracted to the interposition of an iron plate, and is almost 

 terminated by it, for bodies further off are not attracted by the magnet 

 so much as by the iron plate as also that this power is essentially 

 different from gravity, " and in receding from the magnet decreases 

 not in the duplicate, but almost in the triplicate proportion of the 

 distance*," a result which has been shown to be perfectly consistent 

 with experiments. Newton however has been supposed to have had 



very inaccurate ideas of magnetic phenomena f ;" it would be very 

 difficult however to show from the little which this great author has 

 advanced upon this subject in his grand work, the Principia, in 

 what his views of magnetic action were defective ; they appear on 

 the contrary to be in most perfect accordance with experimental 

 facts. la associating magnetic action with a law of the " centri- 

 fugal forces of particles terminating in particles next them," Newton 

 never pretended to offer any theory of magnetism, but says with 

 his usual diffidence, " whether elastic fluids do really consist of par- 

 ticles so repelling each other is a physical question," and " which he 

 leaves philosophers to determine," On the other hand, a large 

 amount of experimental research by Hawksbee, Brook Taylor, 

 Whiston, Muschenbroek, and other eminent men, has been sup- 

 posed by Dr. Robison as unworthy of confidence, and ill adapted to 

 the object for which it was designed;]:. The same learned writer 

 thinks that magnetic attractions and repulsions are not the " proper 

 phenomena for declaring the precise law of variation." Yet was it 

 by these same attractions and repulsions that Lambert, and more 

 especially Coulomb, deduced what this accomplished author con- 

 siders as being the true law of force. The author of this commu- 

 nication is led to believe, that all the results of these inc|uiries, in- 

 cluding the deduction of Newton, are not only consistent with, but 

 necessary consequences of, the laws of induced magnetic forces, as 

 he has endeavoured to prove, and that the action of magnetism as 

 commonly observed is something different from what has been 

 usually imagined. That future inquiries may lead to the identity 

 in origin of magnetic and gravitating force he thinks not impro- 

 bable ; there may be some diffuse emanation through space, the 



* Principia, Books 2 and 3. f Ediub. Eucy. vol. xiii. p. 270. 



X ;Mechaui£al Philosopiiy, vol. iv. p. 217. 



