Hoyal Societtj. "497 



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

 terminated by it, for bodies further ofFare 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. In 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 inquiries, 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 

 source of gravity, and other central forces ; and it is not impossible 

 but that the relations of this medium to the particles of common 

 matter may admit of considerable modification or change, and which 

 may be the source of that peculiar power we find displayed in those 

 bodies we consider as being magnetic and call magnets. It has been 

 occasionally supposed that in the reciprocal force between magnets 

 and iron there is a peculiar agency in operation, the law of which is 

 disturbed by the new forces of induction liable to ensue in changing 

 the distances. The author however is of opinion that such a notion 

 is inconsistent with the course of nature ; it is induction which con- 

 stitutes magnetic action, there is no other form of action ; when in- 

 duction is not present there is in fact no action ; we must hence look 

 to these very changes for an explanation of variable magnetic force. 



* Principia, Books 2 and 3. f Edinb. Ency. vol. xiii. p. 270., . 



% Mechanical Philosophy, vol. iv. p. 217. 'ir>; jjqiJ} fe!AVT[<>gdo ^H 



