PORTA' s THEORIES. 237 



the magnet approaches the more strongly the force extends 

 its sphere," which to him is confirmatory of the light 

 analogy. 



The ingenuity which could evolve this conception was 

 not slow to perceive the consequences. Iron is visibly at- 

 tracted when placed in that sphere of virtue, but does any- 

 thing else happen to it? Here is the answer, "Not alone 

 by adhesion does the magnet diffuse its virtue to the iron, 

 but, what is more wonderful, within the radii of its own 

 virtue it causes virtue in the iron. For, if you approach a 

 magnet to iron, so that the latter may be in the sphere of 

 virtue, this iron will attract another iron, and the one so 

 attracted will draw another, and thus you may see a chain 

 of needles or rings in the air hanging." (The Samothra- 

 cian rings, with the upper one not in contact with the 

 lodestone but magnetized therefrom by induction.) "But 

 while the chain exists, if you gradually remove the magnet 

 for a short distance, the last ring falls, and then the next, 

 and so on in succession all fall, and thus you see the stone 

 is able to cause its virtue in the iron without contact." 



It is curious to note how this led Porta into a false con- 

 clusion. He supposed that the sphere of virtue around the 

 magnet had a definite outer limit whereat the radii of virtue 

 ended; and that, so long as it overlapped a certain num- 

 ber of rings, all would remain suspended; but, if its centre 

 were retracted, so that the last ring were left out of the 

 sphere, then that ring would fall. Consequently, he says, 

 if you try to magnetize a bar three feet long with a stone 

 having a sphere of only two feet radius, you cannot do it 

 except over two feet of the bar ; the protruding one foot 

 will be inert. Neither Porta nor Sarpi (seen through him) 

 appears to have had any idea of the virtue extending from 

 pole to pole, or to have made any deductions from the 

 positions assumed by the inclined needles of Peregrinus or 

 Norman. But their knowledge of a field of force and of 

 magnetic induction due thereto was certainly well defined. 



The many other discoveries which Porta records may 



