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POPULAR SCIENCE MONTHLY. 



the attraction manifests itself 'all over the periphery.' Following the 

 same analogy, he calls the line joining the two poles the axis of the 

 magnet, and the equator the line equally distant from them. With the 

 aid of his steel versorium, he recognizes that similar poles are mutually 

 hostile, whilst opposite poles seize and hold each other in friendly em- 

 brace. He also satisfies himself that the energy of magnets resides not 

 only in their extremities, but that it permeates 'their inmost parts, 

 being entire in the whole and entire in each part.' This is exactly what 

 we say ; it is nothing else than the molecular theory proposed by Weber, 

 extended by Ewing and universally accepted. 



At any rate, Gilbert is quite certain that whatever magnetism may 

 be, it is not, like electricity, a material, ponderable substance ; he ascer- 

 tained this by weighing in the most accurate scales of a goldsmith a rod 

 of iron before and after it had been rubbed with the lodestone, and 

 then observing that the weight is precisely the same in both cases, being 

 'neither less nor more.' He discovers also that not only the magnet, 

 but all the space surrounding it, possesses magnetic properties ; for the 

 magnet 'sends its force abroad in all directions, according to its energy 

 and quality.' This region of influence he calls orlis virtutis, a sphere, 

 or, as we call it, a field of force. With wonderful intuition, he sees this 

 space filled with lines of magnetic virtue passing out radially from his 



spherical lodestone, and he calls 

 these lines radii virtutis magnet- 

 icae, rays of magnetic force. 



When Faraday spoke of field 

 of force, magnetic field, lines of 

 electric and magnetic induction, 

 some thought the idea new, 

 whereas not only the idea, but also 

 the very terms occur with appro- 

 priate illustrations in De Mag- 

 nete. 



Clerk Maxwell was so fasci- 

 nated with that beautiful concept 

 that he made it the work of his life 

 to study the field of force due to 

 electrified bodies, to magnets and to conductors conveying currents ; his 

 powerful intellect visualized those lines and gave them accurate mathe- 

 matical expression in the great treatise on electricity and magnetism 

 which he gave to the world in 1873. 



Gilbert observes that the lodestone may be spherical or oblong; 

 'whatever the shape, imperfect or irregular, verticity is present, there are 

 poles,' and the lodestones 'have the selfsame way of turning to the poles 

 of the world.' We find Gilbert working even with a ring of iron. He 



Fig. 1. Gilbert's Spherical Lodestone and 

 Field of Magnetic Force. 



