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 embrace. 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 ascertained 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 orbis 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
Fig. 1. Gilbert's Spherical Lodestone and Field of Magnetic Force.radii virtutis magneticae, 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 appropriate illustrations in De Magnete.
Clerk Maxwell was so fascinated 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 mathematical 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
