GILBERT OF COLCHESTER. 345 



strokes it with a natural magnet and feels certain that he has mag- 

 netized it ; and he assures ns that 'one of the poles will be at the point 

 rubbed and the other will be at the opposite side ;' and how does he con- 

 vince himself that the ring is really magnetized? He cuts it across at 

 the point opposite the one rubbed, opens it out, and finds that the ends 

 exhibit polar properties. 



A favorite piece of apparatus with Gilbert was a lodestone ground 

 down into globular form. He called it a terrella, a miniature earth. 

 He used it extensively for reproducing the phenomena described by 

 magnetizers, travelers and navigators as observed in their compass 

 needles. He breaks up terrellas, in order to examine the magnetic con- 

 dition of their inner parts. There is not a doubtful utterance in his 

 description of what he finds; he speaks clearly and emphatically. "If 

 magTietic bodies be divided, or in any way broken up, each several part 

 hath a north and a south end;"' i. e., each part will be a complete 

 magnet. 



We find him also comparing magnets by Avhat is known to us as 

 the 'magnetometer method.' He brings the magnetized bars in turn 

 near a compass needle and concludes that the magnet or the lodestone 

 which is able to make the needle go round is the best and strongest. 

 He also seeks to compare magnets by a process of weighing, similar to 

 what is called, in laboratory parlance, the 'test-nail' method. He also 

 inquires into the effect of heat upon his magnets, and finds that 'a 

 lodestone subjected to any great heat loses some of its energy.' He 

 applies a red-hot iron to a compass needle and notices that it 'stands 

 still, not turning to the iron.' He thrusts a magnetized bar into the fire 

 until it is red-hot and shows that it has lost all magnetic power. He 

 does not stop at this remarkable discovery, for he proceeds to let his 

 red-hot bars cool while lying in various positions, and he finds : ( 1 ) that 

 the bar will acquire magnetic properties if it lie in the magnetic merid- 

 ian; and (2) that it will acquire none if it lie east and west. These 

 effects he rightly attributes to the inductive action of the earth. 



Gilbert marks these and other experiments with marginal asterisks ; 

 small stars denoting minor and large ones important discoveries of his. 

 There are in all 21 large and ITS small asterisks, as well as 84 illustra- 

 tions in De Magnete. This implies a vast amount of original work, and 

 forms no small contribution to the foundations of electric and magnetic 

 science. 



Gilbert clearly realized the phenomena and laws of magnetic induc- 

 tion. He tells us that "as soon as a bar of iron comes within the lode- 

 stone's sphere of influence, though it be at some distance from the 

 lodestone itself, the iron changes instantly and has its form renewed ; 

 it was before dormant and inert; but now is quick and active." He 

 hangs a nail from a lodestone ; a second from the first, a third from the 



