272 THE INTELLECTUAL RISE IN ELECTRICITY. 



there is no force acting to rotate the poised stone, while 

 of course, in the Copernican system, a rotary heaven has 

 no place. 



There is, however, to be added Gilbert's further 

 hypothesis that every magnet, the earth included, is sur- 

 rounded both by an "orb of virtue" which includes the 

 whole space through which the magnetic action extends ; 

 and by an "orb of coition" which includes all that space 

 through which the smallest magnetic body is moved by 

 the magnet, and beyond which, in other words, the magnet 

 can produce no motion in solid matter. In modern terms 

 the orb of virtue may be regarded as the whole magnetic 

 field capable of recognition as such, and the orb of 

 coition that part of the field in which a selected extremely 

 small magnetic body is attracted. These orbs or spheres 

 which Gilbert speaks of as "effused," and as produced 

 directly from the earth's exhalations, are magnetic be- 

 cause so generated ; but such phenomena are by no means 

 limited to our globe alone. Thus, he considers that all 

 heavenly bodies, and especially the sun and moon, have 

 such effused spheres, which are capable of acting upon 

 other bodies and other effused spheres. Hence, not only 

 does the earth, as has been said, remain in its place by its 

 own magnetic virtues, but "by a confederation of the 

 adjacent globes through the connected effluent strengths, 

 it is directed harmoniously to its neighbors ;" it is moved 

 by "the conspiracy of motions of other bodies and by 

 their effused forms moving together, especially by the sun 

 and moon, by which it is bounded and limited." 



It seems therefore that Gilbert, besides apprehending the 

 existence of a field of force around the earth, also pictured 

 to himself the action of that field upon other fields, and 

 of other fields upon it; a conception so far in advance of 

 his age that nothing but his unequivocally direct state- 

 ments make one willing for a moment to entertain the 

 belief that he ever harbored it ; a conception which finds 

 an every-day illustration in the electric motor in fact, 



