formally.''" Such magnetic powers are the property 

 of all parts of the eanh;'^° they give the earth its 

 rotating motion '"' and hoki ihe earth los^ether in 

 spite of this motion."'" 



Indeed, each of the main stellar bodies, smi, moon, 

 stars, and earth, has such a form or principle unique to 

 itself that causes its i)arts not only to conform with 

 itself liut to revolve."" Thus, if one removes a piece 

 of the moon from this l)ody, it will tend to align itself 

 with the moon and then to return to its proper place; 

 and a fragment of the sun would similarly tend to 

 return after pro|)er orientation.'"" Moreover, there is 

 a farther-ranging, though weaker, mutual action of 

 the heavenly bodies so that one has a causal hierarchy 

 of these specific conforming powers. The form of the 

 sun is superior to that of the inferior globes and is 

 responsible for the order and regularity of planetary 

 orbits.'^' In like manner, the moon is responsible for 

 the tides of the ocean. '^'- 



By virtue of the causal hierarchy of forms, the 

 loadstone acquires its magnetic powers from the 

 earth. '^' As the earth has its natural parts, so has 

 the stone. '^■' Although the geometrical center of a 

 terrella is the center of the magnetic forces, '^^ objects 

 do not tend to move to the center but to its poles, '^' 

 where the magnetic energy is most conspicuous.'" 

 However, in a sense, the energy is everywhere equal : 

 the virtue is spread throughout the entire mass of the 

 loadstone, '°* and all the parts direct the forces to the 

 poles. '^' The poles become the "thrones" of the 

 magnetic powers.'* On the other hand, the directive 

 force is stronger where coition is weaker and accord- 

 ingly, verticity is most prominent at the equator.'*' 



"<5M: pp. 67, 105, 179, 183. 



'<« M: pp. 101, 105, 217. 



'<"M: pp. 179, 304, 305, 311, 322, 326, 328, 330-334, 338- 

 343. 



'♦'* M: pp. 142, 179; see also electric attraction, p. 97. 



'<»M: pp. 308, 317-343. 



ISO M: pp. 106, 340. 



'51 M: pp. 308, 309, 311, 330, 333, 344, 347. 



>" M: pp. 136, 334, 345. 



153 M: pp. 184-186, 190, 232. This is not quite the same 

 argument as that the powers of the loadstone are identical 

 with those of the earth. See footnote 78. 



1" M: pp. 125, 180. 



1" M: p. 151. 



i5«M: pp. 121, 150. 



i"M: pp. 115, 151, 165. 



15* M: pp. 106, 118, 151, 191, 205, 221, 243. 



'5»M: pp. 116, 117, 119, 131, 183, 188, 221. 



iMM: p. 31. 



i«i M: pp. 116, 151, 200. 



The strength of a loadstone depends upon its shape 

 and mass. A bar magnet has greater powers than a 

 spherical one because it tends to concentrate the 

 magnetic jjowers more in the ends.'*^ For a given 

 purity and shape, the heavier the loadstone, the 

 greater its strength.'*' A loadstone has a maximum 

 degree of magnetic force that cannot be increased.'*^ 

 However, weaker ones can be strengthened by stronger 

 ones.'*^ Similarly, the shape and weight of the iron 

 determine the magnetic force in coition.'*" 



The formal forces of a loadstone emanate in all 

 directions from it,'" but there is a bound to it that 

 Gilbert called the "orbis virtutis."'** The shape of 

 this "orbis virtutis" is determined by the shape of the 

 stone."'' This insensible effusion is analogous to the 

 spreading of light that reveals its presence only by 

 opaque bodies.'™ Siinilarly, the magnetic forms are 

 effused from the stone,''' and can only reveal their 

 presence by coition with another loadstone or by 

 "awakening" magnetic bodies within the "orbis 

 virtutis."'''^ Unmagnetized iron that comes w'ithin 

 the "orbis virtutis" is altered, and the magnetic virtue 

 renews a form that is already potentially in the iron.'"' 

 The formal energy is dravwi not only from the stone 

 but from the iron.''* This is not generation, or altera- 

 tion in the sense of a new impressed quality, but 

 alteration in the sense of the entelechy or the activa- 

 tion of a form potentially present.'"^ Those bodies 



'«2 M: pp. 131, 132, 153-158. 



'« M: pp. 141, 152, 153, 158, 161, 191, 222. 



'«* M: p. 146. 



'"M: p. 165. 



i«9M: p. 153. 



i«' M: pp. 121, 123, 124, 304, 305, 306, 307, 309. 



"" Gilbert defined the orbis virtulis in the glossary at the 

 beginning of his treatise as, ". . . totum illud spatium, per 

 quod quaevis magnetis virtus e.xtenditur." This is the core of 

 the difference between electric and inagnetic forces. The sub- 

 stantial form of an electric could not be "effused," but was 

 "imprisoned" in matter (as the Ncoplatonic soul in the human 

 body); while the primary form of a magnet did not require 

 a material carrier and its effusion was similar to the propaga- 

 tion of a species in light. 



'«» M: pp. 124, 150, 151. 



1™ M: pp. 123, 307. 



■"' M: pp. 304-307. -See also p. 310, where it is stated that 

 the sun and earth could awaken souls. 



1" M: pp. 101, 110, 112, 123, 148, 149, 304, 305. This 

 awakening of the iron within tlte "orbis virtutis" is comparable 

 (pp. 216, 350) to the birth of a child under the influence of 

 the stars. 



•"' M: pp. 110, 111,112,189,216,217. See also footnote 36. 



1'* M: p. 106. 



i"M: pp. 106, 109, no. 



136 



BULLETIN 218: CONTRIBUTIONS FROM THE MUSEU.M OF HISTORY .^ND TECHNOLOGY 



