54 Electric and Magnetic Science 



body, may be increased and diminished ; and is sometimes far 

 stronger, for the quantity of matter, than the power of gravity ; 

 and in receding from the magnet, decreases not in the duplicate, 

 but almost in the triplicate proportion of the distance, as nearly 

 as I could judge from some rude observations." 



The edition of ihePrincipia which was published in 1742 by 

 Thomas Le Seur and Francis Jacquier contains a note on this 

 corollary, in which the correct result is obtained that the 

 directive couple exercised on one magnet by another is 

 proportional to the inverse cube of the distance. 



The first discoverer of the law of force between magnetic 1 

 \ poles was John Michell (b. 1724, d. 1793), at that time a young 

 Fellow of Queen's College, Cambridge,* who in 1750 published 

 A Treatise of Artificial Magnets ; in ivhich is shown an easy 

 and expeditious method of making them superior to the lest 

 natural ones. In this he states the principles of magnetic 

 theory as followsf : 



" Wherever any Magnetism, is found, whether in the Magnet 

 itself, or any piece of Iron, etc., excited by the Magnet, there are 

 always found two Poles, which are generally called North and 

 South ; and the North Pole of one Magnet always attracts the 

 South Pole, and repels the North Pole of another: and wee versa" 

 This is of course adopted from Gilbert. 



"Each Pole attracts or repels exactly equally, at equal 

 distances, in every direction." This, it may be observed, over- 

 throws the theory of vortices, with which it is irreconcilable. 

 " The Magnetical Attraction and Eepulsion are exactly equal to 

 each other." This, obvious though it may seem to us, was really 

 a most important advance, for, as he remarks, " Most people, who 



* Michell had taken his degree only two years previously. Later in life he was 

 on terms of friendship with Priestley, Cavendish, and William Herschel ; it was 

 he who taught Herschel the art of grinding mirrors for telescopes. The plan of 

 determining the density of the earth, which was carried out by Cavendish in 1798, 

 and is generally known as the " Cavendish Experiment," was due to Michell. 

 Michell was the first inventor of the torsion-balance ; he also made many valuable 

 contributions to Astronomy. In 1767 he became Rector of Thornhill, Yorks, 

 and lived there until his death. 



t Loc. cit., p. 17. 



