8 The Theory of the Aether 



the line along which the needle set itself. Then laying the 

 needle on other parts of the stone, he obtained more lines in 

 the same way. When the entire surface of the stone had been 

 covered with such lines, their general disposition became evident; 

 they formed circles, which girdled the stone in exactly the same 

 way as meridians of longitude girdle the earth ; and there were 

 two points at opposite ends of the stone through which all the 

 circles passed, just as all the meridians pass through the Arctic 

 and Antarctic poles of the earth.* Struck by the analogy, 

 Peregrinus proposed to call these two points the poles of the 

 magnet : and he observed that the way in which magnets set 

 themselves and attract each other depends solely on the position 

 of their poles, as if these were the seat of the magnetic power. 

 Such was the origin of those theories of poles and polarization 

 which in later ages have played so great a part in Natural 

 Philosophy. 



The observations of Peregrinus were greatly extended not 

 long before the tune of Descartes by William Gilberd or Gilbertf 

 (6. 1540, d. 1603). Gilbert was born at Colchester: after 

 studying at Cambridge, he took up medical practice in London, 

 and had the honour of being appointed physician to Queen 

 Elizabeth. In 1600 he published a work* on Magnetism and 

 Electricity, with which the modern history of both subjects 

 begins. 



Of Gilbert's electrical researches we shall speak later : in 

 magnetism he made the capital discovery of the reason why 

 magnets set in definite orientations with respect to the earth ; 

 which is, that the earth is itself a great magnet, having one of 

 its poles in high northern and the other in high southern 

 latitudes. Thus the property of the compass was seen to be 

 included in the general principle, that the north-seeking pole of 



* " Procul dubio oranes lineae hujusmodi in duo puncta concurrent sicut omnes 

 orbes meridian! in duo concurrunt polos mundi oppositos." 



t The form in the Colchester records is Gilberd. 



J Gulielmi Gilberti de Magnete, Magneticisque corporibus, et de magno magnete 

 tellure : London, 1600. An English translation by P. F. Mottelay was published 

 in 1893. 



