2IO THE INTELLECTUAL RISE IN ELECTRICITY. 



Peregrinus, of the dip or inclination 1 of the magnetic 

 needle. 



In that same letter, Hartmann describes the precise 

 mode of identifying the north and south poles of a mag- 

 net, by placing it in a wooden bowl, floating on water, 

 which Peregrinus gives; and revealing the source of his 

 information beyond peradventure, he says 



"I have received an old parchment book of the time of 

 the wars of the Contadini, in which I have found men- 

 tioned the force of a magnet, and the mode of constructing 

 by means of a magnet, an instrument which moves auto- 

 matically in equal form, time and manner as does the 

 heavens : so that, as the sky, every 24 hours, makes its 

 revolution around the terrestrial circle, so also this instru- 

 ment in the same period completes its revolution. I have 

 not been able to believe it." 



It is easy to identify, from the foregoing, the first of 

 Peregrinus* perpetual motions, and to recognize in the 

 " parchment book" a manuscript of Peregrinus' letter; 

 for no printed edition of it had at that time appeared. 

 The error which Hartmann makes as to the extent of the 

 dip, is easily accounted for by the fact that his needle was 

 arranged on a vertical, instead of a horizontal pivot ; and 

 hence was impeded in inclining. 2 



'The term "Variation" expresses the action of the earth's magnetic 

 force in a horizontal plane, but that force has another action upon a 

 freely suspended needle. Only along the line of the magnetic equator 

 (which varies but little from the earth's true equator), does the needle lie 

 in a horizontal plane; proceeding northward, the north pole of the needle 

 is drawn downward at an increasing angle called the "dip" or "inclina- 

 tion," until it reaches a value of 90 at the magnetic pole; but proceeding 

 southward the north end of the needle is tilted upward. In some modern 

 compasses sliding weights on the frame which carries the needle are used 

 to counteract this inclining tendency. 



'See Bertelli : Memoria Sopra P. Peregrinus, p. in, quoting from 

 Hartmann's letter. Dove : Repertorium der Physik, Berlin, 1838, Band 

 ii. Also, Volpicelli : Intorno alle prime scoperte della propriety che 

 appartengono al magnete, Atti dell. Accad. Pontif. de' Nuovi Lincei, 

 Vol. XXX., 8 March, 1866. 



