PEREGRINUS' MOTOR. 183 



netic metal so as not to interfere with the magnetic needle. 

 Yet the currents generated in a non-magnetic conductor 

 moved in a field of force cause it seemingly to meet resist- 

 ance as if the field contained some retarding medium, so 

 that a copper bar or disk has been applied to the needle 

 of a modern galvanometer to utilize this retarding effect 

 to prevent undue vibration of that needle. The non-mag- 

 netic needle in Peregrinus' compass may have had the 

 same effect. Its retarding influence might not be sufficient 

 to interfere with the impressed force of the earth's magnet- 

 ism upon the iron needle, and yet enough to check the 

 vibrations of the latter due to inertia ; so that Peregrinus 

 may thus have unwittingly stumbled upon a phenomenon 

 the discovery of which belongs to recent years. 



The last chapter of this famous letter relates to the 

 supposed perpetual motion for the understanding of which, 

 by his friend Sigerus, all of these discoveries have been 

 made and described. We have no contemporary record 

 of any earlier attempt to construct a self-moving ma- 

 chine, although Peregrinus in the very beginning says that 

 others have vainly tried to make them. The description 

 which he presents of his own conception is incomprehen- 

 sible : and in this respect it is the prototype and exemplar 

 of all the subsequent so-called elucidations of the myster- 

 ious and power-generating "motors" which have been 

 devised since his day. It had a ring of silver, which he 

 rendered light by perforating it in various places. This 

 he supported in some way so that it would rotate on its 

 center. In the ring he arranged a series of iron teeth, the 

 sides of which were at different inclinations, something 

 after the fashion of the teeth of a ratchet wheel. The 

 magnet was placed at the extremity of a radial arm dip- 

 posed within the ring, with its end close to the teeth. 

 The magnet was fixed. The description of the operation 

 is unintelligible, but presumably Peregrinus expected that 

 the magnet would draw the prominent portion of each 

 tooth to itself, and then the momentum of the wheel would 



