TERRESTRIAL MAGNETISM. 169 



polarity, the phenomena of Arago's magnetism of rotation, 

 and of Faraday's induced currents, on the other hand, shew 

 the probability that all terrestrial substances are capable of 

 assuming transitory magnetic relations. According to the 

 rotation experiments of the former of these two great 

 physicists, water, ice, ( 147 ) glass, carbon, and mercury, affect 

 the vibrations of a needle. Almost all substances shew them- 

 selves in a certain degree magnetic when they are acting as 

 conductors ; that is to say, when a current of electricity is 

 passing through them. 



Although a knowledge of the attracting power of the 

 loadstone, or of naturally-magnetic iron, appears to have 

 existed from time immemorial among the nations of the 

 West, yet it is a well-established and very remarkable his- 

 torical fact, that the knowledge of the directive power of a 

 magnetic needle, resulting from its relation to the magnetism 

 of the Earth, was possessed exclusively by a people occupying 

 the eastern extremity of Asia, the Chinese. More than a 

 thousand years before our era, at the obscurely known 

 epoch of Codrus and the return of the Heraclides to the 

 Peloponnesus, the Chinese already employed magnetic cars, 

 on which the figure of a man, whose moveable out- 

 stretched arm pointed always to the south, guided them 

 on their way across the vast grassy plains of Tartary ; 

 and in the third century of our era, at least 700 years 

 before the introduction of the compass in the European 

 seas, Chinese vessels navigated the Indian Ocean ( 148 ) with 

 needles pointing to the south. I have shewn in another 

 work( 149 ) what great advantages in topographical know- 

 ledge the magnetic needle gave to the Chinese geogra- 

 phers over their Greek and Roman contemporaries, to whom* 



