TERRESTRIAL MAGNETISM. 173 



Almost all substances show themselves to bo, in a certain 

 degree, magnetic when they are conductors, that is to say, 

 when a current of electricity is passing through them. 



Although the knowledge of the attracting power of native 

 iron-magnets or loadstones appears to be of very ancient 

 date amongst the nations of the West, there is strong historical 

 evidence in proof of the striking fact, that the knowledge of 

 the directive power of a magnetic needle and of its relation to 

 terrestrial magnetism was peculiar to the Chinese, a people 

 living in the extremest eastern portions of Asia. More than 

 a thousand years before our era, in the obscure age of Codrus 

 and about the time of the return of the Heraclidse to the 

 Peloponnesus, the Chinese had already magnetic carriages, on 

 which the movable arm of the figure of a man continually 

 pointed to the south, as a guide by which to find the way 

 across the boundless grass plains of Tartary ; nay, even in the 

 third century of our era, therefore at least 700 years before 

 the use of the mariner's compass in European seas, Chinese 

 vessels navigated the Indian Ocean* under the direction of 

 magnetic needles pointing to the south. I have shown, in 

 another work, what advantages this means of topographical 

 direction, and the early knowledge and application of the 

 magnetic needle gave the Chinese geographers over the 

 Greeks and Romans, to whom, for instance, even the true 

 direction of the Apennines and Pyrenees always remained 

 unknown.f 



The magnetic power of our globe is manifested on the ter- 

 restrial surface in three classes of phenomena, one of which 

 exhibits itself in the varying intensity of the force and the 

 two others in the varying direction of the inclination, and in 



Humboldt, Examen critique de VHist. de la Geographic, t. iii. 

 p. 36. 



+ Asie Centrale, t. i. Introduction, pp. xxxviii-xlii. The Western 

 nations, the Greeks, and the Romans, knew that magnetism could be com- 

 municated to iron, and that that metal would retain it for a 'length of 

 time. (" Sola hoec materia ferri vires a magnete lapide accipit, retinet- 

 que longo tempore." Plin. xxxiv. 14.) The great discovery of the ter- 

 restrial directive force depended therefore alone on this, that no one in 

 the West had happened to observe an elongated fragment of magnetic 

 iron stone, or a magnetic iron rod, floating, by the aid of a piece of wood, 

 in water, or suspended in the air by a thread, is such a position an U 

 admit of free motion. 



