180 COSMOS. 



first-mentioned of these great physicists, water, ice, glass, and 

 carbon affect the vibrations of the needle entirely in the samo 

 manner as mercury in the rotation experiments.* Almost all 

 substances show themselves to be, in a certain degree, mag- 

 netic 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 

 among the nations of the West, there is strong historical evi- 

 dence 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 Pel- 

 oponnesus, 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 Oceanf under the direction of 

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

 another work, what advantages this means of topographical di- 

 rection, and the early knowledge and application of the mag- 

 netic 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. J 



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 



* Arago, in the Annales de Chimie, t. xxxii., p. 214 ; Brewster, Treat- 

 ite on Magnetism, 1837, p. Ill; Baumgartner, in the Zeitschrift fu r 

 Phys. vnd Mathem., bd. ii., 8. 419. 



t Humboldt, Examen Critique de VHist. de la Geographic, t. iii.,p. 36. 



\ Asie Centrale, t. i., Introduction, p. xxxviii.-xlii. iTie Western 

 nations, the Greeks and the Romans, knew that magnetism could bo 

 communicated to iron, and that that metal would retain it for a length oj 

 tine. (" Sola hacc materia ferri vires, a magneto lapide accipit, retina- 

 q*e 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 magnet- 

 ic 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, in such a positioa 

 a* to admit of free motion. 



