BOOK V. 



MAGNETISM AND ELECTRICITY. 

 CHAPTER I. 



THE COMPASS. 



I. THE DECLINATION COMPASS. ITS USES. 



LONG before the laws of magnetic phenomena were known, the 

 compass was used to navigate the open sea, when the sky, con- 

 cealed by clouds and fogs, gave no astronomical indication of the 

 direction the ships should follow. It is one of the most striking 

 examples of an application of physical phenomena long before the 

 discovery of the laws or the theory. " A thousand years and more, 

 before our era, 1 ' says Humboldt. " and at so obscure a time as that of 

 Codrus and of the return of the Heraclides to the Peloponnesus, the 

 Chinese had already their magnetic balances, one arm of which carried 

 a human figure which always pointed to the south ; and they made 

 use of this compass to direct them across the vast steppes of Tartary. 

 In the third century of our era, that is to say, seven hundred years at 

 least before the introduction of the compass to the European waters 

 the Chinese junks navigated the Indian Ocean by the pointing of 

 the magnet to the south." 



The south-pointing chariots of which Humboldt speaks consisted 

 of a little statuette turning on a vertical pivot, one of whose out- 

 stretched arms pointed to the south because it contained a magnetic 

 needle of which the south-seeking pole was towards the hand, while 



