130 TERRESTRIAL MAGNETISM. 



but although the needle passed twice in the twenty-four hours 

 through the direction regarded as the mean magnetic 

 meridian of the place, and although for fully two months 

 (April and May) no Aurora was seen, yet the times of the 

 principal elongations varied from four to six hours ; nay, more, 

 the mean epochs of the maxima and minima of west declina- 

 tion from January to May were only one hour apart ! The. 

 amount of variation was so great that on some days the 

 declination varied from 14 to 6 and 7; (within the 

 tropics the differences hardly reach as many minutes. ( 158 ) 

 Not only within the polar circle, but also within the tropics, 

 e. (/. at Bombay (lat. 18 56'), there is great complexity in 

 the periods of the horary variation of the declination. They 

 there fall under two principal classes, being very different 

 from April to October and from October to December ; and 

 these again subdivide into two minor periods, which are 

 far from being well defined. ( 159 ) 



European nations knew from their own experience nothing 

 respecting the direction of the magnetic needle in the 

 southern hemisphere until the second half of the 15th 

 century, when through the adventurous voyages of Diego 

 Cam and Martin Behaim, Bartholomew Diaz and Yasco 

 de Gama, some slight notice on the subject reached Europe. 

 The importance which, as we learn from their early writers, 

 was attached to the south end of the magnetic needle by the 

 Chinese (who, as well as the inhabitants of Corea and of 

 the islands of Japan, guided themselves at sea as well as 

 on land by the compass as early as the 3rd century of our 

 era), was ho doubt occasioned principally by the circumstance 

 that their navigation was mainly directed to the south and 

 south-west. On these southern voyages it had not escaped 



