TERRESTillAL MAGNETISM. 125 



Erman, also found, after an earthquake on Lake Baikal on 

 the 8th of March, 1828, no disturbance either in the amount 

 of the declination or in its periodic variation. ( l48 ) 



Declination.. 



I have already touched, in the earlier part of the present 

 volume, on the historical facts of the earliest recognition of 

 phenomena relating to the third element of terrestrial 

 magnetism, the declination. In the 12th century of our 

 era, the Chinese were not only acquainted with the fact of 

 the deviation of a horizontal magnetic needle, suspended by 

 a cotton thread, from the geographical meridian, but they 

 also knew how to determine the amount of this deviation. 

 When afterwards, by the intercourse of the Chinese with 

 the Malays and Indians, and of these with the Arabs and 

 the Moorish pilots and navigators, the use of the mariners' 

 compass became common among the Genoese, Majorcans, 

 and Catalans in the Mediterranean, on the west coast of 

 Africa, and in the Northern Seas, indications of the varia- 

 tion (or declination) came to be introduced in nautical 

 charts of different parts of the ocean, even as early as 

 1436. ( 149 ) The geographical position of a line of no 

 variation, on which the needle pointed to the true north or 

 pole of the Earth's rotation, was determined by Columbus 

 on the 13th of September, 1492 : it even did not escape 

 him that the knowledge of the magnetic declination might 

 serve to determine the geographical longitude. I have 

 shown elsewhere, from his ship's journal, that on his second 

 voyage (April 1496), when uncertain about his ship's 

 reckoning, he sought to aid himself by observations of 



