TERRESTRIAL MAGNETISM. 55 



This element, so indispensable for the correction of ships' 

 reckonings, was, at that early period, determined less often 

 by sunrise and sunset than by the pole-star, and in either 

 case with much uncertainty ; it was, however, entered on 

 nautical charts, for example, on the rare chart of Andrea 

 Bianco, sketched in 1436. Columbus, who was certainly 

 not the first who recognised the fact of magnetic decli- 

 nation, any more than Sebastian Cabot, of whom it has 

 sometimes been stated, has the praise of having been the 

 first who determined the position of a line of no declination, 

 which he did astronomically, in 2j degrees east of the Island 

 of Corvo (one of the Azores), on the 13th of September, 

 1492. In traversing the western part of the Atlantic 

 Ocean, he was the first who observed the "variation" 

 change gradually from north-east to north-west. He was 

 led thereby to conceive the idea, which has so often engaged 

 the attention of navigators in later centuries, of making use 

 of the declination lines, which he imagined to be parallel 

 to the meridians, for finding the- longitude. We learn 

 from his journals, that, on his second voyage (1496), when 

 uncertain of his position, he sought it by means of declina- 

 tion observations. A view of the possibility of such a 

 method was also, without doubt, that infallible secret of the 

 longitude at sea, which, on his death-bed, Sebastian Cabot 

 boasted of possessing through special divine revelation. 



In the excitable imagination of Columbus, there were 

 connected with the Atlantic line of no declination other 

 and rather fanciful ideas of supposed change of climate, 

 anomalous figure of the terrestrial spheroid, and extraor; 

 dinary views respecting the movements of the heavenly 

 bodies, in all of which he found reasons for proposing the 



