OCEAWIC DISCOVERIES. 657 



that the mode of measuring the amount of western declina- 

 tion had long been understood. The merit due to Columbus 

 is not to have made the first observation of the existence of 

 magnetic variation, since we find, for example, that this is 

 set down on the chart of Andrea Bianco, in 1436, but that 

 he was the first who remarked, on the 13th of September, 

 1492, that " 2^ east of the island of Corvo, the magnetic 

 variation changed and passed from N.E. to N.W." 



This discovery of a magnetic line without variation marks 

 a memorable epoch in nautical astronomy. It was celebrated 

 with just praise by Oviedo, Las Casas, and Herrera. We can- 

 not assume with Livio Sanuto, that this discovery is due to 

 the celebrated navigator, Sebastian Cabot, without entirely 

 losing sight of the fact, that Cabot's first voyage, made at 

 the expense of some merchants of Bristol, and distinguished 

 for its success in reaching the continent of America, was not 

 accomplished until five years after the first expedition of Co 

 lumbus. The great Spanish navigator has not only the merit 

 of having discovered a region in the Atlantic Ocean, where 

 at that period the magnetic meridian coincided with the geo- 

 graphical, but also that of having made the ingenious observa- 

 tion, that magnetic variation might likewise serve to deter- 

 mine the ship's place with respect to longitude. In the 

 journal of the second voyage (April, 1496), we find that the 

 Admiral actually determined his position by the observed de- 

 cimation. The difficulties were, it is true, at that period 

 still unknown, which oppose this method of determining longi- 

 tude. especially where the magnetic lines of declination are 

 BO much curved as to follow the parallels of latitude for consi- 

 derable distances, instead of coinciding with the direction of the 

 meridian. Magnetic and astronomical methods were anxiously 

 sought, in order to determine on land and at sea, those points 

 which are intersected by the ideal line of demarcation. The 

 imperfect condition of science, and of all the instruments 

 used at sea in 1493, to measure space and time, were un- 

 equal to afford a practical solution to so difficult a problem. 

 Under these circumstances, Pope Alexander VI. actually ren- 

 dered, without knowing it, an essential service to nautical 

 astronomy and the physical science of terrestrial magnetism, 

 by his presumption in dividing half the globe between two 

 powerful states. From that time forth tne maritime powers 



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