TERRESTRIAL MAGNETISM. 145 



in some places almost parallel to each other, more rarely 

 returning into themselves, and, where they do so, form- 

 ing closed systems of an oval form, especial attention has 

 been given to the lines on which the declination is 0, 

 at all points of which the direction of the compass- 

 needle is that of true or geographical north and south, 

 and in receding from which lines on either side, 

 opposite declinations are found (i. e. east declinations 

 on one side, and west on the other), the amount of 

 declination increasing unequally with increased dis- 

 tance. ( 185 ) I have shown elsewhere how Columbus 's 

 first discovery of a "line without variation" in the 

 Atlantic Ocean, on the 13th of September 1492, gave an 

 impulse to the study of terrestrial magnetism, - which, 

 however, for two centuries and a half was directed 

 solely towards the improvement of ships' reckonings. 



Greatly as in modern times the higher scientific 

 training of navigators, and the improvements which 

 have taken place in instruments and in methods, have 

 extended our knowledge of detached parts of the lines 

 of no declination in the north of Asia, the Indian 

 Archipelago, and the Atlantic Ocean, yet in this de- 

 partment of knowledge, where the want of a general 

 or cosmical view is felt, there is still reason to com- 

 plain of slowness of progress, and absence of the desired 

 completeness in the research. I am aware that a count- 

 less number of observations, made during accidental 

 passages across the lines of no declination, have been 

 entered into ships' journals ; but comparison and com- 

 bination of the materials are wanting ; nor can the 

 results of such a labour (even supposing it to be under- 

 taken), either on this subject, or on that of the position 



VOL. IV. L 



