THE GENERAL BEARINGS OF MAGNETIC OBSERVATIONS. 1 



By Ettrick W. Creak. 



If necessity be the surest prompter of invention, it is not too much 

 to say that the necessity of the navigator has been a most potent factor 

 in producing the observer of the elements of terrestrial magnetism. 

 The traveler on land might rest during darkness until daylight enabled 

 him to resume his journey; but the seaman on the trackless ocean was 

 dependent upon the indications of his compass by day and night; and 

 after the discovery of Columbus that the magnetic declination or varia- 

 tion of the needle from the direction of the geographical north varied 

 in amount with the latitude and longitude, a new impetus was given 

 to observation. 



The publication of Gilbert's grand discovery that the earth is a 

 magnet and the director of the freely suspended needle, followed by the 

 discovery of the secular change in the value of the declination, natur- 

 ally added to the desire of both landsmen and seamen to know as 

 much as possible concerning that great magnet, both from purely 

 scientific reasons and to meet the practical ends of the navigator. 

 Thus the seventeenth and eighteenth centuries were remarkable for 

 the number of observers both of the magnetic dip and declination. 



So important had a correct knowledge of the declination become to 

 the requirements of navigation, as early as the close of the seventeenth 

 century, that Halley, under the immediate auspices of the Government, 

 made his celebrated voyage over the Atlantic Ocean in a man-of-war, 

 in order that intelligent observation should set at rest much that was 

 doubtful. The results of this voyage, combined with the observations 

 of previous navigators, were embodied in his celebrated chart of lines of 

 equal value of magnetic variation or declination, the first of its kind 

 and of so convenient a form that charts of equal values of the three 

 magnetic elements are to this day the most acceptable form for repre- 

 senting the combined results of magnetic observations made over large 

 areas of sea and land, as well as of the special magnetic surveys which 

 in recent years have been made in various countries. 



1 From Science Progress, Vol. V, No. 26, April, 1896. 



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