The Compass Needle 



We have specialized in magnetic investigations only as an inci- 

 dent in the manufacture of the more accurate instruments in which 

 scientific optics and precise mechanics control the issue. 



The discovery of the north-seeking needle- is traceable back to 

 rather remote antiquity among the Chinese, who observed thisspecific 

 property in polarized steel but were never able to apply the phe- 

 nomenon to the science of terrestrial magnetism which is now of 

 equal importance, in the art of navigation, with the needle itself. 

 As a means of rapid approximate orientation a more serviceable 

 instrument has never been devised, but as a means of conducting 

 land or mining surveys it must be regarded as belonging distinctly 

 to the past generations. The U. S. Gov't. will not permit mining 

 claims or new township locations to be surveyed with the compass. 



The needle was introduced in Europe about the year 1300, but 

 the natural laws affecting its behavior were not studied until 1541 

 in Paris. The needle declines to remain stationary for any length 

 of time on any of the habitable portions of the earth's surface. In 

 London, for instance, it changed its direction 24 28' in the 155 

 years preceding the war of 1812. Shortly afterwards it began to 

 swing back again and has since changed its position more than 10. 

 The change is not so rapid in the U. S. as in Europe, but it is never- 

 theless sufficient to cause a serious error in surveys conducted with 

 it, if its rate of change were not known. A boundary line of a 

 mile, fixed with the needle in Maryland in 1802, would now have 

 the one end more than 500 ft. away from its original location if 

 retraced by this same method. Again, the mean declination, for 

 instance, in the State of Illinois for 1870 was 6 15' E. By 1900 the 

 average declination was 4 E. , showing that in 30 years it had changed 

 at the rate of 4' 30" per annum. 



Points on the surface of the earth showing equal magnetic 

 variation may have drawn through them irregularly cur\ed lines 

 known as isogonic lines. The lines of no declination are known as 

 the agonic lines. In 1855 the Coast Survey published its first 

 isogonic chart. The one reproduced in Fig. 46 is taken from the 

 Forest Service Manual. The lines will have to be replotted in the 

 course of time, for the entire system is slowly changing its position. 

 The American agonic line passes through the U. S. near Charleston, 

 S. C., Cincinnati, O., and Lansing, Mich. The Kuroptran agonic 

 line passed eastward through London in 1657, Paris in 1669 and is 

 iu>w in the vicinity of Petrograd, as shown in Fig. 47. 



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