94 



The National Geographic Magazine 



1802, 39 minutes west, and in 1900, 5 

 degrees west. A street laid out in Bal- 

 timore in 1670 so as to run parallel to 

 the compass direction prevailing at that 

 time would have its north terminus one- 

 tenth of a mile too far west in 1802. 



This change in the compass direction, 

 known as its secular change, was discov- 

 ered by Gellibrand, an English mathe- 

 matician, in 1634. 



Even in the course of a da}' the fitful 

 needle changes its direction by an 

 amount sufficient to be taken into ac- 

 count. This amount, according to the 

 season of the year, may cause a dis- 

 crepancy of from 5 to 20 feet at the 

 terminus of a line a mile long run by 

 the compass in the morning and re-run 

 in the afternoon. 



Again, at times the needle's direc- 

 tion, by some subtle force, is abruptly 

 changed out of its course. This is the 

 case during magnetic storms which 

 make their influence felt over a very 

 large portion of the globe at practically 

 the same instant of time. Thus, in 

 November, 1882, during the period of 

 maximum number of sun spots, oc- 

 curred a magnetic storm which caused 

 the needle at Los Angeles, California, to 

 change its direction by more than a de- 

 gree and a third. At the same time 

 General Greely, at Lady Franklin Bay, 

 in the Arctic region, noted a deflection of 

 20 degrees and 48 minutes. Frequent^ 

 these magnetic storms are accompanied 

 by brilliant displays of polar lights. 



There are, in addition, many minor 

 fluctuations, depending upon the posi- 

 tion of the sun and the moon with refer- 

 ence to the earth and to each other. 



We can follow with our eyes the sun 

 in its apparent motion around the earth, 

 and can behold many of the manifold 

 changes ever taking place in our starry 

 firmament, but here is something in the 

 earth, invisible to us, that we call mag- 

 netism, which day by day, year in, 

 year out, passes through its cycle of 

 changes — a force powerful enough to 

 bend every bit of magnetized steel out 



of its regular course and to compel the 

 needle to march in perfect obedience to 

 its will ! 



We thus see that it is possible to por- 

 tray the state of the earth's magnetic 

 condition, as represented by magnetic 

 maps, only for a definite moment of 

 time. The tides, the trade winds, while 

 subject to definite periodic fluctuations, 

 nevertheless will not change their gen- 

 eral character for thousands of years, but 

 a few years suffice to materially change 

 and make useless a cartographical rep- 

 resentation of the magnetic lines. 



The second great object of a magnetic 

 survey must, therefore, be to provide 

 such means, in order that a continuous 

 record be obtained of the countless fluc- 

 tuations and vagaries of the magnetic 

 needle. It is then possible to always 

 bring our magnetic charts up to date, 

 and to provide the surveyor and mariner 

 with the precise amount of change be- 

 tween any two given dates. 



The Coast and Geodetic Survey has 

 made an exhaustive and careful compi- 

 lation of all the available data for the 

 past three centuries as obtained from 

 various sources, and the practical in- 

 formation which it is in the position to 

 furnish in reference to inquiries from 

 lawyer and surveyor is regarded as 

 final and authoritative throughout the 

 country. The amount of money saved 

 to land-owners in the prevention of 

 costly litigations by the information thus 

 furnished exceeds many times the total 

 amount spent b}' the government for 

 magnetic work. 



Furthermore, in connection with the 

 magnetic survey, certain base stations 

 are established, where are erected 

 magnetic observatories, in which are 

 mounted sensitive magnetic instru- 

 ments, recording photographically, day 

 and night, the variations or changes of 

 the magnetic needle. Such a station 

 has been established 16 miles southeast 

 of the city of Washington, at Chelten- 

 ham, Maryland, far removed from arti- 

 ficial disturbing influences, such as elec- 



