232 



THE NATIONAL GEOGRAPHIC MAGAZINE 



trave;ling in pe;rsia on camei. 



alter materially, and the lapse of half a 

 century or a century to obliterate and 

 completely remodel the form and situa- 

 tion of those lines on its surface which 

 geometers have supposed to be drawn in 

 order to give a general and graphical 

 view of the direction and intensity of the 

 magnetic forces at any given epoch." 



We thus see the need of repetitions of 

 magnetic surveys. 



MAGNETIC STORMS OCCASIONALLY PARA- 

 LYZE) the; te;le:graph wires and 



PREVENT THE TRANSMISSION 

 OE TELEGRAMS 

 By this time the reader will doubtless 

 have received the impression that the 



earth's magnetism, at best, is certainly 

 a most unsatisfactory source of guid- 

 ance, and so it undeniably is. But mat- 

 ters are still worse than already shown. 

 The magnetic needle is subject to all 

 sorts of fluctuations in addition to the 

 one already described. Fortunately 

 most of them are of too short duration 

 or of too small an amount to require 

 attention for the practical purposes of 

 navigation. However, in time of a 

 severe magnetic storm, as occurred on 

 September 25, 1909, the compass may 

 change its direction 5 degrees and more 

 in a quarter of an hour, even at a place 

 so far distant from the North Magnetic 

 Pole as is the city of Washington. But 



