228 



THE NATIONAL GEOGRAPHIC MAGAZINE 



kaie;te;ur i'alls, British guiana (741 

 FEET high) 



the true poles are. In this case the 

 needle would everywhere point exactly 

 north and south ; or, as the mariner 

 would say, "there is no variation of the 

 compass." The problem of navigation 

 would then be extremely easy : if you 

 wished to go due east, for example, all 



you would have to do is to set your 

 course east by the compass, and once set 

 it would remain set for the whole cruise. 



Assume, now, that the magnetic poles 

 are no longer coincident with the true 

 poles, but are displaced by equal amounts. 

 This case represents an earth uniformly 

 magnetized about a diameter making 

 some angle with the axis of rotation. 

 The magnetic poles are still truly oppo- 

 site to each other, but, needless to say, 

 a more complicated condition of affairs 

 for navigation is presented. We must 

 now deal with a "variation of the com- 

 pass," for no longer does the compass 

 point truly north and south. If the mar- 

 iner knew the location of either magnetic 

 pole, he could readily figure out, for an 

 earth thus simply magnetized, how the 

 compass actually pointed at any place for 

 which the latitude and the longitude were 

 known. 



Ralph Walker, of Jamaica, published 

 a book in 1794 in which elaborate tables 

 of the compass direction were given. 

 He believed that the "poles of the ecliptic 

 are the magnetic poles." After exalting 

 the Supreme Architect for having given 

 us this great gift of the earth's magnet- 

 ism, by which "He enables us to behold 



WOODEN PLOW USED IN THE INTERIOR OF GUATEMALA 



