THE EAKTH's magnetism BAUEK. 205 



might have spared themselves considerable pains had they previously 

 familiarized themselves more thoroughly with Halley's w^ork. 



When we to-day speak of the earth's magnetic poles, it is generally 

 recognized that those points on the earth's surface are meant where 

 the dipping needle stands precisely vertical and where tlifi magnetic 

 dip is accordingly 90°. This definition permits, with the aid of the 

 dipping needle, of a precise determination of the magnetic poles, 

 though, of course, it must not be understood that these poles are 

 mathematical points; the area over which the dip may be found to 

 be 90°, within the instrumental means of determination, may^ in fact, 

 be several miles square. A more or less extensive magnetic survey of 

 the region round about would be required to eliminate the possibility 

 of disturbing influences owing to local deposits of iron ore. At these 

 " Poles," since the magnetic force exerted by the earth is all up and 

 down, with no side component, a compass needle would have no 

 directive force acting upon it. Some distance before reaching the 

 magnetic pole it would become sluggish, and directly over the pole 

 itself it would be of no more use than a brass needle to indicate any 

 definite direction, (For chart of the magnetic meridian, see pi. 5.) 



Excluding for the present the purely " local magnetic poles " (pi. 8, 

 fig. 2) caused by extraordinary local deposits of attracting masses, all 

 observations to date show that there are but two such points (or areas) 

 where the dipping needle stands vertical, one in the Northern Hemis- 

 phere, located by Capt. James Clark Ross in June, 1831, in latitude 

 70°. 1 north and longitude 96°. 8 west (pL 6, fig. 2),^ and the other in 

 the Southern Hemisphere, lying, according to the observations of the 

 recent Antarctic expeditions, about in latitude 72°. 7 south and longi- 

 tude 156° east. The magnetic polesj therefore, are, on the average, 

 about 1,200 miles from the geographical poles. Owing to the asym- 

 metrical distribution of the earth's magnetism, the magnetic poles are 

 not diametrically opposite each other, even if the positions given 

 applied to the same year ; in fact, the perpendicular distance from the 

 earth's center to the chord connecting the magnetic poles is about 

 750 miles. 



Let us suppose, now, that one explorer starts out from Oxford, 

 where the compass points at present about 16° west, and follows 

 always the direction shown by the north end of the compass needle, 

 whereas another starts north from Washington, where the compass 

 bears about 5° west, and follows likewise the direction of the compass 

 needle. The paths thus traced out by them are the so-called " mag- 

 netic meridians," which, owing to the irregular way in which the 

 earth is magnetized, would not be straight lines or arcs of great 



^ During Capt. Amundsen's completion of the Northwest Passage, 1903-1907, he also 

 tuade observations with a view to locating the north magnetic pole, but the resulting 

 position has not yet been published. 



