24 LIGHT SCIENCE FOR LEISURE HOURS. 



Can it be doubted that while these changes are 

 taking place, the magnetic poles also are slowly shifting 

 round the true pole ? Must not the northern pole, for 

 instance, be further from Paris now that the needle is 

 inclined more than 23 from the vertical, than in 1671, 

 when the inclination was only 15 ? It appears obvious 

 that this must be so, and we deduce the interesting 

 conclusion that each of the magnetic poles is rotating 

 around the earth's axis. 



But there is another peculiarity of the needle which 

 is as noteworthy as any of those I have mentioned. 

 I refer to the intensity of the magnetic action the 

 energy with which the needle seeks its position of rest. 

 This is not only variable from place to place, but from 

 time to time, and is further subject to sudden changes 

 of a very singular character. 



It might be expected that where the dip is greater, 

 the directive energy of the magnet would be propor- 

 tionately great. And this is found to be approximately 

 the case. Accordingly, the magnetic equator is very 

 nearly coincident with the equator of least intensity,' 

 but not exactly. As we approach the magnetic poles 

 we find a more considerable divergence, so that instead 

 of there being a northern pole of greatest intensity 

 nearly coincident with the northern magnetic pole, 

 which we have seen lies to the north of the American 

 continent, there are two northern poles, one in Siberia 

 nearly at the point where the river Lena crosses the 

 Arctic circle, the other not so far to the north only 

 a few degrees north, in fact, of Lake Superior. In 



