30 LIGHT SCIENCE FOR LEISURE HOURS. 



his object, but each time he was stopped by a barrier 

 of land. He approached so near, however, to the pole, 

 that the needle was inclined at an angle of nearly 

 ninety degrees to the horizon, and he was able to 

 assign to the southern pole a position in 75 S. lat., 

 154 E. long. It is not probable, we should imagine, 

 that either pole is fixed, since we shall now see that 

 the inclination, like the declination of the magnetic 

 needle, is variable from time to time, as well as from 

 place to place ; and in particular, the magnetic equator 

 is apparently subjected to a slow but uniform process 

 of change. 



Arago tells us that the inclination of the needle at 

 Paris has been observed to diminish year by year since 

 1671. At that time the inclination was no less than 

 75 ; in other words, the needle was inclined only 15 

 to the vertical. In 1791 the inclination was less than 

 71. In 1831 it was less than 68. In like manner, 

 the inclination at London has been observed to diminish, 

 from 72 in 1786 to 70 in 1804, and thence to 68 at 

 the present time. 



It might be anticipated from such changes as these 

 that the magnetic equator would be found to be 

 changing in position. Nay, we can even guess in which 

 way it must be changing. For, since the inclination is 

 diminishing at London and Paris, the magnetic equator 

 must be approaching these places, and this (in the 

 present position of the curve) can only happen by a 

 gradual shifting of the magnetic equator from east to 

 west along the true equator. This motion has been 



