274 - POPULAR LECTURES AND ADDRESSES. 



the dip (true north pole downwards) increased by 

 11 in the hundred years from 1751 to 1851; it 

 has been decreasing ever since, and is still steadily 

 decreasing. 



Besides these great changes in the distribution of 

 terrestrial magnetism from century to century, 

 there are small diurnal and annual fluctuations 

 depending in some regular manner upon the sun's 

 influence. It seems also that there are still smaller 

 periodical fluctuations depending on the moon. 

 Besides all these small periodic variations, the 

 greatest of which does not amount to more than a 

 small fraction of a degree in the direction whether 

 of the compass or of the dipping-needle, or to 

 more than a small fraction of one per cent, of the 

 magnitude of the directing force, there are also the 

 great irregular disturbances of terrestrial magnetism, 

 called by Humboldt magnetic storms, amounting 

 sometimes to as much as a degree or two on the 

 direction, and to two or three per cent, on the 

 magnitude, of the terrestrial magnetic force. A 

 magnetic storm is never merely local, but is always 

 experienced simultaneously over the whole earth 

 and generally, perhaps always, at the same time 



