220 POPULAR LECTURES AND ADDRESSES. 



But now, I say, we look with hopefulness to the 

 practical telegraphist for data towards a solution 

 of this grand problem. The terrestrial magnetism 

 is subject, as a whole, to the grand secular varia- 

 tion which I have indicated. But, besides that, 

 there are annual variations and diurnal variations. 

 Every day the needle varies from a few minutes 

 on one side to a few minutes on the other side of 

 its mean position, and at times there are much 

 greater variations. What are called " magnetic 

 storms" are of not very unfrequent occurrence. 

 In a magnetic storm the needle will often fly 

 twenty minutes, thirty minutes, a degree, or even 

 as much as two or three degrees sometimes, from 

 its proper position if I may use that term its 

 proper position for the time ; that is, the position 

 which it might be expected to have at the time 

 according to the statistics of previous observations. 

 I speak of the needle in general. The ordinary 

 observation of the horizontal needle shows these 

 phenomena. So does observation on the dip of 

 the needle. So does observation on the total in- 

 tensity of the terrestrial magnetic force. The- 



