492 ADDRESS DELIVERED BEFORE THE BRITISH ASSOCIATION 



been found to pass from a maximum to a minimum, and back 

 again, in about that time. 



But besides these slow and regular changes, there are others 

 of a different class, which recur at irregular intervals, and which 

 are characterized by a large deviation of the magnetic elements 

 from their normal state, and generally also by rapid fluctuation 

 and change. These phenomena, called by Humboldt " magnetic 

 storms," have been observed to occur simultaneously in the most 

 distant parts of the earth, and therefore indicate the operation of 

 causes affecting the entire globe. But, casual as they seem, they 

 are found to be subject to laws of their own. Professor Kreil was 

 the first to discover that, at a given place, they recurred more 

 frequently at certain hours of the day than at others ; and that, 

 consequently, in their mean effects, they were subject to periodical 

 laws, depending upon the hour at each station. 



The laws of this periodicity have been ably worked out by 

 General Sabine, in his discussion of the results of the British 

 Colonial Observatories; and he has added the important fact, 

 that the same phenomena observe also the two other periods 

 already noticed namely, the annual and the decennial periods. 

 He has further arrived at the very remarkable result, that the 

 decennial magnetic period coincides, both in its duration and 

 in its epochs of maxima and minima, with the period ob- 

 served by Schwabe in the solar spots ; from which it is to be 

 inferred that the Sun exercises a magnetic influence upon the 

 earth, dependent on the condition of its luminous envelope. 



We are thus in the presence of two facts, which appear at first 

 sight opposed namely, the absolute simultaneity of magnetic dis- 

 turbances at all parts of the earth, and their predominance at 

 certain local hours at each place. General Sabine accounts for this 

 apparent discrepancy by the circumstance, that the hours of maxi- 

 mum disturbance are different for the different elements ; so that 

 there may be an abnormal condition of the magnetic force, operat- 

 ing at the same instant over the whole globe, but manifesting 

 itself at one place chiefly in one element, and at another place in 

 another. I would venture to suggest, as a subject of inquiry, 

 whether the phenomena which have been hitherto grouped together 

 as "occasional" effects may not possibly include two distinct 

 classes of changes, obeying separate laws one of them being 

 strictly periodic, and constituting a part of the regular diurnal 



