4&Q BRITISH SCIENTIFIC ASSOCIATION. 



terrestrial magnetic force which re-awakened the attention of the scientific world 

 to the subject. It was in the Committee rooms of this Association that the first 

 stop was taken towards that great magnetic organization which lias borne so much 

 fruit. It was here that the philosophical sagacity of Herschel guided its earlier 

 career ; and it was here again that the cultivators of the science assembled, from 

 every part of Europe, to deliberate about its future progress. It was natural, 

 therefore, that the results obtained from such beginnings should form a prominent 

 topic in the addresses which have been annually delivered from this chair ; and 

 the same circumstances will plead my excuse if I now revert to some of them 

 which have been already touched upon by my predecessors. It has been long 

 known that the elements of the earth's magnetic force were subject to certain 

 regular and recurring changes, whose periods were, respectively, a day and a year, 

 and which, therefore, were referred to the sun as their source. To these periodical 

 changes Dr. Lamont of Munich, added another of ten years: the diurnal range of 

 the magnetic declination having 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 thus to indicate a cause operating upon, 

 the entire globe. But, casual as they seem, these effects are found to be subject 

 to laws of their own. Prof. 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 facts, 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 decennial period observed 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 disturbances 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 maximum 

 disturbance are different for the different elements ; so that there may be an 

 abnormal condition of the magnetic force, operating 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 change ; while the other is strictly abnormal, and simultaneous at all parts 

 of the globe. If this be so, it would follow that we are not justified in separating 

 the larger changes from the rest, merely on the ground of their magnitude, and 

 that a different analysis of the phenomenon is required. The effects hitherto con* 



