British Association, ^ 419 



other periods already noticed, — namely, tlie annual and the decennial pe- 

 riods. He has further arrived at the very remarkable result, tliat the de- 

 cennial magnetic pei'iod coincides, both in its duration and in its epocljs 

 of maxima and minima, Avith 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 the lu- 

 minous 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 discre2)ancy 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 w^ould 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 considered are all referable to the sun as their cause. Prof. 

 Kreil discovered, however, that another body of our system — namely, 

 our own satellite — exerted an effect upon the magnetic needle, and that 

 the magnetic declination underwent a small and very regular variation, 

 whose amount was dependent on the lunar hour-angle, and whose period 

 was therefore a lunar day. This singular result was subsequently con- 

 firmed by Mr. Broun in the discussion of the Makerstoun Observations; 

 and its laws have since been fully traced, for all the magnetic elements, 

 by General Sabine, in the discussion of the results obtained at the Colo- 

 nial Magnetic Observatories, 



The foregoing facts bear closely upon the debated question of the 

 causes of the magnetic variations. It has been usual to ascribe the pe- 

 riodica] changes of the earth's magnetic force to the thermic action of 

 the sun, operating either directhj upon the magnetism of the earth, or 

 affecting it indirecfhj by the induction of the thermo-electric currents. 

 Here, however, we have a distinct case of magnetic action, unaccompa- 

 nied by heat ; and the question is naturally suggested, whether the larger 

 diurnal change may not also be independent of temperature. The most 

 important fact, in its beariiig upon this question, is the exis 

 annual ineqmUty in the diurnal variation, dependent on the sun's decli- 

 nation, which has been recently pointed out by General Sabine. If we 

 deduct the ordinate of the curve which represents the mean diurnal vari- 

 ation for the entire year, from those of the summer and winter half-yearly 

 curves respectively, the differences are found to be equal and opposite; 

 and the curves tliemselves, consequently, are similar, but oppositely jjlaced^ 

 TvUi^ ^^r^^^^i. 4.^ xi,. .,.:^ ^4? l^>^Q/»;oQoo From this. General Sabine draws 



existence of an 



^ith respect to the axis of abscissa?. 



I 



