MAGNETIC STORMS. 397 



conclusion was obtained by comparing with each other the term-days 

 at the colonial observatories, situated in parts of the globe most dis- 

 tant from one another. The days of disturbance still appeared to be 

 of casual occurrence, but were now recognized as affections common to 

 the whole globe, showing themselves simultaneously at stations most 

 widely removed from each other. When distant stations were com- 

 pared, as, for example, stations in Europe with those of America, and 

 either or both with Tasmania, discrepancies in the amount of particu- 

 lar perturbations, similar to those which had been found in comparing 

 the European stations with each other, presented themselves, but larger 

 and more frequent, and extending occasionally even to the reversal of 

 the direction of the simultaneous disturbance. Instances were not 

 unfrequent of the same element, or of different elements, being dis- 

 turbed at the same observation-instant in Europe and America ; and 

 on the other hand, there were perturbations, sometimes of considera- 

 ble magnitude, on the one continent, of which no trace was visible on 

 the other. Hence it was concluded, with the increased confidence due 

 to this additional and more extensive experience, that various forces 

 proceeding from different sources were contemporaneously in action ; 

 and it was further inferred that the most suitable and promising mode 

 of pursuing the investigation was by an endeavor to analyze the effects 

 produced at individual stations, and to resolve them, if possible, into 

 their respective constituents. 



The hourly observations which had been commenced at the colonial 

 stations in 1841 and 1842, and continued through several subsequent 

 years, furnished suitable materials for this investigation, the first 

 fruits of which were the discovery that the disturbances, though casual 

 in the times of their occurrence, and most irregular when individual 

 perturbations only were regarded, were, in their mean effects, strictly 

 periodical phenomena; conforming, in each element and at each sta- 

 tion, on a mean of many days, to a law dependent on the solar hour; 

 thus constituting a systematic mean diurnal variation distinct from the 

 regular daily solar-diurnal variation, and admitting of being separated 

 from it by proper process of reduction. This conformity of the dis- 

 turbances to a law depending on the solar hours was the first known 

 circumstance which pointed to the sun as their primary cause, while at 

 the same time a difference in the mode of causation of the regular and 

 of the disturbance-diurnal variations seemed to be indicated by the 

 fact that in the disturbance-variation the local hours of maximum and 

 minimum were found to vary, (apparently without limit,) in different 

 meridians, in contrast to the general uniformity of those hours in the 

 previously and more generally recognized regular solar-diurnal va- 

 riation. 



This first reference of the magnetic storms to the sun as their pri- 

 mary cause was soon followed by a far more striking presumptive 

 evidence of the same, by a further discovery of the existence of a 

 periodical variation in the frequency of occurrence and amount of ag- 

 gregate effects of the magnetic storms, corresponding in period and 

 coincident in epochs of maximum and minimum with the decennial 

 variation in the frequency and the amount of the spots on the sun's 



