322 Royal Society : — 



America ; and on the other hand, there were perturbations, sometimes 

 of considerable 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 contem- 

 poraneously in action ; and it was further inferred that the most 

 suitable and promising mode of pursuing the investigation was by an 

 endeavour to analyse 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 station, on a mean of many days, to a law de- 

 pendent 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 processes of 

 reduction. This conformity of the disturbances to a law depending 

 on the solar hours was the first known circumstance which pointed to 

 the sun as their primary cause, whilst at the same time a difference, in 

 the mode of causation of the regular- and of the disturbance-diufW 

 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 variation. 



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

 primary 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 

 aggregate effects, of the magnetic storms, corresponding in period, 

 and coincident in epochs of maximum and minimum, with the de- 

 cennial variation in the frequency and amount of the spots on the 

 sun's disk, derived by Schwabe from his own systematic observations 

 commenced in 1826 and continued thenceforward. The decennial 

 variation of the magnetic storms is based on the observations of the 

 four widely distributed Colonial Observatories, and is concurred 

 in by all. This remarkable correspondence between the mag- 

 netic storms and physical changes in the sun's photosphere, of 

 such enormous magnitude as to be visible from the earth even by 

 the unassisted eye, must be held to terminate altogether any hypo- 

 thesis which would assign to the cause of the magnetic disturbances 

 a local origin on the surface or in the atmosphere of our globe, or 

 even in the terrestrial magnetism itself, and to refer them, as cos- 

 mical phenomena, to direct solar influence ; leaving for future solu- 

 tion the question of the mode in which that influence produces the 

 effects which we believe we have thus traced to their source in the 

 central body of our system*. 

 * The existence of a decennial period of the magnetic storms was not, as some 



