640 



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-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 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- 

 mica! 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 

 have supposed, a fortuitous discovery ; but a consequence of a process of exami- 

 nation early adopted^and expressly devised, by the employment of a constant sepa- 

 rating value, to make known any period of longer or shorter duration which might 



