492 



and in the magnitude of their mean effects, by several 

 periodical laws of a highly systematic character, admit- 

 ting of well-assured determination. Perhaps the most 

 important of these periodical laws is the one which has 

 been honoured by M. de Humboldt's notice more than 

 once in the present volume, because its discovery first 

 established the connection of the terrestrial magnetic 

 variations with the periodical variations in frequency 

 and amount of the solar spots, thus affording a more 

 certain evidence than any which had preceded it, of a 

 direct magnetic influence emanating from the central 

 body of our system, and producing phenomena at the 

 surface of our planet, in the production of which it 

 was scarcely possible to imagine that the solar magnetic 

 influence acted through the instrumentality of varie^ 

 tions of temperature. The decennial period, as it has 

 been termed (though some uncertainty still prevails 

 as to its precise length), is apparently a solar period, 

 inasmuch as we perceive it to form a recurring cycle in 

 the physical appearance of the solar disk, whilst on our 

 earth it has no other physical manifestation or accom- 

 paniment than those magnetic variations, which in other 

 ways also (as in their annual and diurnal periods) mani- 

 fest their dependence upon the sun. The epoch of 

 greatest obscuration in the solar disk by the dark spots, 

 is characterised on our globe by a maximum prevalence 

 of the magnetic disturbances, by their greater frequency, 

 duration, and occasional magnitude, and consequently 

 by the greater amount of their aggregate effects ; whilst 

 the epoch of minimum in the solar obscuration is analo- 

 gously characterised by a corresponding minimum in 

 the magnetic affections. The decennial period is shown 



