410 



cannot, therefore, well imagine how any combination of auroras and 

 earth-currents could cause the period of seven hours, which the 

 magnetic curves exhibit ; while at the same time we cannot fail to 

 associate the rapidly reversed earth-currents with those serrated 

 appearances which the curves present. 



The author suggests the following explanation of these pheno- 

 mena. The earth itself may be likened to the soft iron core of a 

 Ruhmkorff 's machine, and the lower strata of the atmosphere to an 

 insulating material interposed between the earth and the upper strata 

 of the atmosphere, which, being very rare, become conducting, and 

 form as it were the secondary coil of this arrangement. 



Now suppose a primary current, probably in our luminary, to in- 

 fluence the earth, and suppose its general direction to remain the 

 same for at least seven hours. This current would act on the 

 magnetic matter of the earth in the same manner during these seven 

 hours ; and would, therefore, account for the magnetic wave of seven 

 hours' duration. 



But although this current has been supposed to remain in the 

 same direction for seven hours, yet we may suppose that its inten- 

 sity, especially if it have an atmospheric origin, is of a fluctuating 

 character. Now any sudden increase or diminution in the intensity 

 of this current, heightened by the iron core on which it acts, i. e. the 

 earth, will produce secondary currents, 



1st. Along the surface of the earth, which is sufficiently con- 

 ducting for this purpose. 



2ndly. Along the upper strata of the atmosphere, which are also 

 sufficiently conducting. 



These will be the earth-currents and auroras which, according to 

 this hypothesis, are therefore due to the fluctuating nature of this 

 primary current ; while, on the other hand, the magnetic disturbances 

 are due to its absolute intensity. 



