438 Prof. Balfour Stewart on the Cause of the 



doubtedly rarified air is a conductor of electricity, yet it is 

 not a good conductor ; and where can we look for sufficient 

 potential to drive currents through these upper atmospheric 

 regions? To this I would reply that as a matter of fact we 

 know that there are visible electric currents in the upper 

 atmospheric regions which occur occasionally at ordinary 

 latitudes, and which are very frequent, if not continuous, in 

 certain regions of the Earth. I allude to the Aurora, which 

 is unquestionably an electric current, and must therefore 

 influence the magnetic needle. Furthermore, both with 

 respect to its times of occurrence and to the disposition of its 

 beams, the Aurora manifests a close connection with the phe- 

 nomena of terrestrial magnetism, occurring at ordinary 

 latitudes only when there are great magnetic disturbances, 

 and the disposition of its beams having a distinct reference to 

 lines of magnetic force. We are therefore justified in asserting 

 that there is no impossibility in conceiving a set of electrical 

 currents, intimately associated with certain phenomena of 

 terrestrial magnetism, to exist in the upper region of the 

 Earth's atmosphere. 



In the next place, the objection may be raised, How is it 

 possible to suppose any cause that will make such currents 

 one and a half times more powerful at times of maximum 

 than at times of minimum sun-spot frequency ? Can it be 

 imagined that the radiant energy of our luminary varies in 

 this proportion ? Now it has been remarked by Professor 

 Stokes that an increase in the radiative power of the Sun would 

 most probably imply, not only an increase in general radia- 

 tion, but a special and predominant increase in such actinic 

 rays as are probably absorbed in the upper regions of the 

 Earth's atmosphere. These regions will therefore greedily 

 absorb the new rays, their temperature will rise, and, as is 

 known to be the case for gases, the electrical conductivity of 

 the strata will be increased *. Thus, even if we imagine the 

 general atmospheric current to remain constant, a greater 

 proportion of it would be thrown at such times into those 

 heated regions which had become good conductors ; but then 

 it is likewise probable that the current itself would be in- 

 creased as well as increasingly diverted into a particular 

 channel. 



Such an explanation appears to receive support from the 

 curious and suggestive fact that the lunar influence on the 



* It seems not impossible, too, that the constitution of these strata 

 with respect to aqueous vapour, may he different at times of maximum 

 and at times of minimum sun-spot frequency; hence their absorbing 

 power may he different. 



