THE AURORA. 3 



plays ; and in itself, it is so full of interest and so 

 suggestive, that our physicists already recognise it as 

 one of the most important which have been made in 

 recent times. 



A few brief words in explanation of the progress 

 which had been effected in the study of auroral phe- 

 nomena, will serve to render the interest and import- 

 ance of the discovery we have to. describe more 

 apparent. 



Let it be premised, then, that physicists had long 

 since recognised in the aurora a phenomenon of more 

 than local, of more even than terrestrial, significance. 

 They had learned to associate it with relations which 

 affect the whole planetary scheme. Let us inquire 

 how this had come about. 



So long as men merely studied the appearances pre- 

 sented by the aurora, so long in fact as they merely 

 regarded the phenomenon as a local display, they could 

 form no adequate conception of its importance. The 

 circumstance which first revealed something of the true 

 character of the aurora was one which seemed to 

 promise little. 



Arago was engaged in watching from day to day, 

 and from year to year, the vibrations of the magnetic 

 needle in the Paris Observatory. He traced the 

 slow progress of the needle to its extreme westerly 

 variation, and watched its course as it began to retrace 

 its way towards the true north. He discovered the 

 minute vibration which the needle makes each day 

 across its mean position. He noticed that this vibra- 



B 2 



