THE AURORA. 3 



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 vibration is 

 variable in extent, and so he was led to watch it more 

 closely. Thus he had occasion to observe more atten- 

 tively than had yet been done the sudden irregulari- 

 ties which occasionally characterise the daily movements 

 of the needle. 



All this seems to have nothing to do with the 

 auroral streamers ; but we now reach the important 

 discovery which rewarded Arago's patient watchful- 

 ness. 



In January 1819 he published a statement to the 

 effect that the sudden changes of the magnetic needle 

 are often associated with the occurrence of an aurora, 

 I give the statement in his own words, as translated 

 by General Sabine : ' Auroras ought to be placed in 

 the first rank among the causes which sometimes dis- 

 turb the regular march of the diurnal changes of the 

 magnetic needle. These do not, even in summer, 

 exceed a quarter of a degree, but when an aurora 



B 2 



