10 LIGHT SCIENCE FOR LEISURE HOURS. 



variation, and watched its course as it began to retrace 

 its way toward the true north. He discovered the 

 minute vibration which the needle makes each day 

 across its mean position. He noticed that this vibra- 

 tion is variable in extent ; and so he was led to, watch 

 it more closely. Thus he had occasion to observe 

 more attentively than had yet been done the sudden 

 irregularities which occasionally characterize 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 watch- 

 fulness. 



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 

 appears, the magnetic needle is often seen to move in 

 a few instants over several degrees." "During an 

 aurora," he adds, "one often sees in the northern 

 region of the heavens luminous streamers of different 

 colors shoot from all points of the horizon. The 

 point in the sky to which these streamers converge is 



