274 Mr. Christie on the Aurora Borealis. 



needle during a long series of years. Taking this view of the 

 subject, Capt. Foster*, by a series of observations made at 

 Port Bowen, in 1825, and published in the Philosophical 

 Transactions for 1826, showed that the changes in the hori- 

 zontal intensity of the needle might be referred to changes in 

 the dip, the terrestrial intensity in the direction of the dip 

 remaining constant, or nearly so. If then we consider that, 

 during the time of the aurora, the absolute terrestrial intensity 

 remained constant, the change which I observed in the direc- 

 tion of the needle indicating a diminution in the horizontal 

 intensity, it will follow that, during this aurora, the force 

 developed was such as to cause an increase in the dip of the 

 needle. 



The change that took place in the direction of the needle 

 was so considerable (more than 21 in less than 15 minutes), 

 that it required no nicety of observation to mark its progress. 

 I have before mentioned, that I was so fortunate as to have 

 Mr. Faraday with me at the time : the changes were so mani- 

 fest, that he could observe them at a short distance from the 

 instrument, at the same time that I was noting them more 

 minutely with the assistance of a glass ; and he agreed with me, 

 that the effects could not be more decisive of the influence 

 exerted upon the needle during the aurora. 



It has been stated, that aurorse have occurred, during which 

 no effect has been observed on the needle ; that this was re- 

 markably the case in Capt. Foster's observations at Port 

 Bowen ; and that these observations are ' a refutation of the 

 supposed connexion between tremors of the needle and aurora 

 borealis.' 



With regard to these observations we may remark, in the 

 first place, that the needle was observed to be continually in a 

 state of tremor, so that it must have been difficult to decide 

 whether any effects were produced on the needle during the 

 time of an aurora ; and, in the second, that, although magnetic 

 effects may, in all cases, be simultaneous with the aurora, yet 

 the direction of the horizontal needle may not invariably be 



* By the untimely death of this meritorious and estimable officer, science has 

 lost an able, zealous, and indefatigable auxiliary his friends one whom they must 

 Jong continue to deplore. 



