Aurora Borealis on the Magnetic Needle. 191 



arch (of three leagues extent and about as much in widtli) that 

 is to say, it presented all the conditions necessary to agitate the 

 magnetic needle.'* -j^» .j ■* 



In the preceding passage it is distinctly asserted as M. Ara-: 

 go's opinion, 



1. That the Auroras Boreales, whereiier they are visible, im- 

 press motions on the magnetic needle. 



2. That the Aurorae Boreales, wherever they are prevented 

 from being visible by clouds, impress similar motions upon 

 the magnetic needle. 



3. That the Aurora Boreales impress the same motions upon 

 the needle, even when the needle is observed at places where 

 the Aurorae Boreales are never seen. 



4. That all M. Arago's predictions of Aurorae Boreales from 

 the movements of the magnetic needle at Paris have been ful- 

 filled, with one singular exception, which Mr Dalton's letter 

 has removed. 



We shall now proceed to examine the truth of these general 

 propositions ; and if we do not entirely overturn them in their 

 imposing generalities, we anticipate at least the thanks of our 

 readers for having shown them the other side of the question, 

 which is so completely concealed in the preceding communica- 

 tion to the Academy of Sciences. 



There can be no doubt, as stated by M. Arago, that Cel- 

 sius, Hiorter, Father Cotte, and Cassini, to whom we add 

 Wargentin, and our celebrated countryman Canton, observed 

 that the magnetic needle was often agitated during an Aurora 

 Borealis ; but we cannot agree with M. Arago, that such ob- 

 servations are a certain proof that they attributed to magnetism 

 the phenomena of these aurorce. Two events may accompany 

 one another even always^ and yet they may not have to each 

 other the relation of cause and effect. The agitation of the 

 needle; and the Aurora Borealis, may be the concomitant effects 

 of a more general cause ; and this general cause may produce 

 the one effect without producing the other, or it may produce 

 both of them, or it may itself exist without producing either of 

 them. This, indeed, is the opinion of Mr Canton himself, 

 who considers the Aurora as the light arising from the electri- 



