AURORA BOREAILS. 



219 



dian ; and tliat the most elevated points of each arch 

 are exactly in this meridian.* 



It does not appear that every Aurora disturbs the 

 magnetic needle ; as Captains Foster and Back both 

 describe very splendid displays of the phenomenon, -which 

 did not appear to produce any tremor or deviation upon 

 their instruments, f 



Some sudden and violent movements have been from 

 time to time observed to take place in suspended 

 magnets; and since the establishment of magnetic 

 observatories in almost every part of the globe, a very 

 remarkable coincidence in the time of these agitations 

 has been detected. They are frequently connected with 

 the appearance of Aurora borealis ; but this is not con- 

 stantly the case. These disturbances have been called 

 magnetic storms ; and over the Asiatic and European 

 continent, the islands of the Atlantic and the western 

 hemisphere, they have been proved to be simultaneous. 



From observations made at Petersburg by Kupffer, 

 and deductions drawn from the observations obtained by 

 the Magnetic Association, it appears probable that these 



* Arago : Annales de Chimie, vol. xxxix. p. 369. On the vari- 

 able Intensity of Terrestrial Magnetism and ike Influence of the 

 Aurora Borealis upon it; by Robert Were Fox. Philosophical 

 Transactions, 1831, p. 199. 



f " Brilliant and active coruscations of the Aurora Borealis," 

 says Captain Back, " when seen through a hazy atmosphere, and 

 exhibiting the prismatic colours, almost invariably affected the 

 needle. On the contrary, a veiy bright Aurora, though attended 

 by motion, and even tinged with a dullish red and a yellow in a 

 clear blue sky, seldom produced any sensible change, beyond, at 

 the most, a tremulous motion. A dense haze or fog, in conjunc- 

 tion with an active Aurora, seemed uniformly favourable to the 

 disturbance of the needle, and a low temperature was favourable 

 to brilliant and active coruscations. On no occasion during two 

 winters was any sound heard to accompany the motions. The 

 Aurora was frequently seen at twilight, and as often to the east- 

 ward as to the westward ; clouds, also, were often perceived in 

 the day-time, in form and disposition very much resembling the 

 Aurora." Narrative of the Arctic Land Expedition. 



