instructions for Observer s of the Aur&ra Borealis. 35 



Periods of the Aurora jfforeaZw.— Another object for inquiry, 

 is the existence of recurring periods of frequency and brilliancy 

 in the Aurora. About fifty years since, its appearances were nir- 

 merous and splendid ; thirty years later they became more rare, 

 till at length they were almost unseen in any part of Britain. 

 This was particularly remarked in the Shetland Islands, where 

 the inhabitants complained of the loss of this " useful light," and 

 the necessity of substituting artificial illumination for ordinary 

 farming purposes ! Since that time the Aurora has reappeared, 

 and perhaps in 1829 and 1830 may have reached a new maxi- 

 mum. It would be important to know whether similar changes 

 have been noticed in America, and other parts of the globe. 



Effect of the Aurora Borealis on the Magnetic Needle. — An 

 opinion, founded probably on the resemblance between some 

 electric phenomena and the Aurora, that this meteor exerts an 

 influence on the magnetic needle, has long prevailed. This 

 seems to be established by the numerous experiments of M, 

 Arago, Mr Christie, and Sir John Franklin ; but is opposed 

 by the failure of Sir Edward Parry and others to detect such 

 an effect. It is therefore a question, under what circumstances 

 the needle is affected by it. Mr Farquharson (who considei*3 

 the fact as unquestionable) is disposed to think it a necessary 

 condition to its occurrence, that the streamers should reach the 

 plane of the magnetic dip, that is, in Britain, a point in the mag- 

 netic meridian 70° alt. above the southern horizon. It may 

 be added, that this action of the Aurora takes place when it is 

 not visible ; at Paris, where M. Arago detected it, this meteor is 

 scarcely ever seen. 



Mode of Observation recommended. — As uniformity of obser- 

 vation is essential to obtain the results desired by the Associa- 

 tion, the following suggestions are offered to such gentlemen as 

 may wish to take a share in the undertaking. 



Each observer should, at 10 p. m., Greenwich time (which 



should be used throughout), examine the sky, and enter in a 



journal the Aujoral phenomena that may occur, as also the ab- 



. sence of any, should that be the case. This examination should 



be continued till 11 *. It is also desirable that he should en- 



• It happens sometimes, that the most interesting appearances of i\\2 au- 

 rora begin and cease at earlier hours. It was particularly remarked at Yort, 



c2 



