THE AURORA BOREALIS. 477 



nous (he does not say how), gave rise, at some distance from the earth, 

 to the phenomena of the aurora.* 



A large library would hardly be sufficient to hold all the memoirs 

 and notices that have been published during the past sixty years on 

 the subject of the aurora boreal is, to say nothing of the numerous trea- 

 tises on physics, meteorology, and astronomy which have devoted one 

 or more chapters to it. Some authors have limited themselves to the 

 simple description of what they have perceived, or to a mere exposi- 

 tion of their theories, while others have done more. Alexander von 

 Humboldt has drawn in his " Cosmos " an excellent outline of the 

 ideas which science entertained on the subject in his time ; and the 

 " Popular Astronomy " of Arago contains valuable details, well classi- 

 fied and arranged, on the same question. 



About 1850, M. de La Rive, a Genevese physicist, endeavored to 

 found a definite theory of the aurora borealis, and with this view ar- 

 tificially reproduced the phenomenon with considerable success. A 

 prime point, which is still far removed from being fixed, is the approxi- 

 mate height of the meteor above the ground. Sometimes two ob- 

 servers, in the neighborhood of a thousand miles apart, will affirm that 

 they have seen the same aurora at the same time and under the same 

 aspect ; at other times, the phenomenon is visible only within a radius 

 of a few leagues. Mairan, basing his calculations on data that are not 

 without value, concluded an elevation of two or three hundred leagues ; 

 Bravais proposed one hundred and fifty kilometres as a mean value. 

 Other authors have supposed that the highest flashes soar to an eleva- 

 tion of eight hundred kilometres. 



M. de La Rive has made a table of all former data, and represents 

 that the aurorjB boreales, very low in reality, hardly pass beyond the 

 zone of clouds. They have been perceived (by Parry) projected on 

 the flanks of mountains. Contradictions of this view are also not want- 

 ing. In support of his opinion that the meteor is low in height, M. de 

 La Rive cites the well-established cases in which sounds have been 

 heard during the manifestations. Sometimes a sulphurous odor has 

 been perceived. The crackling occasioned sometimes by slow electric 

 discharges and the odor of electrified oxygen or ozone are quite analo- 

 gous. Explorers and aeronauts have pretended, according to M. de La 

 Rive, to have gone through the aurora or through the mist that gives 

 rise to it. 



Arago had conceived the electric nature of the meteor, and assumed 

 to predict its appearance by consulting the compass. Other facts, 

 proving a connection between auroras and magnetic phenomena, are 

 abundant. Jessan, in 1878, sailing on the Venus, relates that during 

 a fine aurora all the compasses of the vessel were disordered, and they 



* In this Euler made use of Newton's corpuscular theory of light, though he was an 

 adversary of it. 



