258 On the Aurora Borealis. 



ON THE AURORA BOREALIS. 



BY DAVID WALKEK, ILD., F.L.S. 



An" appearance so remarkable as the Aurora could not fail to 

 attract the attention of early observers, and afford cause for 

 much conjecture. 



Aboiit the earliest theory respecting its origin, supposed 

 that it was produced by the refraction of the sun's rays; 

 another, that it depended on a mixture of the atmosphere of 

 the sun and earth ; while many ascribed it to the effects of the 

 magnetic fluid. But as the science of electricity became better 

 known and more fully developed, when its luminous effects were 

 shown, and especially when a resemblance was traced between 

 the luminosity displayed by the passage of an electric current 

 through a partially exhausted tube, and the appearance of 

 Aurora, all previous hypotheses were abandoned, and the 

 theory of Cavendish pretty generally adopted, which supposed 

 that Aurora is dependant on electricity, transmitted through 

 regions where our atmosphere is in a very rarefied state ; at 

 the same time it considered that some connection could be 

 traced with the magnetic force of the earth. Since the laws of 

 meteorology have been more fully understood, and the prac- 

 tice of recording meteorological observations more widely ex- 

 tended, tbe appearance of Aurora has attracted proportionate 

 attention, especially in its connection with the local variations 

 of the magnetic needle, and the disturbances noticed in the 

 atmospheric electrometers. Such observations have shown, 

 among other facts, that an Auroral light has been simulta- 

 neously perceived over a very extended space, e. g. the Auroral 

 light and. magnetic disturbances of 1831, 1839, and 1859, were 

 noticed at the same time, not only in the northern hemisphere, 

 but also in the southern. Tables of the comparative frequency 

 of the appearance of Aurora in different places, however, indi- 

 cate the neighbourhood of the Arctic zone as that in which 

 these phenomena most frequently occur. 



Electricians and astronomers have endeavoured to ascertain 

 the height of the Aurora above the earth by measurement of its 

 arc, but the results of their observations , taken from different 

 points of view, and perchance not directed to the same Aurora 

 — each observer seeing his own particular arc — are discordant. 

 Thus, of two observers who calculated the height of an Aurora 

 in January 1831, one made it eighteen miles, the other ninety- 

 six. The ancients believed it to be very great, even beyond 

 the limits of our atmosphere. Cavendish supposes its usual 

 elevation to be about seventy-one miles above the earth, at 



