AURORA ROREALIS. 309 



coldness of winter in Vermont. General Martin 

 Field has recorded, in the American Journal of 

 Science, that, during the year 1830-31, the winter 

 of which was excessively rigorous, the aurora 

 was perceived on fifty-six nights ; and that 

 during ten years previously, it was observed 

 eighteen nights on an average, annually, at 

 Fayetteville in the above State. Besides which, 

 it must have existed during many cloudy nights, 

 when it was invisible. 



The arched appearance of the aurora is an 

 optical illusion, produced by the limits of vision, 

 according to the laws of perspective ; for the 

 same reason that the sky presents a vaulted 

 appearance or, that clouds present the appear- 

 ance of a canopy, while in reality they are sus- 

 pended in horizontal strata. The different ap- 

 parent heights of the aurora are owing to their 

 various distances from the points of observation. 

 When the lower atmosphere between the ob- 

 server and the lights is hazy, they appear 

 irised or rose-coloured ; probably for the same 

 reason that the sun rises and sets red, when the 

 lower air is filled with vapours. 



Siberia, Lapland, and the Norwegian Alps, 

 are distinguished by the frequent display of this 

 beautiful and mysterious phenomenon, which I 

 have thus endeavoured to explain in accord- 

 ance with the best established principles of 

 Meteorology. After all, it is quite probable, 



B B 



