THUNDER AND LIGHTNING. 



propagated reciprocally between the centre and the circumference, 

 like those which are observed in marine animals called Medusae. 

 The phenomena become gradually more faint, and generally 

 disappear altogether on the appearance of twilight. Sometimes, 

 however, the aurora continues after the commencement of day- 

 break, when the light is so strong that a printed book may be 

 read. It then disappears, sometimes suddenly ; but it often 

 happens that, as the daylight augments, the aurora becomes 

 gradually vague and undefined, takes a whitish colour, and is 

 ultimately so mingled with the cirrho-stratus clouds, that it is 

 impossible to distinguish it from them. 



Some of the appearances here described are represented in figs. 

 4, 5, 6, 7, copied from the memoir of M. Lottin. 



Fig. 6. 



Fig. 7. 



The height of the auroras has not certainly been ascertained ; 

 but as they are atmospheric phenomena, and scarcely above the 

 region of the clouds, and as they certainly partake of the diurnal 

 motion of the earth, it does not seem probable that their elevation 

 in any case can exceed a few miles. 



192 



