OF THE AURORA BOREALIS. 51 



itself to my mind, is that which connects it with the zodiacal light, which is known 

 to maintain a nearly constant position with respect to the sun. Mairan. as before 

 remarked, 1 thought he had discovered the true cause of the greater frequency of 

 auroras during the winter than during the summer months, in the fact that in that 

 half of the annual revolution, the earth is on the side of its perihelion, while during 

 the summer months, it is on the side of the aphelion ; consequently, it would plunge 

 deeper into the zodiacal light in winter than in summer. But the result of our 

 collation of the exhibitions recorded in the Regents Repiorts, indicates that in respect 

 to number the aggregate for the winter months is less than for the summer, although 

 the balance is in favor of the winter for the greatest exhibitions. If the comparison, 

 when instituted from more extensive and more accurate observations, should continue 

 to afford the same result, then the argument of Mairan could not be held as conclu- 

 sive ; but should a more reliable collation of auroras give the same result as that 

 which appeared so uniform^ to attend the researches of that distinguished philoso- 

 pher, then the argument will gain weight and. be deserving of much consideration. 

 Since only extraordinary displajs of the phenomenon would be recorded in history, 

 probably the comparison of Mairan was instituted in respect to these alone, and of 

 those the balance still appears to be in favor of the winter months. Yet the argu- 

 ment is weakened by the fact that auroras of the first class have sometimes (though 

 rarely) occurred in the summer, as that of July 1, 1837. It is, however, an acknow- 

 ledged fact that the exhibitions of summer are generally much feebler than those of 

 winter; nor can this be ascribed merely to the shorter duration of the nights, for 

 in winter the displays occur chiefly at hours when they might be seen equally well 

 in summer nights. We are still much in the dark respecting the true cause of the 

 annual periodicity, but shall anticipate a more full and satisfactory explanation of 

 it, when it has longer engaged the attention of philosophers, and afforded more full 

 and accurate data for investigation. 



The cause of the secular periodicity appears less obscure, since, referring the 

 phenomenon, as we do, to a cosmical origin, and proving, as we think, the existence 

 of a nebulous body in space which affords the material of the aurora, we recognize 

 at once in the necessary astronomical relations of such a body reasons for the 

 recurrence of the secular periods. 



3. Can any explanation be given of the sensible appearances of the aurora, such as 

 the luminous phenomena, their remarkable motions, and definite arrangement in columns, 

 arches, and coronas? It would not be incompatible with the views I entertain of 

 the origin of the aurora borealis, to suppose that the luminous appearances attending 

 it are due to electricity, since foreign matter through which the earth plunges with 

 immense velocity, might naturally be expected to develop electrical effects in the 

 atmosphere; and I have mentioned a few cases where the auroral flashes did, in 

 fact, look much like those produced by the passage of electricity through rarefied air. 

 But a more general cause of the auroral light is, I think, due to condensation, from 

 the rapid transit of the auroral matter through the air, as tinder is ignited in the 



Vide supra. 



