THE AURORA BOREALIS. 269 



to support it, but on the contrary almost every thing to show its im- 

 probability. 



2. Dr. Halley supposed that "there is a constant circulation of the 

 magnetic fluid of the earth, from the north to the south pole through 

 the air ; which is counterbalanced by a circulation from the south to the 

 north i)ole, through the pores of the earth. The magnetic effluvia, dart- 

 ing upwards from the north pole into the higher regions of the atmos- 

 phere, acquire such an impetus as to render the circumambient ether lu- 

 minous;" and give rise to the phenomena of the Aurora Borealis. But 

 this theory is contradicted by the phenomena of the Aurora Australis, in 

 which the streamers, instead of being directed towards the south as the 

 theory would lequire, move from the south towards the north. 



3. M. de Mairan "ascribed this phenomenon to the impulse of the 

 zodiacal light upon the earth's atmosphere." The zodiacal light is at- 

 tributed to the atrnosphere of the sun, which "extends sometimes as far 

 as the earth's orbit. When the earth is immersed in it, a quantity of 

 the luminous matter falls, by the force of gravity, upon the earth's at- 

 mosphere, and, by the centrifugal force, is driven from the equator to- 

 wards the poles." A fatal objection to this theory likewise is the fact 

 that the aurora actually moves from the poles tov/ards the equator. 



4. Euler ascribed the aurora to the luminous particles of our own 

 atmosphere, driven beyond its limits by the light of the sun, sometimes 

 ascending to the height of several thousand miles! The objection to 

 this theory is that it lacks even a moderate share of plausibility. 



5. M. Monge maintained that the phenomenon consists merely iu 

 clouds illumined by solar light reflected from others placed at diflerent 

 distances in the heavens. It is difficult to see how all the phenomena 

 can be accounted for by this theory. 



6. Captain Ross has maintained that the aurora is due to atmospheric 

 vapors illumined by light reflected from fields of ice in high latitudes, 

 and that he has seen auroras between two separate icebergs. But a 

 fatal objection to this theory is, that some of the most brilliant auroras 

 take place during the long polar winter, in the total absence of the sun's 

 light from those regions in which tliey occur in their greatest splendor. 

 Another objection, which applies with equal force to the theory of 

 Monge, is the fact, determined by Brewster, Biot, and our own country- 

 man Henry, that the light of the aurora is not reflected but direct or ori- 

 ginal light. "No trace of polarization can be discovered in it," which 

 would be the case if it were reflected light. 



7. M. Blot's theory maintains that the aurora is composed of real 

 but very attenuated vapor, whose particles are obedient to the earth's 



