Causes of the Aurora Borealis. 237 



vorable circumstances, during twenty seven consecutive months, and 

 because his observations are uninfluenced by imagination. He por- 

 trays the shifting figures, the spires and pyramids, the majestic arches, 

 and the sparkling bands and stars which appeared within the arctic cir- 

 cle, as surpassing his powers of description. They are sufficient to 

 enlist the superstitious feelings of any people not fortified by religion 

 and philosophy. 



To give the reader a comprehensive view whereby to graduate his 

 estimate of the probable causes of this phenomenon, some of the hy- 

 potheses which have been advanced will be first noticed, and then an 

 examination, whether the aurora is not attributable to causes and 

 agencies not commonly suspected. 



Hypotheses which have been advanced explanatory of the Aurora. 



One of the first conjectures on record was that inflammable sul- 

 phureous vapors ascended from the earth ; but this was objected to 

 by Dr. Halley, on account of the extent of the phenomenon, and 

 the inadequacy of the cause to the effect. It was his opinion that 

 the poles of the earth were in some way connected with the aurora ; 

 that the earth was hollow, having within it a magnetic sphere, and 

 that the magnetic effluvia, in passing from the north to the south, 

 might become visible in the northern hemisphere. 



Dr. Priestly states "that Beccaria deemed the aurora borealis the 

 regular and constant circulation of the whole mass of the etherial 

 electric fluid from north to south, springing from many sources in the 

 northern hemisphere, and rendered luminous on its near approach 

 to the earth."* 



Mairan imagines " that the aurora proceeds from particles thrown 

 off from the atmosphere of the sun by the centrifugal force. "f 



Various theories, at different periods, attempting to shew that elec- 

 tricity must be the cause, have occupied the minds and pens of phi- 

 losophers. By some, it is ascribed to a destruction of the electrical 

 equilibrium ; by others, to a great excess of the electric fluid in the 

 upper regions of the air ; and again, to the passage of magnetic efflu- 

 via in arches of great circles, rising to immense heights, and send- 

 ing off forms like radii, perpendicular to the surlace of the earth. 





* Edinburgh Encvc. t lb. 



Vol. XIX.— .No. 2. 31 



