THE AURORA BOREALIS 



At the time when M. Biot promulgated his theory, it was necessary for him 

 to assign an adequate source whence the electricity was derived, to which 

 he ascribed the aurora ; and he accordingly supposed it to proceed from the 

 polar volcanoes. In the progress of electrical discovery, so many new sources 

 of electricity have, however, been since disclosed, that this part of his hypothe- 

 sis has become needless. 



The following hypothesis has been suggested by Professor Faraday (Erp. 

 Research. 192) : 



" I hardly dare venture, even in the most hypothetical form, to ask whether 

 the aurora borealis and australis may not be the discharge of electricity , thus 

 urged toward the poles of the earth, whence it is endeavoring to return 

 by natural and appointed means above the earth to the equatorial regions. The 

 non-occurrence of it in very high latitudes is not at all against the supposition ; 

 and it is remarkable that Mr. Fox, who observed the deflections of the magnetic 

 needle at Falmouth, by the aurora borealis, gave that direction of it which per- 

 fectly agrees with the present view." The manner in which the electricity 

 above alluded to is urged toward the poles, belongs to another division of our 

 subject, " Magneto Electricity." If the above view is correct, may it not help 

 us in the difficult question of atmospheric electricity 1 



The mode adopted to illustrate the electrical nature of the aurora, is to ex- 

 haust a tall glass tube by means of the air-pump, and then to pass a succession 

 of electric sparks down the interior of the tube, from the prime conductor of 

 the machine. The effects produced by a powerful machine are most brilliant ; 

 a close inspection shows that the whole tube is at times filled with a mass of 

 miniature flashes of lightning ; the color varies from the usual bright electrical 

 light to a vivid violet. The most exalted effects have been produced by means 

 of the hydro-electric machine. The tension of this machine is equal to a 

 spark of twelve or fourteen inches in the atmosphere, and therefore of power 

 to pass readily through four or five feet of partial vacuum, and its quantity is 

 equivalent to a charge of eighty feet of coated surface in ten seconds. A pe- 

 culiar effect attending this powerful discharge is, that sometimes the aurora 

 appears with a bright line of light proceeding from each end of the tube, and a 

 revolving spiral embracing the lower part. 



The falling star is an experiment of the auroral character often introduced 

 in books on electricity. Cavallo says (vol. ii., p. 101), " When the receiver is 

 not exhausted, the discharge of a jar through some part of it will appear like a 

 small globule exceedingly bright." Whence we often hear it said, that the dis- 

 charge of a battery will produce a ball of light passing from one end to the 

 other of the exhausted receiver. If this really were the case, ij; would be a 

 most important experiment ; for if the ball were seen to pass from one end to 

 the other, it would follow that its direction had been actually seen ; and, if so, 

 the one-fluid theory would have been demonstrated. But very little reflection 

 will suffice to show the impossibility of such an appearance ; for, admitting 

 the actual existence of a ball, though we are more inclined to suppose that any 

 such thing would be like an oblong spheroid, the extreme velocity of electricity 

 would take it to the end of its course before the impression of its first appear- 

 ance on the retina had subsided ; just, indeed, as the rotating wheel, having 

 red radii, appears entirely red during the period of rapid rotation ; and so, in- 

 stead of seeing a ball, if such really were there, the eye would recognise a 

 continuous line of light. And this is actually the case. We have ourselves 

 repeated the experiment under very favorable circumstances, and in the pres- 

 ence of very competent witnesses, and one and all agreed in perceiving in 

 every case a distinct continuous line of light, but no appearance of a ball or 

 falling star. 



