114 



AURORA BOREALIS. 



The celebrated academician M. de Mairan, in 1731, 

 published a treatise upon the aurora borcalis, in which 

 he ascribes this phenomenon to the impulse of the 

 zodiacal light upon the atmosphere of the earth. -The 

 zodiacal light is a luminous train, which is visible at 

 certain seasons, a little before sun-rise, or after sun- 

 set, in the shape of a pyramid or lens, stretching 

 along the zodiac. It was first discovered by Cassini, 

 who conceived it to be the atmosphere of the sun, 

 formed by a very rare fluid luminous in itself, or il- 

 luminated by the sun's rays, but not equally through- 

 out ; being much more luminous, and more extended 

 around the equator of the sun ; in which direction it 

 forms a very oblate spheroid, or rather lens, of which 

 the transverse section coincides with the plane of the 

 sun's equator. See Astkonomy Index. 



" It is proved by observation," says M. dc Mairan, 

 " that this solar atmosphere extends sometimes as far 

 as the earth's orbit, and even farther. When, there- 

 fore, it is at its greatest extent, the earth will be im- 

 mersed in it ; in which case a quantity of the luminous 

 matter, influenced by gravitation, falls upon the earth's 

 atmosphere, and descends more or less within it ac- 

 cording to its weight ; each luminous particle descend- 

 ing till it meet with a stratum of air, with which it 

 will be in equilibrium. But as the equatorial regions 

 have a greater centrifugal force than the polar, on 

 account of their greater velocity during the earth's 

 diurnal rotation, the luminous particles of the zodia- 

 cal light must be driven by this centrifugal force, 

 from the equator towards the poles ; and it is then 

 that they form those luminous arches which we call 

 the aurora borealis. ( See Tract. Phys. and Hist, 

 del Aurore Bur. ) Besides this express treatise, there 

 are several papers on the subject of the aurora horea- 

 lis, by M. de Mairan, in the Memoirs of the French 

 Academy ; one in particular for the year 1733, in 

 which he records a variety of his observations on the 

 zodiacal light, made with a view to confirm his theory ; 

 and another for the year 171-7, in which he defends 

 his theory against the attacks of Elder, who wrote a 

 treatise in order to refute it, and establish a new doc- 

 trine of his own. 



Euler justly observes, that the theory of M. de 

 Mairan is not only exposed to the objection of resting 

 upon hypothesis, rather than upon observed facts, but 

 that it is inconsistent with the direction in which the 

 aurora is constantly observed to move ; which is not 

 from the equator towards the poles, but conversely 

 from the poles towards the equator. He himself as- 

 cribes the aurora, not to the zodiacal light, but to the 

 luminous particles of our own atmosphere, driven be- 

 yond its limits by the light of the sun, and sometimes 

 ascending to the height of several thousand miles. 

 This, it must be owned, is no very brilliant specimen 

 of the philosophical acumen of this celebrated ma- 

 thematician. See Mem. Acad. Berlin, 1746, p. 117. 



As soon as the phenomena of electricity, and the 

 by wi.ieh they are governed, were tolerably un- 

 -Looc 1 ., phi' ,.ily had recourse to 



this agent, as affording a satisfactory explanation of 

 the aurora borcalis. The brilliancy of its light, the 

 rapidity of its motions, and the instantaneous chan- 

 ges of fi . ;,t, all seemed plainly to 

 to this powerful element as the cause of these 



striking phenomena. Mr Hawkesbee, too, had 

 very early shewn, that the electrical fluid assumes an 

 appearance resembling the aurora borealis, when it 

 passes through a vacuum or highly rarefied atmo- 

 sphere. If a glass tube, resembling a Florence flask 

 in size and shape, be exhausted of air by means of a 

 stop-cock and syringe fixed to its mouth, and be then 

 excited by friction, it will appear filled with a pale 

 light, resembling the aurora borealis, which will go 

 and come at intervals, sending forth brilliant flashes, 

 exactly as this meteor does in the heavens. If either 

 end of the flask be presented to the conductor of an 

 electrical machine, the other beiug held in the hand, 

 a constant stream of pale light will be transmitted 

 through it, proceeding from the conductor. Mr 

 Canton, also, contrived to exhibit an imitation of the 

 aurora borealis, by means of electricity transmitted 

 through the Torricellian vacuum, formed in a glass 

 tube about three feet long, and hermetically sealed. 

 When one end of the tube is held in the hand, and 

 the other applied to the conductor of an electrical 

 machine, the whole tube is illuminated from end to 

 end, and will continue luminous for a considerable 

 time after it has been removed from the conductor. 

 If, after this, it be drawn through the hand either 

 way, the light will be uncommonly intense, extending 

 without the least interruption, from one hand to the 

 other, even throughout its whole length. By this 

 operation, however, a great part of the electricity is 

 discharged ; nevertheless the tube will flash at inter- 

 vals, if held at one extremity and kept quite still ; 

 but if it be grasped by the other hand at the same 

 time in a different place, strong flashes of light will 

 hardly ever fail to dart from one end to the other, 

 which will continue twenty -four hours and longer, 

 without any fresh excitation. An arched double ba- 

 rometer, of a considerable height, exhibits these phe- 

 nomena in a still more striking manner. 



Thus we find that a small quantity of electricity, 

 excited in a highly rarefied atmosphere, or in a me- 

 dium approaching to a perfect vacuum, will exhibit 

 luminous appearances entirely resembling the aurora 

 borealis, for a very considerable space of time. With 

 respect to the variations of colour which we find in 

 the aurora borealis, these seem fairly ascribable to the 

 different degrees of rarefaction of the air ; for the 

 same electricity which appears white in a very rare 

 medium, becomes blue, purple, or red, in a medium 

 of increased density ; as is fully evinced by the fol- 

 lowing experiment. Let an electrical machine and 

 an air pump be so disposed, that while the machine is 

 worked a succession of strong sparks shall be com- 

 municated from the prime conductor to a metallic 

 knob attached to the top of the receiver of the air 

 pump. Let now the exhaustion of the receiver pro- 

 ceed, and we shall soon perceive the electricity for- 

 cing itself through the air within it, in a visible, 

 stream. At first this stream is of a deep purple co- 

 lour ; but, as the exhaustion advances, it changes to 

 blue ; and at length to an intense white, with which 

 the whole receiver becomes completely filled. 



This experiment would appear to establish the iden- 

 tity of the aurora boreabs with electric light ; and it 

 may be mentioned as collateral proofs of this iden- 

 tity, that the atmosphere is found, by the electrome- 



Aurai-a 

 Borealis. 



