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accumulation of eleflricity in a lighter and kfs condenfed 

 form, which produces the wonderfully diverfified ftreams and 

 corufcations of the aurora borealis. 



Many attempts have been made to aflign the caufe of this 

 phenomenon. Dr. Halley firft imagined that the watery 

 vapours, or effluvia, rarefied exceedingly by fubtsrraneous 

 fire, and tinged with fulphureous ftrcams, which many na- 

 turalifts have fuppofed to be the caufe of earthquakes, 

 misjht have alfo been the caufe of this appearance. But this 

 hypothelis was not fufficient to account for the immenfe ex- 

 tent of thefe phenomena over the furface of the earth, and 

 for their being always feen on the north lide of the horizon, 

 and never to the fouth. Abandoning this hypothefis, he 

 conceived that the aurora borealis is produced by a kind of 

 fubtlle matter, or magnetic effluvia, freely pervading the 

 pores of the earth, and which, entering into it near its 

 foutheni pole, paffes out again with a like force into the 

 settler at the fame diftance from the northern ; the obliquity 

 of its direction being proportioned to its diftance from the 

 pole. This fubtile matter, by becoming fome way or other 

 more denfe, or having its velocity increafed, may be capable 

 of producing a fmall degree of light, after the manner of 

 effluvia from eleClric bodies, which, by a itrong and quick 

 friclion, emit light in the dark : to which fort of light this 

 feems to have a great affinity. If Dr. Halley had known, 

 that an eleftrical ftroke would give polarity to a needle de- 

 ilitute of it, and reverfe the poles of one previoudy endued 

 with it, he would have been led of courfe to conclude the 

 eleftric and magnetic effluvia to be the fame, and that the 

 aurora borealis v/as this fluid performing its circulation from 

 one pole of the eartli to another ; and he would thus have 

 anticipated the hypothefis of Sign.__Beccaria. See Mr. Cotes's 

 defcription of this phenomenon, and method of explaining 

 it, by ftreams emitted from the heterogeneous and ferment- 

 ing vapours of the atmofphere, in Smith's Optics, p. 69, 

 &c. or Phil. Tranf. Abr. vol. vi. part ii. 



The celebrated M. de Mairan, in an exprefs treatife on 

 the aurora boreahs, pubhlhed in 1731, afligns its caufe to 

 be the Zodiacal %/jt, which, according to him, is no 

 other than the fun's atmofphere ; this light happening, on 

 fome occafions, to meet the upper parts of our air, on the 

 fide of the limits where univerfal gravity begins to aft more 

 forcibly towards the earth than towards the fun, fails into 

 our atmofphere, to a greater or lefs depth, as its fpccific 

 gravity is greater or lefs, compared with the air through 

 which it paffes. Although the whole atmofphere of the 

 earth be incolved in the fo'ir atmofphere, it is thrown off 

 both ways from the equatorial to the polar regions. This 

 projeftion is owing partly to the centrifugal force arifing 

 from the diurnal motion of the eaith, which, being greateft 

 at the equator, and decrcafing towards the poles, turns afide 

 the zodiacal matter towards each pole ; fo that by his hypo- 

 thefis he anticipates the difcoveiy of aurone autlrales : and 

 partly to the progreffive motion of the earth in its annual 

 orbit. In this cafe the light (hould dart from the equator 

 to the poles, and not, as it really does, from the poles to 

 ^the equator. Vide Traft. Phyf. & Hiil. del Aurore Boreal. 

 Suites des Mem. de I'Acad. R. des Scienc. ann. 1731. p. 3, 

 feq. There is an abftraft of Mr. Mairan's Phyfical and 

 Hillorical Treatife of the aurora borealis, in the Pliil., Tranf. 

 N° 433. or Abridg. vol. viii. p. 45O. 



M. Euler thinks the caufe of the aurora boreahs not 

 owing to the zodiacal 'light, as Mr. de Mairan fuppofes ; 

 but to particles of our atmofphere, driven beyond its li- 

 mits by the impulfe of the light of the fun. On this fup- 

 pofition he endeavours to account for the phenomena ob- 

 fervcd concerning this hght, He fuppofes the zodiacal light, 



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and the tails of comets, to be owing to a fimilar caufe. 

 See Tail of Comets, and Zodiacal Light. 



Ever fince the identity of hghtning and of the eleftric 

 matter has been afcertained, philofophers have been natu- 

 rally led to fcek the explication of aerial meteors in the prin- 

 ciples of eleftricity ; and there is now no doubt but mod 

 of them, and efpecially the aurora borealis, are electrical 

 phenomena. Belide the more obvious and known appear- 

 ances which conllitute a refemblance between this meteor 

 and the eleftric matter vi'hereby lightning is produced, it 

 has been obfervcd, that the aurora occafions a very fenlible 

 fliiftuation in the magnetic needle ; that the atmofphere 

 yields, at the time of its occurrence, a quantity of eleftric 

 fire ; and that, when it has extended lower than ufual into 

 the atmofphere, the flaflies have been attended with various 

 founds of rumbling and hifiing, already mentioned, and at- 

 tributed by Dr. Blagden (ubi fupra) to fmall ftrcams of 

 eleftric matter running off to the earth from the great 

 maffes, or accumulations, of eleftricity, ,by which he fup- 

 pofes both meteors and the noithern lights to be produced. 

 Befides, the aurora borealis may be partly imitated by means 

 of artificial eleftricity. J)r. Hamilton, of Dublin, (Phil. 

 Eff. efT. iii.) feems to have been the firft perfon who at- 

 tempted to difcover any pofitive evidence of the elcftrical 

 quality of the aurora boreahs ; but the only proof he pro- 

 duces is an experiment of Mr. Hawkfbee, by which the 

 eleftrical fluid is fhewn to affume appearances refembhng 

 the aurora borealis, when it pafTes through a vacuum. He 

 obferved, that when the air was moft perfectly exhaufted, the 

 ftreams of eleftric matter were then quite white ; but when a 

 fmall quantity of air was let in, the light afiiimed more of 

 a purple colour. The flaftiing of this light, therefore, 

 from the denfe regions of the atmofphere into fuch as are 

 more rare, and the tranfitions through mediums of differ- 

 ent denfities, he confiders as the caufe of tlie aurora bore- 

 ahs, and of the different colours it affumes. Mr. Canton, 

 foon after he had obtained eleftricity from the clouds, of- 

 fered a conjefture, that the aurora borealis is occafioned by 

 the flaftiing of eleftric fire from pofitive towards negative 

 clouds at a great diftance, through the upper part of the 

 atmofphere where the refiftance is leail. And he fuppofes, 

 that tiie aurora, which happens at the time whfn the mag- 

 neric needle is difturbed by the heat of the earth, is the 

 eleftricity of the heated air above it ; and this appears 

 chiefly in the northern regions, as the alteration in the heat 

 of the air in thofe parts will be the greateft ; nor is this hy- 

 pothefis, he faya, improbable, when it is confidered, that elec- 

 tricity is the known caufe of thunder and lightning ; that it 

 has been extrafted from the air at the tim.e of an aurora bore- 

 alis (fee Conden-eur) ; that the inhabitants of the northern 

 countries obferve itto be remarkably ftrong when a fudden 

 thaw fucceeds fevere cold weather ; and that the tourmalin 

 is known to emit and abforb the eleftric fluid only by the 

 increafe or diminution of its heat. Pofitive and negative 

 eleftricity in the air, with a proper quantity of moiilure to 

 ferve as a conduftor, will, he conceives, account for this and 

 other meteors fomttimes feen in a ferene ftf y. Mr. .Canton af- 

 terwards contrived to exhibit this meteor by means of the 

 TorrtceUian vacuum, 'in a glafs tube about three feet long, and 

 fealed hermetically. Vv'hen one end of the tube is held in the 

 hand, and the other applied to the conduftor, the whole 

 tube will be illuminated from end to end ; and will continue 

 luminous without interruption for a confiderablc time after 

 it has been removed from the conduftor. \i, aficr this, it 

 be drawn through the hand either way, the light will be 

 uncommonly intenfe, and without the leaft interruption, 

 from one hand to the other, even to its whole length. And 



though 



