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thougli a great part of the ele<ftricity is difcliargcd by this 

 operation, it will flafh at intei-vals, uhen held only at one 

 . extremity, and kept qvite ftill ; but if it be grafpcd by the 

 other hand at the fame time in a different place, ftrone 

 fialhes of light will hardly ever fail to dart from one end to 

 the other, and thefe will continue twenty-four hours and 

 lotiger, without any frc(h excitation. An arched dou- 

 ble barometer of a confiderable lieight is an improvement of 

 this contrivance, for exhibiting the apptarancc of an aurora 

 borcali?, by means of the electric fire. To Mr. Cantop.'s 

 hypothefis it has been objected, that the citftncal fire would 

 flafh in every direction, according to the pofition of the 

 clouds, as Well as from north to fouth ; and that upon his 

 hypothefis, illi-drated by the tourmalin, the aurora borealis 

 ought to be moll frequent in fummer, when the air is mod 

 heated, whereas it is found to be the reverfe. Signior 

 Bcccaria, who purfued his obfervat:ons on atmofpherical 

 tleftricity farther than any of his affociates in thefe inquiries, 

 conjeftiires that there is a conllant and regular circulation 

 of the eleftric fluid from north to fouth, which may be the 

 original courfe of magnetifm in general ; and he thinks, 

 that the aurora boreahs mav be this eleiSric matter perform- 

 ing its circulation in fucb a ftatc of the atmcfphere as ren- 

 ders it vifible, or approaching nearer the earth than ufual. 

 Againft. this fuppofed circulation it has been alleged that 

 it ought to dart irom the horizon towards the zenith in the 

 northern hemifphere, and from the zenith towards the hori- 

 zon in the fouthern one ; whereas Mr. Forltcr, as we have 

 already mentioned, informs us, that the columns fliot up 

 from the horizon towards the zenith as well in the fouthern 

 hemifphere as in the northern ; fo that if the aurora boreahs 

 is to be regarded as the flafhings of elettric matter, its 

 courfe is plainly direfted from both poles towards the 

 equator, and not from one pole to the other. Why the 

 eleftricity of the atmofphere fhould be conftantly found to 

 direft its courfe from the poles towards the equator, and 

 not from the equator to the poles, fnggefts a difficulty 

 which an anonymous writer (Encycl. Brit.) has attempted 

 to folve in the following manner. — AfTuming three axioms ; 

 viz. that all eleftric bodies, when confiderably heated, 

 become conductors of eleftricity ; that, e convcrfo, non- 

 cleclrics when fubjefted to violent degrees of cold, 

 ought to become eledric ; and that cold mull alfo incrtafe 

 the eleftric powers of fuch fubftanccs as are already elec- 

 tric ; it is eafy (fays this writer) to deduce from thtfe prin- 

 ciples the caufe of the aurora boreahs. " The air, all 

 round the globe, at a certain height above its furface, is 

 found to be exceedingly cold, ?.nd as far as experiments 

 have yet determined, excedingly tltclrical alfo. The infe- 

 rior parts of the atmofphere between the tropics, are vio- 

 lently heated during the day-time by the relleflion of the 

 fun's rays from the earth. Such air will therefore be a kind 

 of condudlor, and much more readily part with its eleftri- 

 city to the clouds and vapours floating in it, than the colder 

 air towards the north and fouth poles. Hence the prodi- 

 gious appearances of eleftricity in thefe regions, (hewing 

 itfelf in thunder and other tempeils of the moil ten-ible 

 kind. Immenfe quantities of the eleftric fluid are thus com- 

 municated to tlie earth ; and the inferior warm atmofphere 

 having once exhaufted itfelf, mud necelTarily be recruited 

 from the upper and colder region. This becomes very proba- 

 ble fiom what the French mathematicians obfervcd wlien on 

 the top of one of the Andes. They were otten involved in 

 clouds, which, finking down into the warmer air, appeared there 

 to be highly eleftrified, and difcharged themfelvcs in violent 

 tempefls of thunder and lightning ; while in the mean time, 

 on the top of the mountain, they enjoyed a calm and fereue 



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%• In the temperate and frigid zones, the inferior psrtt 

 of the atmofphere, never being fo ftrongly heated, do not 

 part with their eleftricity fo cafily as in the torrid zone, 

 and coiifequently do not require fuch recruits from the up- 

 per regions ; but not with (landing the dih'erence of heat ob- 

 fervcd in dilTcrent parts of the earth near ti ,■ furface, it is 

 very probable that at confiderable heights the degrees of 

 cold are ncar'.y equal all round it. Were there a like equality 

 in the heat of the under part, there could never be any con- 

 fiderable lofs of equilibrium in the eleftricity of the atmo- 

 fphere ; but as the hot air of the torrid zone is perpetually 

 bringing down vaft quantities of eleftric matter from the 

 cold air that lies diredtly above it ; and as the inferior parts 

 of the atmofphere lying towards the noith and fouth poles 

 do not conduft in any great degree ; it thence lollows, that 

 the upper parts of the atmofpliere lying over the torrid 

 zone will continually require a fupply from the northern 

 and fouthern regions. This eafily fliews the ntceffity of an 

 eleftric current in th.e upper parts of the atmofphere from 

 each pole towards the equator : and thus we arc alfo fur- 

 nilhed with a rcafon why the aurora borealis appears more 

 frequently in winter than in fummer ; namely, btcaufe at 

 that time the eleftric power of the inferior atmofphere is 

 greater on account of the cold than in fummer ; and confe- 

 quently the abundant eleftricity of the upper regions muft 

 go almoft. wholly off to the equatorial parts, it being impof- 

 fible for it to get down to the earth : hence alfo the aurora 

 boreahs appears very frequent and bright in the frigid zones, 

 the degree of cold in tjie upper and under regions of the 

 atmofphere being much more nearly equal in thefe parts 

 than in any other. In fome pvts of Siberia particu- 

 larly, this meteor appears conftantly from Oftober to Chrift- 

 mas, and its corufcations are faid to be very terrifying. 

 Travellers agree, that here the aurora borealis appears in 

 greatefl; perfeftion ; and it is to be remarked, that Siberia 

 is the coldefl country on earth. In confirmation of this, it 

 may alfo be obferved, that, from the experiments hitherto 

 made v.ith the eleftrical kite, the air appears confiderably 

 more eleftrical in winter than in fummer, though the clouds 

 are known to be often mod violently eleftrified in the fum- 

 mer-time ; a proof, that the eleftricity naturally belonging 

 to the air is in funmier much more powei-ful'.y drawn ct? by 

 the clouds than in the winter, owing to the escefs of heat 

 in fummer, as already obferved. 



" A confiderable difficulty, however, ftill remains from 

 the upright pofition which the ftreams of the aurora bore- 

 alis are generally fuppofed to have ; whereas, according to 

 the hypothefis above mentioned, they ought rather to run 

 direftly from north to fouth. This difficulty occurred to 

 Dr. Halley ; but he anfwers it by fuppofing his magnetic ef- 

 fluvia to pafs from one pole to another in arches of great 

 circles, arifing to a vaft height above the earth, and confe- 

 quently darting from the places whence they arofe almoll 

 like the radii of a circle ; in which cafe, being fet off in a 

 direftion nearly perpendicular to the furface of the earth, 

 they muft neceftarily appear creft to thofe who fee them 

 from any part of the furface, as is de.monftnited by mathe- 

 maticians. It is alfo reafonable to think that tliey will take 

 this direftiion rather than any other, on account of thcij- 

 meeting with lefs refiftance in the very high regions of the 

 air than in fuch as are lower. 



" But the greateft difficulty ftill remains : for we hars 

 fuppofed the equilibrium of the atmofphere to be broken in 

 the day-time, and rcftored oiUy in the night ; whereas, con- 

 fidering the immenfe velocity with which the eleftric fluid 

 moves, the equihbrium ought to be reftored in all parts il- 

 moft inftantaneeufly ; yet the aurora boreahs never appear* 

 3 B 2 tueej't 



