AURORA 
Stow abqve-mentioned, an aurora borealis was obferved 
two nights fucceffively, viz. on the 14th and 15th of No¬ 
vember, with much the fame appearances as defcribed by 
Dr. Halley in 1716, and which we now fo frequently ob- 
ferve. Again, the fame was twice feen in Brabant, in 
the year 1575; viz. on the 13th of February and 2Sth 
of September. Its appearances at both thefe times were 
defcribed by Cornelius Gemma, profelfor of medicine in 
the univerfity of Lovain, who compares them to fpears, 
fortified cities, and armies fighting in the air. After this, 
Michael Masftlin, tutor to the great Kepler, allures us, 
that at Baknang, in the county of Wurtemberg, in Ger¬ 
many, thefe pihenomena, which he (files chajmata , were 
feen by himfelf no lefs than leven times in 1580. In 1381, 
they again appeared in an extraordinary manner in April 
and September, and in a lefs degree at fome other times 
of the fame year. In 1621, September 2d, this phenome¬ 
non was obferved all over France, and defcribed by Gaf- 
fiendus, who gave it the name of aurora borealis ; yet nei¬ 
ther this, nor any fimilar appearances pofterior to 1574, 
are defcribed by Englifti writers till the year 1707 ; which, 
as Dr. Halley oblerves, (hews the prodigious neglect of 
curious matters which at that time prevailed. From 1621 
to 1707, indeed, there is no mention made of an aurora 
borealis being feen by any body; and, conlidering the 
number of aftronomers who during that period were in 
a manner continually watching the heavens, we may very 
reafonably conclude that no fuch thing did make its ap¬ 
pearance till after an interval of eighty-fix yeaos. In 1707, 
a fmall one was feen in November; and, during that year 
and the next, the fame appearances were repeated five 
times. The next on record is that mentioned by Dr. Hal¬ 
ley, in March 1713-16, the brilliancy of which attracted 
univerfal attention, and by the vulgar was confidered as 
marking the introduction of a foreign race of princes. 
Since that time thofe meteors have been fo common, that 
no accounts have been kept of them. 
It was for a long time matter of doubt whether this me¬ 
teor made its appearance only in the northern hemifphere, 
or whether it was alio to be obferved near the fouth pole. 
This is now afeertained by Mr. Forfier, who, in his voy¬ 
age round the world with captain Cook, allures us, that 
he obferved them in the higheft fouthern latitudes, though 
with phenomena fomewhat different from thofe which are 
feen here. On February 17th, 1773, as they were in la¬ 
titude fifty-eight degrees fouth, “ A beautiful phenome¬ 
non was obferved during the preceding night, which ap¬ 
peared again for feveral fucceflive nights. It confided of 
long columns of a clear white light, (hooting up from the 
horizon to the eaffward, almoft to the zenith, and gra¬ 
dually fpreading on the whole fouthern part of the fky. 
Thefe columns were fometimes bent fidewife at their up¬ 
per extremities; and, though in molt refpeCts fimilar to 
the northern lights of our hemifphere, yet differed from 
them in being always of a whitilh colour, whereas ours 
affume various tints, efpecially thofe of a fiery and purple 
hue. The fky was generally clear when they appeared, 
and the air lharp and cold, the thermometer (landing at 
the freezing-point.” 
Dr, Halley obferved, that the aurora borealis defcribed 
by him arofe to a prodigious height, it being feen from 
the weft of Ireland to the confines of Ruffia and Poland 
on the eaft ; nor did he know how much further it might 
have been vilible : fo that it extended at leaft thirty de¬ 
grees in longitude, and from latitude fifty degrees north 
it was feen over all the northern part of Europe ; and, 
what was very furprifing, in all thofe places where it was 
vifible, the fame appearances were exhibited which Dr. 
Halley obferved at London. He obferves, with feeming 
regret, that he could by no means determine its height, 
for want of obfervations made at different places. Father 
Bofcovich has, however, determined the height of an 
aurora borealis, which was obferved by the marquis of 
Polini the 16th of December, 1737, and found it was 823 
miles high ; and Mr. Bergman, from a mean of thirty 
BOREALIS. S7E 
computations, makes the average height of the aurora bo¬ 
realis amount to 70 Swediffi, or 4^9 Englifli, miles. But 
Euler ftippofes the height to be feveral thoufands of miles ; 
and Mairan alfo alligns to them a very elevated region. 
Many attempts have been made to determine the caufe 
of this phenomenon. Dr. Halley imagines that the wa¬ 
tery vapours, or effluvia, exceedingly rarefied by fubter- 
raneous fire, and tinged with fulplntreous dreams, which 
many naturalifts have fuppofed to be the caufe of earth¬ 
quakes, may alfo be the caufe of this appearance ; or that 
it is produced by a kind of fubtile matter, freely pervading 
the pores of the earth, and which, entering into it nearer 
the louthern pole, gaffes out again with (ome force into 
the aether, at fome diftance from the northern. This fub¬ 
tile matter, by becoming more denfe, or having its velo¬ 
city increafed, may perhaps be capable of producing a 
fmall degree of light, after the manner of effluvia from 
eleCtric bodies, which, by a ftrong and quick friction, emit 
light in tire dark ; to which fort of light this feems to 
have a great affinity. Phil.Tranf. No. 347. SeealfoMr. 
Cotes’s defeription of this phenomenon, and his method 
ofexplaining it, by dreams emitted from the heterogeneous 
and fermenting vapours of the atmofphere, in Smith’s Op. 
tics, p. 69; or Phil. Tranf. abr. vol. vi. part 2. 
The celebrated M. de Mairan, in an exprefs treatife oil 
the aurora borealis, publifhed in 1731, fuppofes its caufe 
to be the zodiacal light, 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 atmofphere 
about the limits where univerfal gravity begins to act 
more forcibly towards the earth than towards the fun, 
falls into our air to a greater or lefs depth, as its fpecific 
gravity is greater or lefs, compared with the air through 
which it paffes. However, M. Euler thinks the caufe of 
the aurora borealis not owing to the zodiacal light, as 
M. de Mairan fuppofes; but to particles of our atmo¬ 
fphere, driven beyond its limits by the impulfe of the lo- 
lar light. And on this fuppolition he endeavours to ac¬ 
count for the phenomena obferved concerning this light. 
He fuppofes the zodiacal light, and the tails of comets, 
to be owing to a fimilar caufe. 
But ever fince the identity of lightning and the eleCtric 
matter has been determined, philofophers have been na¬ 
turally led to feek for the explication of aerial meteors in 
the principles of electricity ; and there is now no doubt 
but moll of them, and efpecially the aurora borealis, are 
eleftrical phenomena. Belides the more obvious and 
known appearances which conftitute a refemblance be¬ 
tween this meteor and the eleCtric matter by which light¬ 
ning is produced, it has been obferved, that the aurora 
occalions a very fenfible fluctuation in the magnetic needle; 
and that, whenever it has extended lower than ufual in 
the atmofphere, the fiafhes have been attended with va¬ 
rious founds of rumbling and biffing, efpecially in Ruffia 
and the other more northern parts of Europe ; as noticed 
by Sig. Beccaria and M. Meflier. Mr. Canton, foon after 
he had obtained electricity from the clouds, offered a 
conjecture, that the aurora is occafioned by the daftiing 
of eleCtric fire pofitive towards negative clouds at a great 
diftance, through the upper part of the atmofphere, where 
the refiltance is leaft: and he fuppofes that the aurora, 
which happens at the time when the magnetic needle is 
difturbed by the heat of the earth, is the eleCtricity of the 
heated air above it; and this appears chiefly in the north¬ 
ern regions, as the alteration in the heat of the air in thofe 
parts is the greateft. Nor is this hypo&helis improbable, 
when it is confidered, that eleCtricity is the caufe of thun¬ 
der and lightning; that it has been extracted from the air 
at the time of the aurora borealis ; that the inhabitants of 
the northern countries obferve it remarkably ftrong when 
a fudden thaw fucceeds very cold fevere weather; and 
that the tourmalin is known to emit and abforb the elec¬ 
tric fluid only by the increafe or diminution of its heat. 
Pofitive and negative eleCtricity in the air, with a proper 
quantity of moiftuie to fepve A a conductor; will account 
