£j 2 A U R 
for this and other meteors, fometimes feen in a ferene fky. 
Mr. Canton has fince contrived to exhibit this meteor by 
'means of the Torricellian vacuum, in a glafs tube about 
three feet long, and fealed hermetically. When one end 
of the tube is held in the hand, and the other applied to 
the conductor, the whole tube will be illuminated from 
end to end, and will continue luminous without interrup¬ 
tion for a confiderable time after it has been removed from 
the conductor. If, after this, it be drawn through the 
hand either way, the light will be remarkably intenfe 
through the whole length of the tube. And, though a 
great part of the electricity be difcharged by this opera¬ 
tion, it will (till flafli at intervals, when held only at one 
extremity, and kept quite (till; but if, at the fame time, 
ft be grafped by the other hand in a different place, (trong 
flaihes of light will dart from one end to the other; and 
thefe will continue twenty-four hours or more, without a 
frefii excitation. Sig. Beccaria conjectures, that there is 
aconflant and regular circulation of the eleCfric fluid from 
north to fouth;"and he thinks that the aurora borealis 
may be this elcCtric matter performing its circulation in 
fuch a date of the atmofphere as renders it vifible, or ap- 
roaching nearer than ufuai to the earth. Though pro- 
ably this is not the mode of its operation, as the meteor- 
is obferved in the fouthern hemifphere with the fame ap¬ 
pearances as in the northern. Dr. Franklin fuppofes, that 
the eleClric fire difcharged into the polar regions, from 
many leagues of vaporifcd air raifed from the ocean be¬ 
tween the tropics, accounts for the aurora borealis; and 
that it appears fird where it is fird in motion, namely, in the 
mod northern part; and the appearance proceeds fouth- 
ward, though the fire really moves northward. Mr. Kir- 
wan, in the TranfaCtions of the Royal Irifli Academy, 
anno 1788, has fome ingenious remarks on the aurora bo¬ 
realis aujlralis. He gives his reafons for fuppofing the 
rarefaction of the atmofphere in the polar regions to pro¬ 
ceed from them, and thefe from a combudion of inflam¬ 
mable air caufed by elcClricity. He obferves, that after 
an aurora borealis the barometer commonly falls, and high 
winds from the Couth generally follow. 
Mr. John Dalton, profeflbrof natural philofophy in the 
new college, Mancheder, has lately publiflied a treatife on 
the aurora borealis, the phenomena of which he has elafled 
and characterized thus : “ The appearances of the aurora 
come under four different deferiptions. Fird, a horizon¬ 
tal limit, like the morning aurora, or break of day. Se¬ 
cond,’fine, (lender, luminous, beams, well defined, and of 
denfe Imht; thefe continue a quarter, half, or one whole, 
minufe, D lometimes at red apparently, but oftener with a 
quick lateral motion. Third, fiaflies pointing upward, 01- 
in the fame direction as the beams, which they always 
fucceed ; thefe are only momentary, and have no lateral 
motion, but tl ey are generally repeated many times in a 
minute ; they appear much broader, more diffufe, and of 
a weaker light, than the beams; they grow’gradually fainter 
till they difappear : thefe fometimes continue for hours, 
fhifhing at intervals. Fourth, arches, nearly in the form 
of rainbows ; thefe, when complete, go quite acrofs the 
heavens, from one point of the horizon to the oppofite 
point. When an aurora takes place, thofe appearances 
feem to fucceed each other in the following order: fird, 
the faint rainbow-like arches; fecond, the beams; and, 
third, the flafnes : as for the northern horizontal light, it 
will appear to confid of an abundance ot flafhes, or beams, 
blended together, owing to the fmiation of the obferver 
relative to them. 
The luminous beams of the aurora borealis, are cy- 
lindrical, and parallel to each other, at lead over a mode¬ 
rate extent of country. The cylindrical beams of the au¬ 
rora are all magnetic, and parallel to the dipping-needle 
at the places over which they appear. And, as the beams 
‘are fwimming in a fluid of equal denfity with themfelves, 
they are in the fame predicament as a magnetic bar, or 
needle, fwimming in a fluid of the fame fpecific gravity 
with itfelf; but this lad will only reft in equilibrio when 
A U R 
in the direction of the dipping-needle, owing to what is 
called the earth’s magnetifm ; and, as the former alfo reds 
in that pofition only, the eff'eCts being limilar, we mud, 
by the rules of philofophizing, aferibe them to the fame 
caufe. Hence, then, it follows, that the aurora borealis 
is a magnetic phenomenon, and its beams are governed 
by the earth’s magnetifm. The height of the rainbow- 
like arches of the aurora, above the earth’s furface, is 
about 150 Englifii miles. The beams of the aurora are 
dmilar and equal in their real dimenfions to one another. 
The didance of the beams of the aurora from the earth’s 
furface, is equal to the length of the beams, nearly. That 
appearance which w’e have called the horizontal light, and 
which is always fituate near the horizon, is nothing but 
the blended lights of a group of beams, or fiaflies, which 
makes the appearance of a large luminous zone. 
“ We are under the necedity of confidering the beams 
of the aurora borealis of a ferruginous nature, becaufe 
nothing elfe is known to be magnetic, and confequentiy, 
that there exifls in the higher regions of the atmofphere 
an eladic fluid partaking of the properties of iron, or ra¬ 
ther of magnetic decl, and that this fluid, doubtlefs from 
its magnetic property, alfumes the form of cylindric beams. 
It fhould feem too, that the rainbow-like arches are a fort 
of rings of the fame fluid, which encompafs the earth’s- 
northern magnetic pole, like as the parallels of latitude 
do the other poles ; and that the beams are arranged in 
equididant rows round the fame pole. At fird view, in¬ 
deed, it feems incompatible with the known laws of mag¬ 
netifm, that a quantity of magnetic matter (hould afliime 
the form of fuch rings, by virtue of its magnetifm ; but 
it may take place in one cafe at lead, if we fuppofe the 
rings fituate in the middle, between two row’s of beams, 
fo that the attraction on each fide may be equal. As for 
the beams, in their natural date, they mud all be guided 
by the earth’s magnetifm, (I mean the caufe that guides 
the needle, whether it is in the earth or air I know not,) 
and confequentiy all have their north poles downward ; 
but whether any two neighbouring beams have the poles 
of the fame denomination, or of different denominations,, 
aiding upon each other, dill the effect will be the fame, 
and their mutual aCtion upon each other not diflurb their 
parallelifm, nor the pofition of the rings; becaufe, whe¬ 
ther the poles mutually attraCt or repel each other is of 
no moment in this cafe, and the attraction of each pole is 
alike upon the rings. Things being thus dated, I more¬ 
over fuppofe, that this eladic fluid of magnetic matter is,, 
like vaporized air, an imperfect conduCter of eleCtricity; 
and that, when the equilibrium of eleCtricity in the higher 
regions of the atmofphere is didurbed, 1 conceive that it 
takes thefe beams and rings as conductors, and runs along 
from one quarter of the heavens to another, exhibiting all 
the phenomena of the aurora borealis. The reafon why 
the diffufe fiaflies fucceed the more intenfe light of the 
beams is, I conceive, becaufe tlie eleCtricity difperfes the 
beams in fome degree, which colleCt again after the elec, 
trie circulation cea(es.” 
We mud leave to the future obfervers on the heavens 
the talk of ferutinizing the feveral parts of this theory.: 
but, meanwhile, we mud do Mr. Dalton the juflice to 
oblerve, that he has rendered the mechanifm which lie 
deferibes, or the exiflence of parallel beams, far from 
improbable. From the comparifon of his obfervations, 
profefTor Dalton finds the aurora to be a fign of fair wea¬ 
ther ; a faCt which, if confirmed, militates againfl the 
fuppofition that this meteor arifes from the combudion of 
hydrogene air. 
AUROS', a town of France, in the department of the 
Gironde, and chief place of a canton, in the diftriCf of 
Bazas, five miles north-ead of Bazas. 
AUROUX', a town of France, in the department of 
the Lozerre, and chief place of a canton, in the didriCI of 
Langogne, feventeen miles north of Mende. 
AU'RUM, f. [aGr. aur, Welch ; Hypficrates 
fays, from Aurus its inventor; probably from -ntt Heb. 
3 refplendency.] 
