Urbana union. [volume] (Urbana, Ohio) 1862-1872, 1869-06-02

]
has remained unsupported, even by some
Ofthe oldest and most intelligent inhab-
itants.-EfOH1 the Chicago Tribune
Theory of AurOras
The Polar light is a light which is fire.
quentlY seen near the horizon, bearing
some resemblance to the morning twi-
light, whence it has received the name
OfauTOra. In the northern hemisphere
it is usually termed "aurOra bOrealis, y
because it it chiefly seen in the north.
A similar phenomenon is also seen i'll
theSOuthern hemisphere, where it is call
1 ed aurora AustTalis." Each of them
may, with great prOpriety, be called eau
rOrO POlaTiS," :" or Polar Light. They eX-
hibit an endless variety of appearances.
In the United States an aurora is uni
fOTmIy preceded by iazy or slaty ap-
| pearance of the sky, particularly in
the meighbOrhOOd of the northern hOr-
izen. When the aurOraI display COM
mences, this haZy portion of the sky as
sUmeS the form of a dark bank or seq
ment of a circle in the north, rising
ordinarily to the height of from five to
/ ten degrees. This dark segment is not
a cloud, for the stars are seen through
/ it as through a smoky atmosphere
with little diminutiOn of brilliancy
| This dark bank is simply a dense
haze, and itappeaTs darker from the con-
| trast with the luminous arc which rests
upon it.
! In high northern Iatitudes, when the
| aurora covers the entire heavens, the
whole sky seems filled with a dense haze;
and in still higher latitudes, where the
/ aurora is sometimes seen in the south,
! this segment is observed resting on the
| southern horizon ana bOrdered by the
auroraI light.
! AurOraS are sometimes observed sim
ultaneOusly over large portions of the
| globe. The aurora of August 2S 1859,
was seen throughout more than 40 de-
gTees of longitude, from Jamaica on the
south to an unknown distance in British
| America on the north. The aurora of
! September ,, 1859, was seen at the Sand
wich Islands; it was seen throughout
| the whole OfNOrth America and Europe;
and the disturbance of the magnetic
needle indicated its presence throughout
all Asia, although the sky was overcast,
| so that in many places it could not be
seen. An aurora was seen at the same
| time in South America and New HOll-
and. The auraras of September 25, 1871,
and November II, 1913, were almost
| equally extensive.
| The height of a large number of aurO-
ras has been computed, and the average
result for the upper limit the streamers
| is 450 miles. From a multitude of OBJ
servatiOns it is concluded that the aurora
seldom appears at an elevation less than
about 45 miles above the earth's surface,
and that it frequently extends upward to
an elevation of 500 miles. AUrOral arches
| having a well defined border are geneT-
alIy less than 100 miles in hight.
| AUrOTas are very uneqnally distribu-
/ ted over the eaTth's surface. They Oc-
cur must frequently in the higher lati-
| tudes, and are almost unknown within
/ the tropics. At Havana, in latitude %
| degrees, but six aurOTas have been recOr-
ded within a hundred gears, and south
| of Havana auTOras are still more Unfre-
qUent. AS we travel nOTthward from
| Cuba, auTOras increase in frequency and
briIIiancy; they rise higher in the heav-
| ems, and oftener ascend to the zenith.
| Near the parallel of 40 degrees we find
on an everage only ten aurOras annually.
Near the parallel of AZ degrees the aver
age number is twenty annually; near As
degrees the number is forty; and near
the parallel Of 50 degrees it amounts to
eighty annually. Between this point
and the parallel of 63 degrees aurOras,
during the winter, are seen almost every
night. They appear high in the heav-
ems, and as often to the south as to the
north. In regions further north they
are seldom seen eXceptin the south, and
from this point they diminish in frequen-
cy and brilliancy as we advance toward
the pole. Beyond latitude 81 degrees
the average number of aUrOTas is redu-
ced to forty annually. Beyond latitude
81 degrees it is reduced to twenty; and
near latitude rs degrees it is reduced to
ten annually.
AUrOral exhibitions take place in the
upper regions Of the atmosphere, since
they partake Ofthe earth's rotation. All
the celestial bodies have an apparant
motion from east to west, arising from
the rotation of the earth; but bodies be-
longing to the earth, including the at-I
mOsphere and the clouds which float in
it, partake of the earth's rotation, so that
their relative position is not affected by
it. The same is true of aTOTal exhibit
tiOns. Whenver an aurOTal corona is
formed, it maintains sensibly the same
position in the heavens during the whole |
period Of its continuance, although the
stars meanwhile TevOlve at the rate of
Is degrees per hour.
The grOSser part of the earth'S atmOs- |
phere is limited to moderate distance |
from the earth. At the height of lit |
tie over four miles, the density of the air /
is only One-half what it is at the earth's |
surface. AT the height of 80 miles the
atmosphere is Well-nigh inappreciable
i'll its effect upon twilight. The phenOm-
ena of lunar eclipses indicate an appre-
ciable atmosphere at the height of 66
miles. The hephenOmena OfshOOting-stars
an atmosphere at the height of
200 or 800 miles, while the aurora indi-
Gates that the atmosphere does not en-
tiTely cease at the height of 500 miles,
AurOral exhibitions take place, there
fore, in an atmosphere ofeXtreme rarity;
so rare indeed that if, in experiments
with an air-pump, we could exhaust the
air as completely, we should say that
we had obtained perfect vacuum. II
The auTOral beams are simply spaces
which are illumed by the flow of cleo /
tricity through the upper regions Ofthe /
During the aUTOras of 1850 I
these beams were nearly 500 miles in
length, and their lower eXtTemities were
elevated about 45 miles above the sur :
face. Their tops inclined toward the :
south, about II degrees in the neighhor- \
hood OfNew York, this being the position +
which the dipping-needle there assu- +
nes.-PrOfessOr LOOmis, i'll HdyPcJ's
Magazine. =