Perrysburg journal. (Perrysburg, Wood Co., O. [Ohio]) 186?-1965, 1869-06-11
Theories of the Aurora Borealls.

Theories of the Aurora BOrealls.
THE Polar Light is light which is fire.
quently seen near the horizon, bearing
some resemblance to the morning twilight,
whence it has received the name of Au-
rora In the northern hemisphere it is
usually termed aurora Borealis," be-
cause it is chiefly seen in the north. A
similar phenomenon is also seen in the
southern hemisphere, where it is called
aurora Australis." Each Of them may,
with greater propriety, be called "Aurora
Polaris," or Polar Light, They exhibit an
endless variety of appearance. In the
United States an aurora is uniformly pre-
ceded by hazy or slaty appearance of the
sky, particularly in the neighborhood of
the northern horizon. When the auroral
display commences, this hazy portion of
the sky assumes the form of dark bank
or segment of circle in the north, rising
ordinarily to the height Of from five to
ten degrees. This dark segment is not
cloud, for the stars are seen through it as
through smoky atmosphere, with little
diminution of brilliancy. This dark bank
is simply a dense haze, and it appears
darker from the contrast with the lumi-
nous arc which rests upon it. In high
northern latitudes, when the aurora coy
ers the entire heavens, the whole sky
seems filled with dense haze and in still
higher ATTITUDES, where the aurora is some.
times seen in the south, this dark segment
is observed resting on the southern hori-
zon and bordered by the auroral light
Auroras are sometimes observed simuta-
neously over large portions of the globe.
The aurora of August 2S, 1859, was seen
throughout more than 140 degrees of
longitude, from Eastern Europe to Cali
fornia; and from Jamaica on the south to
an unknown distance in British America
on the north. The aurora of September
g 1859, was seen at the Sandwich Islands;
it was seen throughout the whole of North
America and Europe and the disturbance
ence throughout all Northern Asia, al.
though the sky was overcast so that at
many places it could not be seen. An
aurora was seen at the same time in South
America and New Holland. The auroras
of September 20, 1871, and November II,
1848, were almost equally extensive.
The height of large number of auroras
has been computed, and the average re-
suit for the upper limit of the streamers
is 450 miles. From multitude of obser-
vations it is concluded that the aurora sell
dom appears at an elevation less than
about forty-five miles above the earth's
surface, and that it frequently extends up
ward to an elevation of 500 miles. Auroral
arches having well.defined border are
generally less than 100 miles in height.
Auroras are very unequally distributed
over the earth's surface. They occur
most frequently in the higher latitudes,
and are almost unknown within the trop-
les. At Havana, in latitude 23 degrees,
but six auroras have been recorded within
hundred years, and south of Havana
auroras are still more unfrequent. AS we
travel northward from Cuba, auroras in
crease in frequency and brilliancy; they
rise higher in the heavens, and oftener as.
cend to the zenith. Near the parallel of
40 degrees we find on an average only ten
auroras annually. Near the parallel of
AZ degrees the average number is twenty
annually: near 45 degrees the number is
forty; and near the parallel of 80 degrees
it amounts to eighty annually. Between
this point and the parallel Of 81 degrees
auroras, during the winter, are seen al
most every night. They appear high in
the heavens, and as often to the south as
the north. In regions further north they
are seldom seen except In the south, and
from this point they diminish in frequen-
cy and brilliancy as we advance toward
the pole Beyond latitude 62 degrees the
average number of auroras is reduced to
forty annually. Beyond latitude 81 de.
grees it is reduced to twenty; and near
latitude 78 degrees it is reduced to ten an-
nually.
Auroral exhibitions take place in the
upper regions of the atmosphere, since
they partake of the earth's rotation. All
the celestial bodies have an apparent mo-
tion from east TO west, arising from the rota-
tion of the earth; but bodies belonging to
the earth, including the atmosphere and the
clouds which float in 1t, partake of the
earth's rotation, so that their relative posi-
ton is not affected by it, The same is true
of auroTal exhibitions. Whenever the
auroral corona is formed, it maintains sen.
sibly the same position in the heavens
during the whole period of its continuance,
although the stars meanwhile reVolVe at
the rate of Is degrees per hour
The grosser part of the earths atmos-
phere is limited to moderate distance
from the earth. At the height of a little
over four miles, the density of the air is
only one.half what it is at the earth's sur
face. At the height of fifty miles the at.
mosphere is well-nigh inappreciable in its
effect upon twilight The phenomena OF
lunar eclipses indicate an appreciable at
mosphere at the height of sixty.six miles,
The phenomena of shooting stars indicate
an atmosphere at the height Of 300 or 800
miles, while the aurora indicates that the
atmosphere does not entirely cease at the
height of 500 miles. Auroral exhibitions
take place, therefore, in an atmosphere OF
extreme rarity; so rare, indeed, that if,
in experiments with an airpump, we
could exhaust the air as completely, we
should say thAt we had obtained perfect
vacuum.
The auroral beams are simply spaces
which are Illuminated by the flow of cleo
tricity Through the upper regions of the
atmosphere. During the auroras of 1859
these beams were nearly 500 miles in
length, and their lower extremities weze
elevated about 45 miles above the earth's
surface. Their tops inclined toward the
south about ~ degrees in the neighbor.
hood Of New York, thls being the position
which the dipping needle there assumed -
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