The Grenada sentinel. [volume] (Grenada, Miss.) 1868-1955, 1869-06-12
Theories Of the Aurora BOreaIIs.

that fool and his money are soon |
parted. -NsO Orleans Picayuae, May 27 |
Theories of the Aurora BOrealiS.
THE Polar Light is a light which is fre. |
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 "AurQra 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 a 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 eov-
ers the entire heavens, the whole sky
seems filed with dense haze and in still
higher latitudes, 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 simulta-
neously over large portions of the globe.
The aurora Of August %, 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
3, 1850, was seen at the Sandwich Islands;
it was seen throughout the whole of North
America and Europe and the disturbance
of the magnetic needle indicated its pres
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 25, 1841, and November II,
| 1643, 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-de6ned border are
generally less than 100 miles in height.
Auroras are very unequally distributed
over the earths surface. They occur
most frequently in the higher latitudes,
and are almost unknown within the trop-
| les. At Havana, in latitude % degrees,
but six auroras have been recorded within
hundred years, and south Of Havana
auroras are still more unfrsquent. 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
suroras annually. Near the parallel Of
40 degrees the average number is twenty
annually near As degrees the number is
forty; and near the parallel of 30 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 brillisnCy as we advance toward
the pole. Beyond latitude 81 degrees the
average number Of auroras is reduced tt
) forty annually. Beyond latitude 81 de
grees it is reduced to twenty; and neal
! latitude rs degrees it is reduced to ten an
nually.
| Auroral exhibitions take. place in tn
upper regions of the atmosphere, sine
they partake of the earth's rotation. Al
: the celestial bodies have an apparent mo
tion from east to west, arising from the rota
( ton of the earth but bodies belonging Il
! the earth, including the atmosphere and tn
!. clouds which float in 1t, partake Of tn
earth's rotation, so that their relative posi
i ton is not affected by it. The same is tru
of auroral exhibitions. Whenever tn
q auroral corona is formed, il maintains sen
! sibly the same position in the heaven
4 during the whole period Gifts continuance
!. although the stars meanwhile revolve q
the rate Of 15 degrees per hour
a The grosser part of the earth's atmol
4 phere Is limited to a moderate distanc
from the earth. at the height Of littl
4 over four miles, the density Of the air
3 only one. half what it is at the earth's sui
: face. At the height Of fifty miles the a
!. mosphere is well-nigh inappreciable in Il
f effect upon twilight The phenomena {
!. lunar eclipses indicate an appreciable q
\ mosphere at the height Of sixty-six mile
4 The phenomena Of shooting stars indieai
\ an atmosphere at the height Of 200 OF sc
miles, while the aurora indicates that th
atmosphere does not entirely cease at Il
height Of 500 miles. Auroral exhibition
take place, therefore, in an atmosphere
4 extreme rarity 30 rare, indeed, that
|. ID experiments with an air.pump, v
q could exhaust the air as completely, v
II should say that we had obtained a perfe
4 vacuum.
q The auroral beams are simply spac
Il which are illuminated By the flow of ele
b tricity through the upper regions Of n
Il atmosphere. During the auroras OF is
Is these beams were nearly 500 miles
b length, and their lover extremities we
Is elevated about 45 miles above tie eartl
Q. surface. Their tops inclined toward {
4. south about N degrees III the neighbt
/. hood Of New York, this being the positi
ii whieh the dipping needle there assumed
d PrqfsssoF LO6mis, IN Hg'ptT's Aggasi
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