Let us now examine another phenomenon. The Aurora Borealis has been
generally considered to be in some way connected with the magnetism of
the earth, and with the position of the magnetic pole. It is certain
that the appearance of this meteor does affect the needle in a way not
to be mistaken, and (although not invariably) the vertex of the luminous
arch will usually conform to the magnetic meridian. Yet (and this is
worthy of attention), the observations made in the North Polar
Expeditions[29] "appear to prove, that in the immediate vicinity of the
magnetic pole the development of light is not in the least degree more
intense or frequent than at some distance from it." In fact, as the
American magnetic pole is, as stated, in latitude 73d, the central
vortex will seldom reach so high, and, consequently, the aurora ought
at such times to be more frequent in a lower latitude. In a late work by
M. de la Rive, this gentleman expresses the opinion, that the cause of
the aurora is not due to a radiation of polar magnetism, but to a purely
electrical action.[30] His explanation, however, is not so satisfactory
as his opinion. Now, we have examined numerous cases of auroral
displays, and never yet found one which could not be legitimately
referred to the action of ethereal vortices. Generally, the aurora will
not be visible, when the upper surface of the atmosphere of that
latitude in which the vortex is known to be (reckoning in the direction
of the magnetic meridian) is below the horizon, which shows that the
brightest portion is in the atmosphere. In latitude 41d even, it may
show itself when the vortex is three days north, more frequently when
one or two days north; but when the vortex passes centrally, or south,
it rarely is seen, and this is the only difficulty in explaining it by
the theory. But, when we reflect that the ether shoots out in straight
lines, and at an angle corresponding to the magnetic dip, we are at no
loss to perceive the reason of this. If each minute line composing the
light were seen endwise, it would be invisible; if there were millions
such in the same position, they could add nothing to the general effect;
but, when viewed sideways, the case would be different, there would be a
continued reduplication of ray upon ray, until in the range of some
hundreds of miles an effect might be produced amounting to any degree of
intensity on record. Now, this is the case when the aurora is
immediately overhead, it will be invisible to those below, but may be
seen by persons a hundred miles south; so, also, when it is to the
south, it is too oblique to the line of vision to be seen, especially as
all the rays to the northward of the observer can contribute nothing to
increase the effect. That it is of the nature of rays very much
diffused, can hardly be doubted; and, therefore, if only of a few miles
in depth, its impressions are too faint to be sensible. By referring to
the record of the weather in the second section of this work, an auroral
display will be found on July 12th, the central vortex having passed a
little to the northward the same evening, and the next day passing south
_descending_. On that occasion the author saw an inclined column, in
profile, due east, and between himself and a line of bluffs and timber,
about eight miles distant; And, he has not any doubt that the mass of
rays began where he stood. As in a shower, every drop, passing through a
conical surface, whose axis passes through the sun and through the eye,
contributes to form the apparently distant rainbow.
The altitude of this meteor has been much exaggerated, especially of
those rings or luminous arches, which are often detached completely from
the luminous bank. On the 24th of May, a bright aurora was visible at
Ottawa, but the author's attention was engrossed by the most brilliant
arch of light he had ever seen. It was all the time south of the zenith,
and had no visible connection with the aurora north. At 9 hours, 59
minutes, 30 seconds mean solar time, Arcturus was in the exact centre of
the band, at which time it was very bright, and full 7d wide. At the
same time, Prof. G.W. Wheeler observed the aurora in Perryville, in the
State of Missouri, only 1d of longitude to the westward, but did not see
the arch.[31] The difference of latitude between the two places being 3d
30', and the weather, as he states, clear and still, there is only one
reason why he did not see the arch: it must have been too _low_, and had
become merged in the bank of light. At the time mentioned, the altitude
of Arcturus was 68d 30', and, as Prof. Wheeler assigns only 10d as the
altitude of the bank, the maximum elevation of the arch, on the
supposition of its composing a part of the bank, was 43 miles. At
Perryville, the bank and streamers had disappeared at 10 o'clock. At
Ottawa, the arch or bow disappeared at 10 h. 5 m., differing only the
fraction of a minute from the time at Perryville; but, the bank was
still visible, but low and faint, the greatest altitude having been over
30d. To show the rapid fluctuations in width and position of this bow,
we will add a few of the minutes taken at the time with great care, in
hopes some other observer had been equally precise. When first seen,
there were three luminous patches, or elongated clouds of light; one in
Leo, one in Bootes, and another in Ophinchus, all in line. This was
about 9h. 15m. The times following are correct to 30 seconds:
9h. 42m. 30s. Bow complete; south edge 2d north of Arcturus.
9 45 30 Northern edge diffuse south; edge bright, and well
defined; 10d wide in zenith; north edge on Alphacca.
9 47 30 South edge 5d north of Arcturus; north edge close to
Cor. Caroli.
9 53 30 Eastern half composed of four detached bands
_shingling_ over each other.