The second general class of Aurorae are those which propagate themselves
from different points of the compass, between north and west; towards the
opposite points ; sometimes also originating in the S.E. quarter, and extending
themselves towards the N.W. They may be subdivided, like the former,
into the distant arches,' which pass to the southward without much ‘visible
change in their appearance ; and those which discover beams, and separate at
intervals into wreaths, flashes, and irregular segments, exhibiting all the phenomena
described above.: In explaining the mode by which the two general
classes of Auroras are conducted into the horizon, I shall call the motion of the
arches (which is in a plane seldom deviating more than two points from the
magnetic meridian) the direct motion ; and that by which the Aurorae propagate
themselves nearly at right angles to the magnetic meridian, the lateral
motion. Let us suppose a mass of Aurora to be modelled at its birth, in a
longitudinal form crossing the meridians at various angles ; the whol^j to be
impelled with a direct motion towards the magnetic south, but the parts
having different velocities, and each extremity continually removing itself, by
a lateral motion, from the centre, so as to increase the length of the mass.'' If
the centre enter the northern horizon, it will appear like an arch, the real extremities
being invisible; and its direct motion will carry it to the southward
in that form. But if one extremity first enter the horizon, it will extend itself,
by its lateral motion, to the opposite point, passing, at the same time, by its
direct motion to the southward.
On the unequal velocities of the parts, we had many instances, in the direct
motion of the arches, by which the centres were often carried from the zenith
60° or 70° to the southward, while the extremities did not materially alter
their positions. Nor can this be accounted for by any application of the
rules of perspective, because arches exactly similar sometimes changed the
bearings of their extremities in proportion to the advance of their centres ;
and at Cumberland-House, on two occasions, the extremities of arches arrived
at the east and west points of the compass, while their centres remained only
10° above the northern horizon.
There are two other forms of the Aurora which cannot be classed in either
of these descriptions, and the occurrence of which is not frequent.
The first-is a small mass, much elevated above the horizon, sometimes exhibiting
brilliant beams, and then disappearing or passing to the southward.
It has, perhaps, been detached by violent motion from a body below the
horizon. The second is anarch from-north to south. In it the lateral and
direct motions are in the same direction. The flashes and beams are, in my
opinion, generally inclined in the direction of the dipping needle. Their perspective
must be considered in determining this; for, near the magnetic
meridian, they appear perpendicular to the horizon; but when at some distance
to the eastward or westward, they decline towards the perpendicular at
different angles.
The coronae, and other wreaths of the Aurora, sometimes approached so
near, that the beams appeared under an angle of 10° or 12°. No difference
was then distinguishable between them and the flashes before described,
except that the time of their existence was shorter.
The colours, shown by the beams when in rapid motion, were pea-green at
the upper extremities, and purple and violet intermixed at the lower. (It
will be remembered that these were the three colours which Mr. Morgan
produced by introducing an electric spark into an exhausted glass bulb.)
On one occasion, the lower colour was orange, which last was, in every
instance, the prevailing colour at Cumberland-House.
On the 8th of March, 1821, at 5h. 30m. p. m., immediately after sunset,
an arched Aurora was visible, extending from N.W. to S.E.b.S. This was
the earliest period of the day at which we saw it; for although it might, from
the shortness of the days in December and January, have been seen at 3 p.m.,
if present, it seldom appeared before 7 p. m., and was usually most brilliant
at midnight. On the 11th of February, the clouds formed a regular arch
extending N.N.W. to E.; and the needle of a compass, fixed in the house
for the purpose of making observations, receded.20' from the magnetic
meridian to the westward. I saw these clouds disperse, and afterwards
collect in a different form. The disturbance of the compass, is another proof
of the presence of the Aurora during the day ; but, on the whole, there is
reason to conclude that such is not often the case. On the 13th of November
the Aurora was seen, between the clouds and the earth, by Mr. Franklin and
Dr. Richardson. On the 13th of March, I saw an Aurora, which was emanating
in wreaths from the N.W., pass over the lower surface of a stratum of
white clouds. The upper edge of the clouds was 80 feet distant from the lower,
and its Azimuth, S. 35° W. The Aurora passed at the altitude of 70° and