Fuly 23, 1885] 
NATURE 
275 
increasing in force, and in a few moments a broad are of 
light stretches along the north-western sky, resting both 
its bases on the horizon in north-east and west, and whose 
highest point lies a few degrees above the horizon in 
north-north-west. Upwards the light is gradually lost in 
the sky, downwards the intensity is greatest, and the 
lower edge stands sharply out. Solitary, stronger spots 
of light, now here, now there, travel, with an unsteady 
motion, at times right or left through the arc, again to 
disappear in the cloud. Following the arc attentively it 
will be seen to rise gradually, its point of culmination 
travels upwards, and the distance between the two bases 
becomes greater and greater. The colour of the light is 
nearly white, with a weak yellow-green tinge, which is 
easily discovered by comparing it with the cold, white 
light of the Milky Way. Suddenly energy and life be- 
come «manifest inthe phenomenon. The lower edge of 
the arc changes in an instant into a small, intense stream 
of light, which is sharply defined by the dark space below 
—the ‘ dark segment ’—appearing black or faintly violet. 
Higher up the luminosity gathers into a broad, but fainter 
arc, running parallel with the other. Only for a moment 
does the aurora retain this distinct form ; stronger waves 
of light begin to appear in the lower arc, which soon 
generate groups of intense, short, and perpendicular 
streamers, reaching the upper arc, which sway right and 
left, at the same time travelling east or west. Below, the 
ends, strong in light, cut down into the dark segment, 
whose sharp curve is thereby broken. Of the upper arc 
only fragments now remain, while the lower is dissolved 
into quivering bunches of streamers, which die out one 
after another as new ones are being lit in their place. 
They move, here slowly, there quickly, oscillating appa- 
rently to the right or left, but it is impossible to say 
whether it is really the streamers which move horizon- 
tally, or merely the light which passes from streamer to 
streamer without the latter shifting their position. 
“ But this display is only of short duration, the streamers 
soon lose their motion and light, and in a few minutes 
there remain only some pale, diffuse luminosities. Slowly 
these now gather, until another arc is formed. It is not 
so symmetrical, and does not possess the classical rest of 
Fic. 1.—Auroral Arcs, 
Nordenskjéld. 4 
Fic, 2.—Auroral Ares, Nordenskjéld. 
the former. 
intensity until a fresh burst of streamers occurs; in 
two or three places the light shoots up into bunches of | 
long streamers between which the space is nearly dark ; 
in another second the streamers are isolated in groups, 
which, like the former, gracefully sway to and fro, their | 
faint points reaching nearly up to the Pole star. 
The | 
lower ends are broken at various heights, and develop as | 
they move the soft colours of the rainbow. The culminat- 
ing point in the display has been reached. The streamers 
vanish one by one, the light pales, and the remnants in 
the sky again form into a long, low-lying arc. Only for a 
short time it retains this pronounced form, the edges 
become obscure, the centre follows, and finally the last 
faint indications of the aurora sink into the unfathomable 
darkness of space.” 
Now the great variety in the appearance of the aurora | 
depends to a great extent upon the various mixtures of 
Constantly it changes form, position, and | certain component features. These have been designated 
auroral arcs, often very narrow, often degenerating into 
broad bands ; auroral streamers, single or multiple shafts 
of light of various colours, nearly always vertical in direc- 
tion, and long or short, with lateral and vertical motions ; 
the auroral corona, a brilliant point near the zenith, from 
which, in most brilliant displays, streamers seem to radiate 
in every direction, the heavens putting on the appearance 
of a bright ribbed dome; and, finally, auroral clouds, 
which are amorphous and most irregular in their dis- 
tribution. 
Before we proceed further with more detailed descrip- 
tions of these various features, each of which in the main 
is seen more richly from certain positions on the earth’s 
surface than in others, or puts on different aspects, a word 
must be said about the magnetic basis of the whole phe- 
nomenon, since it has long been known to be connected 
with the #zagnetic poles of the earth. y 
In the first place, the mariner’s compass or decli- 
