At Jan Mayen (8) in 1882-1883, out of 177 arcs whose position was accurately determined, 44 were seen in the north, their summits averaging 38.5° above the northern horizon; 88 were seen in the south, their average altitude above the southern horizon being 33.5°; while 45 were in the zenith. At Tasiusak (10) in 1898-1899 the magnetic directions of the principal types were noted separately. The results are given in Table VI.

Table VI.

Direction. Absolute Number for each Type. Percentage
from all
Types.
Arcs. Bands. Curtains. Rays. Patches.
N. 9 16 5 15 4 10
N.E. 9 13 2 20 4 9
E. 3 11 2 26 3 9
S.E. 5 6 1 10 7 6
S. 45 43 1 16 15 24
S.W. 9 9 2 12 13 9
W. 3 11 2 22 6 9
N.W. 2 8 2 8 5 5

Table VI. accounts for only 81% of the total displays; of the remainder 15% appeared in the zenith, while 4% covered the whole sky. Auroral displays generally cover a considerable area, and are constantly changing, so the figures are necessarily somewhat rough. But clearly, whilst the arcs and bands, and to a lesser extent the patches, showed a marked preference for the magnetic meridian, the rays showed no such preference.

At Cape Thorsden (7) in 1882-1883 auroras as a whole were divided into those seen in the north and those seen in the south. The variation throughout the twenty-four hours in the percentage seen in the south was as follows:—

Hour.0-3.3-6.6-9.9-12.
A.M.69554435
P.M.55706565

The mean from the whole twenty-four hours is sixty-three. Between 3 A.M. and 3 P.M. the percentage of auroras seen in the south thus appears decidedly below the mean.

17. The following data for the apparent angular width of arcs were obtained at Cape Thorsden, the arcs being grouped according to the height of the lower edge above the horizon. Group I. contained thirty arcs whose altitudes did not exceed 11° 45′; Group II. thirty arcs whose altitudes lay between 12° and 35°; and Group III, thirty arcs whose altitudes lay between 36° and 80°.

Group.I.II.III.
Greatest width.11.5°12.0°21.0°
Least width. 1.0° 0.75° 2.0°
Mean width. 3.45° 4.6° 6.9°

There is here a distinct tendency for the width to increase with the altitude. At the same time, arcs near the horizon often appeared wider than others near the zenith. Furthermore, Gyllensköld says that when arcs mounted, as they not infrequently did, from the horizon, their apparent width might go on increasing right up to the zenith, or it might increase until an altitude of about 45° was reached and then diminish, appearing much reduced when the zenith was reached. Of course the phenomenon might be due to actual change in the arc, but it is at least consistent with the view that arcs are of two kinds, one form constituting a layer of no great vertical depth but considerable real horizontal width, the other form having little horizontal width but considerable vertical depth, and resembling to some extent an auroral curtain.