advancing towards the zenith, parallel to one another; while in other cases they are altogether absent. The bundles of rays and columns of light assume the most varied forms, appearing either in the shape of curves, wreathed festoons and hooks, or resembling waving pennants or sails."

In the higher latitudes," the prevailing colour of the polar light is usually white, while it presents a milky hue when the Aurora is of faint intensity. When the colours brighten, they assume a yellow tinge; the middle of the broad ray becomes golden yellow, while both the edges are marked by separate bands of red and green. When the radiation extends in narrow bands, the red is seen above the green. When the Aurora moves sideways from left to right, or from right to left, the red appears invariably in the direction towards which the ray is advancing, and the green remains behind it." It is only in very rare cases that either one of the complementary colours, green or red, has been seen alone. Blue is never seen, while dark red, such as is presented by the reflection of a great fire, is so rarely observed in the north that Siljeström noticed it only on one occasion.10 luminous intensity of the Aurora never even in Finmark quite equals that of the full moon.

The

The probable connection which, according to my views,. exists between the polar light and the formation of very small and delicate fleecy clouds (whose parallel and equivalent. rows follow the direction of the magnetic meridian), has met with many advocates in recent times. It still remains a doubtful question, however," whether, as the northern travellers, Thienemann and Admiral Wrangel believe, these parallel fleecy clouds are the substratum of the polar light, or whether

9 Op. cit. pp. 35, 37, 45, 67, 481 ("Draperie ondulante, flamme d'un navire de guerre déployée horizontalement et agitée par le vent, crochets, fragments d'arcs et de guirlandes)." M. Bevalet, the distinguished artist to the expedition, has given an interesting collection of the many varied forms assumed by this phenomenon.

10 See Voy. en Scandinavie (Aur. boréal.), pp. 523–528, 557.

11 Cosmos, vol i, p. 194; see also, Franklin, Narrative of a Journey to the Shores of the Polar Sea in 1819-1822, p. 597; and Kämtz, Lehrbuch der Meteorologie, Bd. iii (1836), s. 488-490. The earliest conjectures advanced in relation to the connection between the northern light and the formation of clouds are probably those of Frobesius. (See Aurora borealis spectacula, Helmst, 1739, p. 139).

they are not rather, as has been conjectured by Franklin, Richardson, and myself, the effect of a meteorological process generated by and accompanying the magnetic storm. The regular coincidence in respect to direction between the very fine cirrous clouds (polar bands) and the magnetic declination, together with the turning of the points of convergence, were made the subjects of my most careful observation on the Mexican plateau in 1803, and in Northern Asia in 1829. When the last named phenomenon is complete, the two apparent points of convergence do not remain stationary, the one in the north-east and the other in the south-west (in. the direction of the line which connects together the highest points of the arch of the polar light which is luminous at night), but move by degrees towards the east and west.12 A precisely similar turning, or translation of the line, which in the true Aurora connects the highest points of the luminous arch, whilst its bases (the points of support by which it rests on the horizon) change in the azimuth and move from east-west towards north-south, has been several times observed with much accuracy in Finmark.13 These clouds ar

12 I will give a single example from my M.S. journal of my Siberian journey:-"I spent the whole of the night of the 5-6th of August (1829), separated from my travelling companions, in the open air, at the Cossack outpost of Krasnajazarki, the most eastern station on the Irtisch, on the boundary of the Chinese Dzungarei, and hence a place whose astronomical determination was of considerable importance. The night was extremely clear. In the eastern sky polar bands of cirrous clouds were suddenly formed before midnight (which I have recorded as 'de petits moutons également espacés, distribués en bandes parallèles et polaires). Greatest altitude 35°. The northern point of convergence is moving slowly toward the east. They disappear without reaching the zenith; and a few minutes afterwards, precisely similar cirrous bands are formed in the north-east; which move during a part of the night, and almost till sunrise, regularly northward 70° E. An unusually large number of falling stars and coloured rings round the moon throughout the night. No trace of a true Aurora. Some rain falling from speckled feathery masses of clouds. At noon on the 6th of August the sky was clear, polar bands were again formed, passing from N.N.E. to S.S.W., where they remained immoveable, without altering the azimuth, as I had so often seen in Quito and Mexico." (The mag netic variation in the Altai is easterly.)

13 Bravais, who, contrary to my own experience, almost invariably observed that the masses of cirrous clouds at Bossekop were directed, like the Aurora borealis, at right angles to the magnetic meridian (Voyages en Scandinavie, Phénomène de translation dans les pieds de

ranged in the form of polar bands correspond, according to the above developed views, in respect to position, with the luminous columns or bundles of rays which ascend in the true Aurora towards the zenith from the arch, which is generally inclined in an east and west direction; and they cannot, therefore, be confounded with those arches of which one was distinctly seen by Parry in bright day-light after the occurrence of a northern light. This phenomenon occurred in England on the 3rd of September, 1827, when columns of light were seen shooting up from the luminous arch even by day.14

It has frequently been asserted that a continuous evolution of light prevails in the sky immediately around the northern magnetic pole. Bravais, who continued to prosecute his observations uninterruptedly for 200 nights, during which he accurately described 152 Aurora, certainly asserts that nights, in which no northern lights are seen, are altogether exceptional, but he has sometimes found even when the atmosphere was perfectly clear, and the view of the horizon was wholly uninterrupted, that not a trace of polar light could be observed throughout the whole night, or else that the magnetic storm did not begin to be apparent until a very late hour. The greatest absolute number of northern lights appears to occur towards the close of the month of September; and as March, when compared with February and April, seems to exhibit a relatively frequent occurrence of the phenomenon, we are here led, as in the case of other magnetic phenomena, to conjecture some connection with the period l'arc des Aurores boréales, pp. 534-537), describes with his accustomed exactitude the turnings or rotations of the true arch of the Aurora borealis, pp. 27, 92, 122, 487. Sir James Ross has likewise observed in the southern hemisphere similar progressive alterations of the arch of the Aurora (a progression in the southern lights from W.N.W.-E.S.E. to N.N.E.-S.S.W.) Voyage in the Southern and Antarctic Regions, vol. i, p. 311. An absence of all colour seems to be a frequent characteristic of southern lights, vol. i, p. 266, vol. ii, p. 209. Regarding the absence of the northern light in some nights in Lapland, see Bravais, Op. cit. p. 545.

14 Cosmos, vol. i, p. 191. The arch of the Aurora seen in bright daylight reminds us by the intensity of its light of the nuclei and tails of the comets of 1843 and 1847, which were recognised in the immediate vicinity of the sun in North America, Parma, and London. Op. cit. vol. i, p. 85, vol. iii. p. 543.

of the equinoxes. To the northern lights which have been seen in Peru, and to the southern lights which have been visible in Scotland, we may add a coloured Aurora, which was observed for more than two hours continuously by Lafond in the Candide, on the 14th of January, 1831, south of New Holland, in latitude 45° 15

The accompaniment of sound in the Aurora has been as definitely denied by the French physicists and Siljeström at Bossekop1 as by Thienemann, Parry, Franklin, Richardson, Wrangel, and Anjou. Bravais estimated the altitude of the phenomenon to be fully 51307 toises (or 52 geographical miles), whilst an otherwise very careful observer, Farquhar son, considers that it scarcely amounts to 4000 feet. The data on which all these determinations are based are very uncertain, and are rendered less trustworthy by optical illusions, as well as by erroneous conjectures regarding the positive identity of the luminous arch seen simultaneously at two remote points. There is, however, no doubt whatever of the influence of the northern light on declination, inclination, horizontal and total intensity, and consequently on all the elements of terrestrial magnetism, although this influence is exerted very unequally in the different phases of this great phenomenon, and on the different elements of the force. The most complete investigations of the subject were those made in Lapland by the able physicists Siljeström and Bravais (in 1838-1839), and the Canadian observations at Toronto (1840-1841), which have been most ably discussed by Sabine.18 In the preconcerted simultaneous observations which were made by us at Berlin (in the Mendelssohn-Bartholdy Garden), at Freiberg below the surface of the earth, at St. Petersburgh, Kasan and Nikolajew, we found that the magnetic variation was affected at all these places by the Aurora borealis, which was visible at Alford in Aberdeenshire (57° 15′ N. lat.) on the night of the 19-20th of December, 1829. At some of these stations, at which 15 Comptes rendus de l'Acad. des Sciences, t. iv, 1837, p. 589.

16 Voyages en Scandinavie, en Laponie, etc., (Aurores boréales,) p. 559; and Martin's Trad. de la Météorologie de Kaemtz, p. 460. In reference to the conjectured elevation of the northern light, see Bravais, Op. cit. pp. 549, 559.

17 Op. cit. p. 462.

18 Sabine, Unusual Magnet. Disturbances, pt. i, pp. xviii, xxii, 3, 54.

the other elements of terrestrial magnetism could be noted, the magnetic intensity and inclination were affected no less than the variation.19

During the beautiful Aurora, which Professor Forbes observed at Edinburgh on the 21st of March, 1833, the inclination was strikingly small in the mines at Freiberg, while the variation was so much disturbed that the angles could scarcely be read off. The decrease in the total intensity of the magnetic force which has been observed to coincide with the increasing energy of the luminosity of the northern light is a phenomenon which is worthy of special attention. The measurements which I made in conjunction with Oltmanns at Berlin during a brilliant Aurora on the 20th of December, 1806, and which are printed in Hansteen's "Untersuchungen über den Magnetismus der Erde," were con

20

19 Dove, in Poggend. Ann. Bd. xx, s. 333–341. The unequal influence which an Aurora exerts on the dipping needle at points of the earth's surface, which lie in very different meridians, may in many cases lead to the local determination of the active cause, since the manifestation of the luminous magnetic storm does not by any means always originate in the magnetic pole itself; while, moreover, as Argelander maintained and as Bravais has confirmed, the summit of the luminous arch is in some cases as much as 11° from the magnetic meridian.

20"On the 20th of December, 1806, the heavens were of an azure blue, with not a trace of clouds. Towards 10 P.M. a reddish-yellow luminous arch appeared in the N.N.W., through which I could distinguish stars of the 7th magnitude in the night telescope. I found the azimuth of this point by means of a Lyræ, which was almost directly under the highest point of the arch. It was somewhat further west than the vertical plane of the magnetic variation. The Aurora, which was directed N.N.W., caused the north pole of the needle to be deflected, ́for, instead of progressing westward like the azimuth of the arch, the needle moved back towards the east. The changes in the magnetic declination, which generally amount to from 2′ 27′′ to 3′ in the nights of this month, increased progressively and without any great oscillation to 26′ 28′′ during the northern light. The variation was the smallest about 9h. 12m. when the Aurora was the most intense. We found that the horizontal force amounted to 1' 37".73 for 21 vibrations during the continuance of the Aurora, while at 9h. 50m. A.M., and consequently long after the disappearance of the Aurora, which had entirely vanished by 2h. 10m. A.M. it was 1' 37".17 for the same number of vibrations. The temperature of the room, in which the vibrations of the small needle were measured, was in the first case 37°.76 F. and in the second 37°.04 F. The intensity was therefore slightly diminished during the continuance of the northern light. The moon presented no coloured rings." From my magnetic journal, see Hansteen, s. 459.