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OBSERVATIONS UPON THE ELECTRICITY OF THE ATMOSPHERE AND THE AURORA BOREALIS, MADE DURING THE SWEDISH EXPEDITION OF 1868 TO THE NORTH POLE.
By Prof. Selim LEMSTRÖM, of the University of Helsingfors, Finland.
There can be no savant now living who is not convinced that polar light is a phenomenon due to electric action in the upper regions of the atmosphere. Of the two theories which have been advanced to explain the pbenomenon, one of which seeks its origin exclusively in variations in the intensity of terrestrial inagnetism, the other in the electricity of the air, the former must give place to the latter, since there are very many convincing proofs in its favor. Unhappily, our knowledge of the electric state of the atmosphere in high polar regions is very limited; could it be extended, all doubts which now exist in regard to the subject would probably disappear. The attempts made to discover the nature of atmospheric electricity in the regions of the extreme north, have in general given only negative results, with the exception, however, of the researches made in the neighborhood of Bossekop by French savants, who, by sending up a kite or an arrow, attached by a conducting wire to an electroscope, to a vertical height of from 30 to 40 yards, have proved the constant presence of positive electricity; but, these observations are too few in number, and were made in a latitude not sufficiently high to be conclusive.
I. One of the most important of my objects in the physical researches of the expedition of 1868 was the study of the phenomena relating to tbe electricity of the atmosphere; but notwithstanding all my care, I obtained only negative results. As I am convinced that in every case I could account for my want of success, I will briefly describe the experiments I attempted, in order to pass to the observations I had occasion to make in regard to the aurora borealis.
The first experiment, made on the 26th and 28th of August, 1868, on a narrow tongue of land at Kobbe Bay, by means of the electrometer, gave no result, although the observations were made several times a day, and even at night, at the same time that I observed the magnetic instruments. Expecting to find the cause of these negative results either in the insensibility of the instrument or in the nature of the locality, which, closed on three sides by mountains, was open only on the side toward the sea, I determined to modify my instrument, and to look for a more open place for my observations. It was not until the 28th of September, while the Sophie was anchored at Southgatt, that I could carry this project into execution.
Having made my electrometer more sensitive, I went, on the day above mentioned, between 11 o'clock and noon, to an island situated at the mouth of the Southgatt, and established iny instrument on the highest point of the island, 600 feet above the level of the sea. Notwithstanding these precautions I still obtained no certain result. This was possibly on account of the violence of the wind, which produced oscillations in the electrometer, but other observations made on the 7th of October at King's Bay were equally unsuccessful.
Although these experiments were too few in number, and too incomplete to draw from them any positive conclusion whatever, I am convinced that this absence of electric manifestation was due to the peculiar constitution of the air in these regions. A glance over the hygrometric observations shows that the air was almost constantly saturated with moisture, and this moisture did not exist merely in the form of insensible vapor, but also as fog. This circumstance rendered it almost impossible to isolate the instrument, and consequently to obtain the effects of the electricity of tension. We way at least conclude that there is no electricity of tension in northern aerial regions which approximate to a plain, but that the electricity rises through damp air into the higher regions of the atmosphere. I am inclined to believe that observations of the electricity of the air made on level ground will always give negative results. Elevated ground should be chosen, and an instrument which may be sent up into the higher strata of the atmosphere, such as the kite used by Franklin.
Setting aside these incomplete experiments, which can only be of use as guides for future efforts, I pass to the observations upon the aurora borealis.
During the last days of September the Sophie was anchored at Southgate, a strait lying between the island called Danes Island and the continent of Spitzbergen, at 790 39' 7" of latitude and 11° 7' of longitude west of Greenwich. The gulf of which this strait is an outlet is surrounded on the north and south by mountains, those on the south aboat 300 meters in height. At the mouth of this gulf lies the island above inentioned; to the east the view is limited by other mountains varying in height. The Sophie was anchored close to the shore of Danskow, a little to the northwest of the island, at the mouth of the strait. On returning from the island, where the ivstruments for the magnetic observations had been deposited, I perceived upon the ridge of the mountain, to the south, a brilliant polar light rising from 100 to 15° above the mountain in undulating rays, distinctly defined, at their base appearing as a diffuse yellowish light, but higher up as vertical orange beams, while at the top they formed a series of sharp points. The rays had an undulating motion, and the crest of the mountain was covered with a light fog, which the wind was moviug from east-northeast to
west-southwest. In a few moments the cloud of mist passed the mount. ain and the rays disappeared, but the crest of the mountain continued to be illuminated by a pale wandering light, which floated along the mountain, and of which it was not easy to determine the character; still I was in no doubt, for the spectral analysis very clearly displayed the yellow ray discovered in polar light by M. Angström. I continued to observe the crest of the mountain, over which foggy vapors were passing, allowing to appear from time to time the pale light I have described.
At 11 o'clock 30 minutes the upper part of the fog, which presented very much the appearance of a cloud with serrated edges, became illuminated with a yellowish white light, in the course of a moment con. verted into yellowish and reddish rays, which extended with an uudulatory motion along the edges of the fog, following the irregularities in their minutest detail. The fog rose in the form of an arch about 100 above the mountain, and the rays attained a height of from 100 to 150, which gives for the wbole phenomenon an elevation of from 200 to 250 above the borizon. At the same time there began to appear at the north an indistinct combination of the brilliant edges of clouds, among which I clearly distinguished one, from which proceeded a distinctlymarked yellow ray, seeming in appearance to connect this cloud with another. The rest of the sky was covered with fillets or bands of light clouds, passing over the zenith from the east to the west and allowing the stars to appear at intervals.
The day following, the 26th of September, having observed the crest of the mountain with attention, I found it was almost entirely covered with snow, except at one or two places, wbich seemed to be those at which the night before the light had appeared with least intensity. The evening of the same day the phenomenon was again manifested, but with some modifications. A little below the horizon to the southwest, almost opposite the promontory or headland which terminates the mountain, appeared a series of clouds whose upper edges were strongly illuminated with a diffuse yellow and white light, which was very intense on the edge of the cloud at the extreme western end of the series, but diminished in brilliancy along the edges of the clouds until at the eastern end, where the last cloud seemed to melt into the headland, it was hardly perceptible. Very soon rays appeared, similar to those observed the night before, which seemed to proceed from a mist lying along the crest of the mountain, but somewhat back of it. This time the phenomenon seemed to take place at a much greater distance than during the preceding observations, but the form and color of the rays were the same; and I again, with the spectroscope, obtained the yellow ray as well from the light emanating from the edge of the clouds as from that proceeding from the rays themselves.
On the 27th of September, after having observed in the mornirg a radiation of yellowish-white light proceeding from one edge of a cloud which stood out prominently from a wall of clouds, I perceived in the evening, at 11 o'clock 30 minutes, a pale, wandering light moving distinctly along the ridge of the mountain. The light appeared for a few moments in the form of rays of a clear and brilliant yellow, following every detail of the sinuosities of the mountain. The pale glimmer of light seemed to follow the ridge of the mountain, and I was convinced, from the movement of the mists, that the luminous phenomenon was formed upon the ridge itself.
On the 30th of September, at 9h. 30 m., I witnessed on the island of Amsterdam a very intense luminous phenomenon, during which every peak and ridge, the most elevated, was illuminated with a pale ligbt, particularly when covered with a vail of mist. We could clearly distinguish the contour of the mountains, and above them an effect of light, which frequently rose to a great height, and ended by gradually diminishing in intensity until lost in the upper strata of the mist. This light appeared during the whole of the harvest season, while we were stationed at Spitzbergen. Upon some peaks, overlooking a glacier which descends to the very bottom of Smeerenburg Bay, the light was still stronger. We even perceived upon one point reddish rays resembling flames, in which the spectroscope evidently indicated the presence of the yellow ray. On the days when this phenomenon was observed the wind was very strong, with a variable direction difficult to determine. We
however, sure that there were two contending currents of air-the one from the north or east, the other from the south or west.
While the Sophie remained at Kingsbay, during the first fifteen days of October, every night fog-like mist covered the summits and ridges of the mountains, and their brilliancy seemed to increase as the season advanced. Besides this general phenomenon, we observed on the 9th of October, at 5h. in the morning, a brilliant polar light in the south, about 1,000 feet above the chain of mountains, which gradually faded away toward the north. A similar light appeared on the 11th of October and on the 12th, beneath a cloud upon the mountains, ready to be dissolved into snow, appeared a pale yellow light with points at its edge. This luminous phenomenon had an undulatory movement, in addition to the forward motion given to it by the course of the clouds toward the west. Soon the light disappeared, and was replaced by a light fall of fine snow. On the 14th and 15th of October, in the evening, a cloud appeared, (on the 14th in the west-southwest, and on the 15th in the southeast, from the upper edge of which, when at a sufficient height above the horizon, emanated an intense yellow light, soon transformed into rays of genuine polar light, yellow at their base and red at the top. These rays, which moved with the cloud, rose with it nearly to the zenith, where they tended to form a crown.
On the entrance of the Sophie into the Norwegian archipelago, on the evening of the 18th of October, we saw some patches of polar light scattered here and there over the sky, in the porth and east, which afterward formed a continuous ring around the horizon. The rays of this ring gradually elongated, and suddenly meeting around the zenith, formed for a few moments a boreal crown of perfect regularity and the most brilliant colors.
On our arrival at Tromsö, I examined with the spectroscope a beautiful polar light, which appeared on the 21st of October, commencing at the north. The first rays clearly displayed the yellow ray in question. The phenomenon becoming more brilliant, a variegated band was formed toward the south, of yellow, red, and green, which gave, 1st, the yellow ray; 2d, in the blue part, a very distinct and very clear ray; 3d, two lines of the breadth of a bair, showing very decided horizontal stripes by the side of the yellow. I ought to say tbat the yellow ray in every case seemed to me peculiar in being variable in intensity; sometimes more, sometimes less vivid. On the 27th of October, we were enabled to determine more accurately the position of the rays, and we found that the yellow line furnished by the yellow light of chloride of sodium was 61.0; in the auroral light, the yellow line was at 79.9; the blue at 65.90. The first of the shaded lines at 125.0; and the second shaded line at about 105.0.
II. Admitting, in general, the most of the opinions advanced in the work of M. Loomis, in regard to the aurora borealis,* which, in many respects, accord with the theory given by M. de la Rive, and supported by experiment, I have still, after the observations described, some doubts in regard to certain parts of this work, especially those which relate to the particular nature of the phenomenon.
The fact that polar light is an electric phenomenon, taking place in the atmosphere, is well established by the analogy which exists between its effects and those of electric currents. It produces, as they do, perturbations of the maguetic needle and currents in a good conductor. The luminous phenomenon itself exactly resembles the light produced by electric discharges in a damp atmosphere, or between two electrodes placed in rarified air. The results of spectral analysis, as well as those discovered by M. Angström, and the new rays I have described, give further proof of the electric origin of polar light; for, in order to obtain a spectrum with a gas, the latter must be incandescent, and electricity is the only source of heat which can produce this incandescence in the aerial molecules and other particles which constitute the atmospbere.
The question as to the height at which polar light is manifested bas been the subject of much controversy. From observations made in America with great care and in several separate localities, M. Loomis calculated the height of the beautiful aurora borealis of the 2d of September to be at its lower limit 45 to 50 English miles, and at its upper limit from 400 to 500. Mr. Potter, in England, in 1833, gave, as height of the aurora, 63 miles, and Dalton, in 1826, estimated it at 100. During the French expedition, from 1838, 1839, observations made simultaneously at two stations-Bossekop and Jupwig-gave 60 to 100 miles
* Smithsonian Report for 1865.