Henry Richardson Procter

“ It is scarcely possible that a light so faint as not even to obscure the stars should be visible in sunlight, and such facts would seem to suggest that the auroral light is developed in cloud or mist of some sort, which may become Connection of aurora with cloudsvisible by reflected light, as well as by its own. ”

“ Should the aurora be flamy, and shoot out like rays, there is good reason for assuming a disruptive discharge of electricity, and then there ought to appear the strongest line in the spectrum of the air, the green, whose wave-length is 500.3. Precisely this has actually been observed by Vogel, and has, moreover, been seen by Angström and others. Finally, should the aurora be observed as it appears at a less height in the atmosphere, then are recognised both the hydrogen lines and also the strongest of the bands of the dark-banded air-spectrum. ”

“ But the apparent convergence of such masses of cloud towards the opposite points of the horizon, which have been so frequently noticed by meteorologists, is an optical deception, produced when they are situated in a plane parallel to that on which the observer stands. These circumstances, says Dr Richardson, are here noticed, because if it shall hereafter be proved that the aurora depends upon the existence of certain clouds, its apparent polarity may, perhaps, with more propriety, be ascribed to the clouds themselves which emit the light ”

“ It is not, however, certain that stellar space is an insulator, and it is possible, moreover, that the auroral currents do not follow the magnetic curves through their whole course, since electric discharge is always in the path of least resistance, and this is modified not only by the magnetic forces, but by atmospheric density, and it is possible that on attaining a certain height the current may proceed horizontally on a stratum of least resistance. ”

“ It must, however, be remembered that the observed frequency of auroræ is much affected in Scotland by the continuous twilight during the summer months. If there be this connection between thunder-clouds and auroras, it is not improbable that the " dark segment" is sometimes a real cloud or mist, situated at a height where the density of the air is too great for luminous discharge; and in several cases Silbermann has seen auroral rays rise from small clouds, which gradually melted entirely away, or left a small non-luminous nucleus when their electricity was discharged. ”

“ It has also been shown that in electrical discharges in air at ordinary pressures, while the spark itself was unaffected by the magnet, it was surrounded by a luminous cloud or aura, which was drawn into the magnetic curves, and which might also be separated from the spark by blowing upon it. It is evident, therefore, that any mechanical force may separate the luminous particles from the electric discharge which produces them. ”

“ There are still some points with regard to the origin of this electricity which are unexplained, and it is uncertain whether the magnetic disturbance causes the electrical phenomena, or vice versa. It has been shown by Prof. Plücker that when an electric discharge takes place through rarefied gas in the field of a magnet, it is concentrated in the magnetic curves, which are the only paths in which it can move without being disturbed by the magnet. This is well shown in De la Rive's well-known experiment, in which an electro-magnet is enclosed in an electric egg. ”

“ To give some idea of the extent of magnetic perturbations, we may mention that during the aurora of 13th May 1869, the declination at Greenwich varied 1°25′, while the vertical force experienced four successive maxima, and the greatest oscillation amounted to 0.04 of its total mean value. The horizontal force at the same time only varied 0.014 of its mean value. During the aurora of the 15th April of the same year the declination at Stonyhurst varied 2°23′14′′" in nine minutes.
The electric currents produced at such times in telegraph wires, though transient, are often very powerful. ”

“ It is assumed by Angström that water vapour is necessarily absent in the higher atmosphere on account of the cold, but when we remember that its molecular weight is lighter than that of oxygen in the, proportion of 9 to 16, it is not unlikely that it may attain great elevations under the very low tensions that prevail at such heights, and it is possible also that both it and other bodies may, by electric repulsion in the auroral beams, be carried up much above the level which they would attain by gravity. ”

“ The natives of the Arctic regions of North America pretend to foretell wind by the rapidity of the motions of the aurora; and they say that when it spreads over the sky in a uniform sheet of light, it is followed by fine weather, and that the changes thus indicated are more or less speedy, according as the appearance of the meteor is early or late in the evening, an opinion not improbable, when it is recollected that certain kinds of cirro-stratus are also regarded by meteorologists as sure indications of rain and wind. ”

“ Relations.We have already remarked the manifest relation between the forms and position of auroræ and the earth’s lines of magnetic force, and in addition to this have noted the disturbance of the magnetic needle during auroral displays. It is not, however, at such times only that the magnetic elements are subject to variation; the total force, declination, and inclination, all are constantly varying both regularly with the hours of the day and the seasons of the year, and irregularly at uncertain times. ”

“ AURORA POLARIS, Aurora Borealis and Australis,Polar Light, Northern Lights, or Streamersan electrical meteor, appearing most frequently in high latitudes, in the form of luminous clouds, arches, and rays, of which the latter sometimes meet at a point near the zenith, and form what is called a boreal crown. ”

“ The aurora was frequently seen without producing any perceptible effect on the needle. At such times its appearance was that of an arch, or an horizontal stream of dense yellowish light, with little or no internal motion. The disturbance in the needle was not always proportionate to the agitation of the aurora, but it was always greater when the quick motion and vivid light were observed to take place in a hazy atmosphere. In a few instances the motion of the needle was observe 1 to commence at the instant a beam darted upwards from the horizon ”

“ He thinks that all clouds are subject to magnetic or diamagnetic polarisation, and states that when the lines converge towards the magnetic pole, fine weather follows; when they are at right angles to this position, wet and stormy. The aurora appears in these latitudes usually to occur at a height much greater than that of ordinary clouds. ”

“ Still, mere negative evidence must be received with caution, and it is very possible that in high latitudes such sounds may occasionally be heard, since the electric discharge seems to originate near the poles. The aurora, too, seems to vary greatly in height, and in lower latitudes is usually at such an altitude that audible sounds from it are quite impossible. ”

“ They sometimes cover the whole sky, and frequently have a strong tremulous motion from end to end. This tremulous motion is sometimes seen also in the arches when near the zenith; and Benjamin V. Marsh mentions a case in which the matter of the arch had the appearance of a rapid torrent flowing from east to west. A rare form of aurora is that in which the rays appear to hang from the sky like fringes or the folds of a mantle. The ordinary colour of the aurora is a pale greenish-yellow, but crimson, violet, and steel-colour are not uncommon. ”

“ In telegraphic wires we cannot observe the existence of a constant terrestrial current, since the conductivity of the soil is infinitely greater than that of the telegraphic wire, and it is only sudden changes that manifest themselves. ”

“ At Edinburgh the annual average is 30, at York 10, in Normandy 5; while at Gibraltar the average is about 1 in ten years.
These curves, which Prof. Fritz calls isochasmen, are nearly normal to the magnetic meridians, and bear a close relation to the curves of equal magnetic inclination, especially with those laid down by Hansteen in 1730, while they noticeably diverge in some places from those of Sabine of 1840. They also approximate to the isobaric curves of Schouw, and Prof. Fritz remarks that the curves of greater frequency tend towards the region of lowest atmospheric pressure. ”

“ The irregular oscillations when violent are called magnetic storms, and it must be noted that auroral display never takes place except during such disturbances, although a large proportion of the most remarkable magnetic storms are unaccompanied by visible auroræ. ”

“ The yearly and eleven-yearly periodicity of auroræ tends to support the theory, but it is a formidable difficulty that, while shooting stars are more frequent in the morning, or on the face of the earth which is directed forwards in its orbit, the reverse is the case with auroræ. ”

“ Not only are auroral displays less frequent in Iceland and Greenland than further south, but it is found that while south of this zone auroræ appear usually to the north of the observer, north of it they are generally to the south, and within it, north or south indifferently. South of this lie other zones approximately parallel to it, and of constantly diminishing frequency. That in which the average yearly number of auroræ is 100 passes through the Drontheim, the Orkneys, and the Hebrides, and reaches the American coast just north of Newfoundland. ”

“ Prof. Loomis has shown that during auroral displays electrical currents traverse the earth s surface in the same general direction, though subject to great variation in intensity and even to reversal. Waves of magnetic disturbance are also propagated in the same direction (ibid., xxxii. 318) .
With regard to the arches it is evident that they are generally circles concentric to the magnetic pole, and it is very probable that they are analogous to the striæ often seen in discharges in rarefied gases. ”

“ Newton finds that, being the distance from the observer to the centre of curvature of the nearest part of this belt (which for England is situated about 75° N. lat., 50° W. long.) , the apparent altitude of the arch, its amplitude on the horizon, its height, , the earth’s radius, and the distance of the observer from the ends of the arch—
He gives the heights of twenty-eight auroræ calculated by this method, ranging from 33 to 281 miles, with a mean of 130 miles. ”

“ This result, of course, assumes a good deal; but if correct, it implies a vacuum attainable with difficulty even with the Sprengel pump. The pressure may, however, be much greater in the path of the auroral beams, since, as Prof. ”