Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/105

Rh of the auroral light is reflected, whether it be its own or derived from some other body, it should be polarised; but so far polariscope observations are deficient, and give no certain information. It is difficult to separate the proper polarisation of the aurora from the mere atmospheric polarisation of the sky. Mr Ranyard, who appears to have used a double-imaged prism and Savart during the great aurora of Feb. 4, 1872, and also to have made some observations on that of Nov. 11, 1871, did not detect polarisation. On the other hand, Prof. Stephen Alexander, in his report on his expedition to Labrador (App. 21, U. S. Coast Survey Rep., 1860), found strong polarisation with a Savart, and, singularly enough, thought it strongest in the dark parts of the aurora. The observations were made in lat. about 60°, in the beginning of July, and near midnight, but he does not state whether there was twilight or any trace of air polarisation at the time, nor does he give the plane of polarisation.

With regard to the height of auroræ, Sir W. R. Grove (Nature, vol. iii. p. 28) states that he saw an aurora some years ago at Chester in which the rays came between him and the houses; and Mr Ladd observed a similar case in which, the lighthouse at Margate was visible through a ray. The evidence, however, appears strong that aurora is usually at a very great height. Dalton calculated the height of an auroral arch, which was seen as far north as Edinburgh, and as far south as Doncaster, and at most intermediate places, from its apparent altitude, as measured by its position in relation to the stars as seen from Kendal and Warrington, 83 miles apart. The resulting height was about 100 miles, and the position slightly south of Kendal. An observation at Jedburgh confirmed this, but some taken at Edinburgh placed it above Carlisle at a height of 150 miles. Dalton, however, considered the former reckoning the more trustworthy. Backhouse has made many calculations, and considers that the average height of auroræ ranges from 50 to 100 miles, and numerous other observers have calculated similar heights. All these observations, however, are liable to the objection, that different observers may really have seen different arches, of which, as has been remarked, there are often several concentric ones. It is not likely that this was really the case in most instances, but it has, no doubt, sometimes occurred, and may account for the heights of 500 to 1000 miles calculated by early observers. This difficulty is met by a method proposed by Prof. H. A. Newton (Sill. Jour, of Sc., 2d ser. vol. xxxix. p. 286) for calculating the height by one observation of altitude and amplitude of an arch. It seems almost certain that the auroral arches are arcs of circles, of which the centre is the magnetic axis of the earth; or, at least, that they are nearly parallel to the earth s surface, and probably also to the narrow belt or ring surrounding the magnetic and astronomical poles, and passing through Faroe, the North Cape, and the north of Nova Zambia, which Loomis and Fritz have found to be the region of most frequent aurora. This being assumed, Prof. Newton finds that, $$\scriptstyle d$$ 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.), $$\scriptstyle h$$ the apparent altitude of the arch, $$\scriptstyle 2a$$ its amplitude on the horizon, $$\scriptstyle x$$ its height, $$\scriptstyle\mathrm{R}$$, the earth’s radius, and $$\scriptstyle c$$ the distance of the observer from the ends of the arch&mdash;

He gives the heights of twenty-eight auror calculated by this method, ranging from 33 to 281 miles, with a mean of 130 miles. The method, of course, rests on the assumption that auroral arches are arcs of circles, but it is decidedly confirmatory both of this assumption and of the heights calculated by other methods. It cannot well be objected that such altitudes are beyond the limits of our atmosphere, since Prof. A. S. Herschel (Nature, vol. iv. 504) gives the height of twenty meteors varying from 40 to 118 miles, with an average of about 70 miles, and it is almost certain that these bodies are rendered incandescent by atmospheric friction. Assuming 0° C. as the temperature at the earth’s surface, and the absolute zero, &minus;273° C., as a minimum for the auroral region, the pressure would be about 0.2 millimetre (0.0078 inch) at a height of 100 kilometres (62 miles) above the earth’s surface. 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. A. S. Herschel suggests, electrical repulsion may carry air or other matter up to a great height. A similar effect is observed in the so-called vacuum tubes, in which the pressure becomes much greater in the narrow central part, while the discharge is passing. It is found that the apparent altitude of the auroral corona is always a little less than that indicated by the dipping needle, owing to the curvature of the lines of magnetic force, or, in other words, because its altitude corresponds with the inclination of the parallel of latitude over which it is actually situated; and Galle has suggested (Pogg. Ann., cxlvi. 133), that from this divergence the height may be calculated, and, indeed, gives a series of heights so determined, which do not differ materially from Prof. Newton’s. It is, however, doubtful if the position of these coronas, and consequently the value of the small angle (not more than 4 or 5), admit of sufficiently accurate determination for such a use.

Early observers, and especially Mr Canton, conjectured that the aurora was an electric discharge in the rarefied upper atmosphere, and the resemblance between it and the phenomena exhibited by discharges in an air-pump vacuum confirmed the idea. Recent spectroscopic observations have thrown some little doubt on this conclusion, or at least have shown that there is still a mystery left unexplained. When the light of any glowing gas is analysed by the prism, it is found to consist of a series of coloured lines and bands, of which the number and position is dependent on the nature of the gas, and which is called its spectrum. The light of the aurora gives a spectrum usually consisting of a single line in the greenish yellow, which does not coincide with a principal line of any known substance,&mdash;a spectrum totally different from those of the gases of the atmosphere. Besides this line there is occasionally visible a sharp line in the red, and several fainter and more refrangible bands. The following table includes most of the principal determinations of the auroral lines, which have hitherto been published:&mdash;