Untitled

Hawke's Bay Times, Volume 15, Issue 759, 7 February 1870, Page 4

THE SECRET OF THE AURORA. (From the Spectator, Oct. 9.) Men of science have long felt that a strange secret lay hidden in the brilliant folds of the Aurora. The magic arch, with its pointed strearr ers 3 shifting silently and swiftly across the heavens, pulsating mysteriously as though illuminated by the fitfully changing glow of some concealed furnace, and rendered surpassingly beautiful by the brilliancy of its colors, had always had strange charms for men of thoughtful mind. And gradually a series of laborious researches had revealed the laws av hich associate this beautiful apparition with disturbances affecting the economy of our whole earth, and not indistinctly connected with the habitudes of the solar system iteelf J>nt recently a discovery has been made which is even more remarkable than any which had before rewarded the labors of physicists,—a discovery at once instructive and perplexing, revealing a bond of union between the aurora and a phenomenon hitherto thought to be quite different in character, but leading us still to learn what the exact nature of that bond of union may be. Let us consider what has been learned respecting auroral displays and their relation to the earth's economy. We had occasion recently to point out that a sudden disturbance in the sun in 1359 had been presently followed by intense magnetic action, the whole electric system of the earth quivering, t,o to speak, under the influences of the solar forces educed by disturbance. And we mentioned that amongst the signs of this magnetic action brilliant displays of the auroral streamers had been witnessed in both hemispheres on the night following the solar disturbance. This circumstance teaches us the true character of the aurora as strikingly as any which astronomers and physicists had patiently been gathering during the past halfcentury. We learn at once that a relation subsists between the aurora, terrestrial magnetism,and the central luminary of our scheme. And even if we have to pause here, we should have learned enough to indicate the significance and importance of the aurora. Our earth, we may be sure, is not the only recipient of that mystic influence which rouses into activity the phenomena we term magnetic. Over the whole realm which it rules, the central orb sends forth the strange electric impulses. When on r are illuminated by the magic streamers, we may be sure that those of Yenus and Mars, of Jupiter and of Saturn, nay, even the skies of those unseen orbs which travel far out in space beyond the paths of Uranus and Neptune, are lit up with auroral displays. When once it has been shown that we owe our auroras to solar action, we recognise the cosmiSftl character of the display, and that,

in a sense, the terrestrial magnetisrn on which it depends is a bond of affinity between our earth and i's sister orbs. But while we were at once taught the true significance of the aurora, we were left in doubt as to the way in which the solar action aroused the electric luminosity in the upper regions of our air. N "We could not even tell at what elevation the light was suspended above our earth The most, eminent physicists differed in [their views as to the possibility of learning where the magic streamers really wave, when we see them most distinctly. While Arago had held that to attempt to measure the height of the aurora was as futile as to attempt to measure the height of a rainbow, Sir John Herschel considered that precisely the same laws of measurement might be adapted to the aurora as to any object, raised high above the earth. One point, however, was well determined. The aurora lights are undoubtedly to be ascribed to electric action taking place at a veryconsiderable height, where the ah is very rare indeed. It became, therefore, a question whether anything could be learned by analyzing the auroral light, as to the condition of that particular part of our atmosphere in which the electric action takes place.

Spectroscopic analysis, that strange and powerful mode of research which has revealed so many unlocked-for facta, was accordingly applied to the light of a brilliant' aurora. The result was rather surprising. Instead of a rainbow-colored streak of lio-ht, such as would have appeared if the aurora were due to the existence of particles excited to luminosity by electric action, a single line of colored streak light appeared This indicated that the light is due to the incandescence of some gas through which the electric discharges in upper air take place, But this was not the circumstance which attracted surprise. Rather, this was to have been looked for. It was the position of the line which astonished our physicists. If the gas had been one which chemists are acquainted with, the bright line would have occupied the position proper to that gas, and would at once have indicated its nature. But there is no known element whose spectrum has a bright line where this one appeared- The observation has been repeated over and over again, by Angstrom, by Otto Struve, and recently by Mr Plummer, always with the same result,- -we cannot tell what the substance may be to whose incandescence or luminosity the aurora owes its brilliancy. But now a most remarkable discovery has been effected. Angstrom has found that the mysterious line of the auroral spectrum exists in the ■spectrum of another object which had been thought to be wholly different in character. Ever since its discovery by Cassini, the zodiacal! light has been an object of interest to astronomers. Gradually a theory had been formed respecting it, which had been sanctioned by the authority of such men as Humboldt and SixJohn T-lerschel. It was held out that this appearance is due to the light reflected from a multitude of minute cosmical bodies travelling around the sun within the orbit ofj our earth.

This theory had never been tested by spectroscopic analysis. Indeed, the zodiacal light shines so faintly that it was hardly hoped that its spectrum could be rendered visible But it was confidently anticipated that if the zodiacal light ever were thus analyzed, its spectrum would be that which the theory required,— that is, a very faint reproduction of the common solar spectrum. Now, at length, we hear from Angstrom that the spectrum of the zodiacal light has been observed, and instead of being, as had been expected, a faint rainbow-colored streak, it presents but a single line. That line is the same that ive see in the spectrum of the auwraf In other words, the light of the zodiacal gleam and that of the auroral streamers are due to the same sort of electric discharge taking place in the same medium. Without pretending to further interpret this startling result, we

may • indicate the promise it affords of explaining a number of phenomena which have long seemed most perplexing. When once we recognise Ihe fact that electric action is effective in producing any of the celestial lights, we have a resource available to remove many difficulties. Astronomers were asking how comets, for example, could exhibit the spectrum of the incandescent vapour of carbon, —that .is, a spectrum indicative of the most intense heat, when, as in the case of Y/innecke's comet (whose spectrum was of this nature), they were further from the sun than the earth is. The action of the sun in exciting electrical discharges would be quite sufficient to account for this and similar phenomena. . Again, it has long been recognised that the peculiarities of comets' tails seem only explicable as due to electric action ; but astronomers were unwilling to adopt such a theory without some positive evidence in its favor. Yv r e now have such evidence; and it is most, probable that the first longtailed comet which is submitted to spectroscopic analysis will establish the view which Euler put forth more than half a century ago, that comets' tails have something in common with the aurora and the zodiacal light. It would indeed be strange if three of the most mysterious phenomena with which men of science; are acquainted should find their ex- 1 planation simultaneously.