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tions. A similar difference of opinion is found in the reports of observations on previous eclipses. If variations in the brightness and extent of the coronal radiations do actually occur, it is favorable rather than opposed to the auroral theory of the corona ; but it is probable that the apparent changes are due to inequalities in the interceptive action of the earth's atmosphere on the light of the corona, and especially on the faint light at its outer boundary.
Professor Harkness, of the U. S. Naval Observatory, in his able Report of Observations on the eclipse, states that the four angles of the trapezoidal outline of the corona were in the middle heliographic latitudes — relying upon the report of another observer; but on a direct examination of the question of the location of these angles, or “star points" of the corona, made since the publication of the Report, he has satisfied himself that their actual position was such as I have above reported it from my own observations. *
In the delineations of the corona given by the observers of previous eclipses, two or more conspicuous outward extensions are generally shown, but the positions of these more projecting parts are seldom given with respect to the equator or poles of the sun. The figure of the eclipse accompanying the Report of P. Prof. Capellotti, of observations on the eclipse of April 15, 1865, made at Chili
, is an exception. It shows three principal points of outstreaming of the corona; two lying very nearly in the plane of the sun's equator, and nearly diametrically opposite to each other, and a third near one of the poles of the sun. In the eclipses of 1858, 1860 and 1868, four such points were seen, distributed at about a quadrant's distance from each other. In the eclipse of 1812, but two were noticed, which were diametrically opposite to each other. In that of 1851, there appears to have been no marked deviation from a general uniformity of radiation.
Relying then upon the only definite knowledge we have of the location of the more conspicous portions of the corona, viz. that obtained in the eclipses of 1865 and 1869, we may say that the corona is brighter and more extended about in the direction of the plane of the sun's equator than in any other direction. This striking fact lends a powerful support to the auroral theory of the corona; for, as we have already seen, the streamers proceeding from the lower latitudes on the sun, on opposite sides of the equator, should converge and intersect in the plane of the equator, and for a certain distance
* Professor Winlock, in his report of observations on the eclipse, says, " the photograph of the corona taken at Shelbyville shows a flattening at the extremities of the sun's axis, and an elevation about the equatı real region.” The photographic impressions obtained of the eclipse, at the different stations show, however, but a small portion of the outward extent of the corona visible to the naked eye.
on either side of this plane, and in consequence the corona should appear to extend farther in the plane of the equator, than in other directions. The convergence of individual rays or lines of emanation, we have already seen, was actually noticed by Professor Eastman. It of course may happen that inequalities in the amount of outstreaming on opposite sides of the equator, may throw the more prominent and conspicuous parts of the corona to the one side or the other of the plane of the equator.
It will be observed that from our present point of view, the extension of the corona in the plane of the sun's equator is a phenomenon kindred to the much greater luminous extension seen in the zodiacal light; the only difference between them being, that in the former the auroral emanations proceed from lower heliographic latitudes, and intersect nearer the sun.
But it may be asked how are we to explain, on the present theory, the "star points” of the corona over the polar regions of the sun. For these, two reasons may be assigned. (1) If we admit a distribution of magnetism on the sun similar to that which prevails on the earth, the auroral streamers should diverge from each other less rapidly in the high than in the low latitudes. (2) Upon opposite sides of a line of no declina
a tion traversing the sun's surface, analogous to that which trav. erses Russia, the natural directions of the streamers prolonged upward would be such as to occasion the convergence and intersections of those proceeding from the opposite sides of this line.
We may say then that the more extended portions of the corona, in the eclipse of 1869, were over those regions of the sun's surface, and those only, where upon the present theory the intersections of streamers might be expected to occur.
In some eclipses distinct luminous curves having the appearance of luminous jets issuing tangentially to the sun's limb, or obliquely inclined to it, and pursuing a course either convex or concave to the limb, have been seen. According to M. Liais these peculiarities were conspicously observable in the eclipse of Sept. 7, 1858. While it is possible that such curves may be the result of the intersections of a mass of straight streamers, it is not improbable that they may be actual luminous jets; for if from any cause any portion of the auroral matter should be projected from the sun in a direction oblique to the surface, it would proceed in a convex hyperbolic curve if repelled by the sun, and in a concave curve if attracted. Now I have shown in a former No. of this Journal (July, 1861), that the portion of cometary matter posited on the convex side of the tail of Donati's Comet was actually repelled by the sun, while that on the concave side had become detached from the head of the
comet, because of a diminished gravitation toward the sun. Upon our fundamental conception that the coronal matter is essentially in the same physical condition as such cometic matter, and subject to the action of the same solar forces, it may well happen that some individual jets will proceed in convex, and others in concave curves, according as the escaping matter is repelled or attracted by the sun.
2. Observations on the physical constitution of the corona with the Spectroscope and Polariscope. The results of the observations made at the sate eclipse, with the spectroscope and polariscope, are strongly confirmatory of the truth of the theory of the corona under discussion. Professor Pickering in the Report of his observations with a polariscope, says, “ The form of polari. scope used was that adopted by Arago in his experiments on sky polarization. It consists of a tube about twenty inches long and two inches in diameter, one end of which is closed by a double image prism of Iceland spar, and the other by a plate of quartz. Looking through the former we see two images of the latter, which when the light is polarized assumes complementary tints. If, now, the corona was polarized in planes passing through the center of the sun (as is generally admitted), when viewed through the polariscope, in one image the upper and lower parts should have appeared blue, and those on the right and left yellow ; while in the second image these colors would be reversed, the yellow being above and below, and the blue on the sides. In reality the two images were precisely alike, and both pure white, but one was on a blue and the other on a yellow back-ground. From this we infer that the corona was unpolarized, or, at least, that the polarization was too slight to be perceptible.'
We may infer from this that the corona is either self lumi. nous or shines by diffuse reflection; since specular reflection produces polarization.*
The testimony of the spectroscope is still more decisive. Profs. Pickering, Harkness, and Young, agree that the spectrum from the light of the corona was a continuous one, or free from dark lines; but containing one or more bright lines. The absence of dark lines indicates that the corona did not shine by the light of the photosphere, reflected either diffusely or specularly from its substance; since such light, after reflection, should, like the direct solar light, have given a spectrum with the Fraunhofer lines. The presence of bright lines, on the other hand, is a direct indication that the corona was self lumipous; and therefore that its light was the result either of combustion or of electric discharges. As it is hardly supposable that an actual combustion could prevail at the distance of tens, and hundreds of thousands of miles from the sun's photosphere, in regions where, if any solar atmosphere exist, the results of recent observations with the spectroscope by Lockyer and Frankland lead us to believe that it can only be the faintest possible trace of it, we must infer that the light of the corona is of electric origin.
* The question whether the light from the corona is in any degree polarized or not cannot be regarded as definitively settled. It is to be hoped that the observations to be made on the eclipse of December will remove all doubt on this point.
In the hands of Prof. Young and Prof. Winlock the spectroscope has obtained direct evidence of a physical correspondence between the solar corona and terrestial auroras.
Prof. Young observed in the spectrum of the corona a bright line the position of which he gives as 1474 on Kirchoff's scale, and which proves to be in coincidence with a small line marked as iron on Kirchoff's and Angström's maps. He remarks that "it turns out also to coincide very closely if it is not (which is much more probable) absolutely identical with a line recently discovered by Prof. Winlock of Cambridge, in the spectrum of the aurora borealis. He also saw two other fainter lines in the spectrum of the corona which coincided quite closely with other lines reported by Prof. Winlock as visible in the spectrum of the aurora. In view of these results of spectroscopic observation he remarks, as follows: “At present it seems pretty likely that the spectra of the corona and the aurora borealis are identical, with only such differences in the intensity of their lines as we might naturally expect, and that very probably the identity extends to the essential nature of the phenomena themselves.
The detection of the same iron line in the aurora and corona, taken in connection with the well established fact that the vapor of iron is present in the photosphere and chromosphere of the sun, and that the magnetic features of the aurora lead to the natural conclusion that some form of ferruginous matter constitutes the substance of auroras, for which no terrestrial origin can reasonably be assigned, conducts to the inference that the terrestrial auroral matter is derived from the sun, and adds to the weight of accumulative evidence in support of the theory I have advocated that the corona is made up of material emanations from the sun.
NOTE.—Some persons have conjectured that the corona might be produced hy the passage of the sun's rays through the earth's atmosphere, but it may readily be shown that this is impossible. When one reflects that the half width of the moon's shadow, in the larger eclipses, is as great as the estimated height of the atmosphere, it will be seen that, to an observer on the central line of the eclipse the line of sight will not fall upon the illuminated portion of the atmosphere exterior to the shadow, unless inclined under a large angle to the line of direction of the centers of the sun and moon. The corona, therefore, if of ierrestrial atmospheric origin ought to present, toward the middle of the eclipse, the appearance of a halo en. tirely detached from the dark body of the moon, and many degrees distant from it. It ought also to increase in brightness from its inner border, for a considerable distance outward.
Others have imagined that the corona might be attributable to the passage of the sun's light through a lunar atmosphere; but since some of the streamers, or rays of the corona, have been seen to extend to a distance greater than the suu's diameter, this would require the lunar atmosphere to be of vast extent; whereas no decisive evidence has yet been obtained of the existence of any lunar atmos. phere capable of producing a sensible refraction, or reflecting a perceptible amount of the sun's light to an observer on the earth.
Perhaps the more prevalent idea, at the present day, is that the corona, with its rays and tufts of light. is a phenomenon of diffraction produced by the passage of the sun's rays along the denticulated edge of the noon. This theory has an air of plausibility, but it is entirely inadequate to account for the great extent of the coronal rays. The fringes produced by the diffraction of light in its passage near the edge of a body appear to the eye of the observer to extend but a small angular distance from the edge. This would be more strikingly true in the case of a distant body, like the moon.
The only remaining supposition is that the corona is either an envelope of some kind permanently connected with the sun, or is made up of material emanations proceeding immediately from the sun. To the large body of indirect evidence that the corona is wholly a solar phenomenon that has been obtained, we may now add that of direct observation, since it appears that “ an examination of the photographs of totality," obtained at the eclipse of 1869. shows that as the moon ad. vanced the corona was progressively covered.
ART. III. — On the Duration of Flashes of Lightning; by O. N.
Rood. (From a letter to Dr. W. GIBBS.)
AFTER the completion of my first set of experiments on the duration of the discharge of a Leyden jar, I became anxious to make some measurements of the duration of a flash of ordinary lightning, which may be considered as equivalent to the discharge of an immense jar with an enormous striking-distance. The results of Feddersen have shown that the duration of the discharge is increased by an addition to the size of the jar, as well as by augmentation of the striking-distance, and as both these quantities are so large with a flash of lightning, it was reasonable to expect that the duration of its discharge would be prolonged in some corresponding ratio. During the violent thunder-storm of last August, which occurred in the evening, I happened to be at a house commanding an unobstructed view of the horizon, and this circumstance taken in connection with the frequency and proximity of the electrical discharges, induced me, although entirely unprovided with apparatus, to attempt a measurement of their duration. A circular disc, five inches in diameter, was hastily cut from white cardboard, while a steel shawl-pin served as an axis, on which it was made to revolve by constantly striking its edge tangentially with the right hand, the pin being held in the left. The maximum velocity attainable in this way was always employed. The general indications at the time were that the rate thus obtained was considerably more uniform than might have been expected, and subsequent quantitative experiments have confirmed this