Chapter VIII: Part 8

But while the comet is shortening its distance and hastening to the sun in the form of a huge globular mass of diffuse light, it is continually encountering another force, increasing in a far more rapid ratio than the law of gravitation. At great distances from the sun, the force of the radial stream was insufficient to detach any portion of the comet's atmosphere; presently, however, the globular form is changed to an ellipsoid, the radial stream begins to strip the comet of that doubly attenuated atmosphere of which we have spoken, and the diameter of the comet is diminished, merely because the luminosity of the escaping ether is terminated at the limit of that atmosphere. Meanwhile the mass of the comet has suffered only an infinitely small diminution; but if the perihelion distance be small, the force may become powerful enough to detach the heavier particles of the nucleus, and thus a comet may suffer in mass by this denudating process. We regard, therefore, the nucleus of a comet to represent the mass of the comet and the coma, as auroral rays passing through a very attenuated envelope of detached particles. The individual gravitating force of these particles to the comet's centre, may be therefore considered as inversely as the squares of the distances, and directly as the density of the particles; and this density will, according to analogical reasoning, be as the distances or square roots of the distances;--grant the last ratio, and the gravitating force of the particles composing the exterior envelope of a comet, becomes inversely as the 2.5th power of the distances from the comet's centre.[45] This being the law of the radial stream, it follows, of course, that a comet's diameter is inversely as the force of the radial stream. It must, however, be borne in mind, that we are speaking of the atomic density, and not of density by compression; for this cometary dust, which renders luminous the escaping ether of the nucleus, must be far too much diffused to merit the name of an elastic fluid. May not the concentric rings, which were so conspicuous in the comet of 1811, be owing to differences in the gravitating forces of such particles, sifted, as it were, and thus arranged, according to some ratio of the distances, by the centripulsive force of the electric coma, leaving vacant intervals, through which the ether passed without becoming luminous? This at least is the explanation given by our theory. We may, indeed, consider it possible that the escaping ether, when very intense, might be rendered luminous by passing into the surrounding ether, and, as it became more diffused by radiation, at last become invisible. In this case, as the law of radiation is as the squares of the distances from the centre inversely, the rays would be more and more bent at right angles, or apparently shortened, as the power of the radial stream increased, and the apparent diameters of the coma would be diminished faster than the ratio of the 2.5th power of the distances. But whichever view we adopt, the diameter would again increase in the same ratio on leaving the sun, if we make allowance for increase of temperature, as well as for diminution of density, for the ordinary distance of a comet's visibility. We, however, regard the change of diameter, as due to both these nodes of action, as best agreeing with the indications afforded by their tails.

From the preceding remarks, it results that the density of the particles producing the nebulous envelope of a comet, renders the variations of diameter only approximate to the law of the radial stream; a comet's own electric energy, or the intensity of the escaping ether, may also modify this expression, and many other causes may be suggested. That the radial stream is the cause, in the way we have pointed out, is proved by the positions of the major axis of the short-period comet, making frequently nearly a right angle with the radius vector of the orbit in 1828. A soap bubble gently blown aside, without detaching it from the pipe, will afford a good illustration of the mode, and a confirmation of the cause. The angles measured by Struve, reckoned from the radius vector, prolonged towards the sun, are subjoined:

November 7 99d.7 | December 7 154d.0
November 30 145 .3 | December 14 149 .4

At this last date, the comet was getting pretty close to the sun. When the angle was greater, as on November 7th, the comet appeared to make almost a right angle with the radius vector; and in this position of the earth and comet, the longer axis of the elliptical comet was directed to the axis of the vortex, as may be verified by experiment. At the later dates, the comet was more rapidly descending, and, at the same time, the axis of the comet was getting more directed towards the earth; so that the angle increased between this axis and the radius vector, and consequently became more coincident with it. We have now to consider the luminous appendage of a comet, commonly called a tail.

The various theories hitherto proposed to account for this appendage are liable to grave objections. That it is not refracted light needs not a word of comment. Newton supposes the tail to partake of the nature of vapor, rising from the sun by its extreme levity, as smoke in a chimney, and rendered visible by the reflected light of the sun. But, how vapor should rise towards opposition in a vacuum, is utterly inexplicable. In speaking of the greater number of comets near the sun than on the opposite side, he observes: "Hinc etiam manifestum est quod coeli resistentia destituuntur."[46] And again, in another place, speaking of the tail moving with the same velocity of the comet, he says: "Et hinc rursus colligitur spatia coelestia vi resistendi destitui; utpote in quibus non solum solida planetarum et cometarum corpora, sed etiam rarissimi candarum vapores motus suos velocissimos liberrime peragunt ac diutissime conservant." On what _principle_, therefore, Newton relied to cause the vapors to ascend, does not appear. Hydrogen rises in our atmosphere because specifically lighter. If there were no atmosphere, hydrogen would not rise, but merely expand on all sides. But, a comet's tail shoots off into space in a straight line of one hundred millions of miles, and frequently as much as ten millions of miles in a single day, as in the case of the comet of 1843. Sir John Herschel observes, that "no rational or even plausible account has yet been rendered of those immensely luminous appendages which they bear about with them, and which are known as their tails." Yet, he believes, and astronomers generally believe, that a comet shines by reflected light. This theory of reflexion is the incubus which clogs the question with such formidable difficulties; for, it follows, that the reflecting matter must come from the comet. But, what wonderful elements must a comet be made of, to project themselves into space with such immense velocity, and in such enormous quantities as to exceed in volume the body from which they emanate many millions of times. This theory may be, therefore, safely rejected.

From what we have already advanced concerning the coma or nebulosity of the comet, we pass by an easy path to an explanation of the tail. In the short-period comets, the density of the elementary atoms is too great to be detached in the gross from the nucleus, or, rather, the density of the atoms composing the nucleus is too great to permit the radiating stream of the comet carrying them to a sufficient distance to be detached by the radial stream of the sun. Hence, these comets exhibit but very little tails. We may also conceive, that the continual siftings which the nucleus undergoes at each successive perihelion passage, have left but little of those lighter elements in comets whose mean distances are so small. Yet, again, if by any chance the eccentricity is increased, there are two causes--the density of the ether, and the heat of the sun--which may make a comet assume quite an imposing appearance when apparently reduced to the comparatively passive state above mentioned.

According to our theory, then, the coma of a comet is due to the elasticity of the ethereal medium within the nucleus, caused both by the diminished pressure of the external ether near the sun, and also by the increased temperature acting on the nucleus, and thus on the involved ether. The tail, on the contrary, is caused by the lighter particles of the comet's attenuated atmosphere being blown off by the electric blast of the radial stream of the solar vortex, in sufficient quantities to render its passage visible. It is not, therefore, reflected light, but an ethereal stream rendered luminous by this detached matter still held in check by the gravitating force of the sun, whose centre each particle still respects, and endeavors to describe such an orbit as results from its own atomic density, and the resultant action of both the acting forces. From the law of density of the ether, the coma ought to be brightest and the radiating stream of the comet's nucleus strongest on the side of least pressure: from this cause, and the fact that the body of the comet affords a certain protection to the particles immediately behind it, there will be an interval between the comet and the tail less luminous, as is almost invariably observed. We thus have an explanation of the fact noticed by Sir John Herschel, "that the structure of a comet, as seen in section in the direction of its length, must be that of a hollow envelope of a parabolic form, enclosing near its vertex the nucleus or head." We have, also, a satisfactory explanation of the rapid formation of the tail; of its being wider and fainter at its extremity; of its occasional curvature; and of its greater length after perihelion than before. But, more especially may we point to the explanation which this theory gives of the fact, that, _ceteris paribus_, the long-period comets, when their perihelion distances are small, have tails of such exaggerated dimensions.

A comet, whose mean distance is considerable, is supposed by the theory to be composed of elements less dense, and, during its long sojourn at its aphelion, it may be also supposed that it there receives continual accessions to its volume from the diffused siftings of the system, and from the scattered debris of other comets. On approaching the perihelion, the rapidity of the change in the density of the ether in a given time, depends on the eccentricity of the orbit, and so does the change of temperature; so that, from both causes, both the length of the tail and the brilliancy of the comet measurably depends on the magnitude of the period and of the eccentricity.

If the nuclei of comets be gaseous as we suppose, and that the smallest stars are visible through them, it is an outrage on common sense, to refer that light, which renders a comet visible at noon-day, within six minutes of space of the sun itself, to the reflected light of the sun. When a small star has been seen through the nucleus of a comet, without any perceptible diminution of light, it indicates perfect transparency; but there can be no reflection from a perfectly transparent body, and therefore, a comet does not shine by reflected light. It is true that Arago discovered traces of polarized light in the comet of 1819, and also in more recent comets, but they are mere traces, and Arago himself admits, that they do not permit "the conclusion decidedly that these stars shine only with a borrowed light." But it still does not follow that a comet (even if independent of reflected light) is in an incandescent state. The auroral light is not polarized, nor any other electric light, neither is it owing to a state of incandescence, yet it is luminous. The intense light of a comet at perihelion is analogous to the charcoal points of a galvanic battery, caused by a rapid current of ether from the nucleus, and assisted by the radial stream of the vortex. This will account for the phenomenon in all its shades of intensity, as well as for the absence of any perceptible phase. It will also account for the non-combustion of such comets as those of the years 1680 and 1843. We shall also be at no loss to understand, why there is no refraction when a ray of light from a star passes through the nebulosity of a comet; and if, as we may reasonably suppose, the gaseous matter composing the nucleus be very attenuated, instruments are yet too imperfect to determine whether these also have any refracting power. On this point, however, it is safest to suspend our judgment, as there may be comets not belonging to our system, with even liquid or solid nuclei, or of matter widely different to those elements composing the members of the solar system.

In addition to what has been already advanced on this subject of a comet's light, we may appeal to the well-known fact that the visibility of a comet is not reciprocally as the squares of the distances from the earth and sun as it ought to be, if shining by reflected light. In Mr. Hind's late work on comets, the fact is stated that "Dr. Olbers found that the comet of 1780 attained its greatest brightness on the 8th of November, thirteen days subsequent to its discovery, whereas according to the law of reflected light, it should have become gradually fainter from the day of its discovery; and supposing the comet self-luminous, the intensity of light should have increased each day until November 26th; yet in the interval between the 8th and 26th of that month, it grew rapidly less." Now this theory teaches, that a comet is neither self-luminous nor dependent on the sun, but on its distance from the axis of the vortex, and a certain amount of elapsed time from the perihelion, varying somewhat in each particular case. This fact is therefore a very strong argument in favor of our theory.

Amidst the many anomalous peculiarities of comets, it has been noticed that a short tail is sometimes seen at right angles to the principal tail, and in a few cases pointing directly towards the sun. Much of this may be owing to perspective, but granting the reality of the fact, it is still explicable on the same general principles.

In speaking of the modifying causes which influence the weather, we mentioned the effect due to the position of the sun with respect to the axis of the vortex. This will be found to have a sensible effect on the action of the radial stream. The natural direction of a comet's electric stream is _towards_ the axis of the vortex, and in the central plane of the vortex it will be also towards the sun. But this stream is met by the stronger radial stream from the axis, and as Mr. Hind describes it, "is driven _backward_ in two streams passing on either side of the head, and ultimately blending into one to form the tail." Now, if the body of the sun be situated between the comet and the axis of the vortex, it will shield the comet from the action of the radial stream, and thus a tail may really point towards the sun.

In 1744 a brilliant comet exhibited six distinct tails spread out like a fan, some seven days after its perihelion passage; its distance from the sun at the time not being more than a third of the earth's distance. The comet was then rapidly approaching the plane of the ecliptic, and if we make the calculation for the position of the sun, we shall find that the body of the sun was on the same side of the axis of the vortex as the comet, and that the comet was then situated at the boundaries of the conical space, enclosed by the radial stream in its deflected passage round the body of the sun. In this position there are numerous cross currents of the stream, and hence the phenomenon in question. As this fact rests on the testimony of one individual, and is an occurrence never recorded before or since, many are disposed to doubt the fact, yet our theory explains even this peculiarity, and shows that there is no necessity for impugning the statement of Cheseaux.

Another unexplained phenomenon is the corruscation of the tail. It has been attempted to explode this fact also, by referring it to conditions of our own atmosphere; and it is generally considered the argument of Olbers, founded on the great length of the tail and the velocity of light, is sufficient to prove that these corruscations are not actually in the tail. Now, it is undoubtedly true, that as light travels less than two hundred thousand miles in a second, and a comet's tail is frequently one hundred millions long, it is impossible to see an instantaneous motion along the whole line of the tail; but granting that there are such flickerings in the tail as are described by so many, it must necessarily be, that these flickerings will be _visible_. It would be wonderful indeed, if a series of waves passing from the comet to the extremity of the tail, should have their phases so exactly harmonizing with their respective distances as to produce a uniform steady light from a light in rapid motion. The argument, therefore, proves too much, and as it is in the very nature of electric light thus to corruscate, as we see frequently in the northern lights, we must be permitted still to believe that not only the tails, but also the heads of comets do really corruscate as described.

With respect to the direction of the tail, astronomers have been forced to abandon the antiquated notion, that the tail always pointed directly from the sun; yet they still pertinaciously cling to the idea, that although this is not always the case, the tail only deviates from this direction _in the plane of the orbit_. As this is a most important question, it is necessary formally to protest against such a conclusion. If the earth should happen to be in the plane of the comet's orbit and the tail appears in that plane, it must of course be in that plane _really_; but if the earth is not in the plane of the comet's orbit, the tail is not _necessarily_ in the same plane, whatever its apparent direction may indicate. It is true there is a tendency of every particle of the tail, moving under the restraining influence of the sun's attraction, to continue in the plane of the orbit; and in certain positions there is no oblique action arising from the force of the radial stream to cause it to deviate from that plane; yet in other positions of the comet, the action of the radial stream may be oblique, forcing it out of that plane, and still such a direction might be assigned to it as to make it conform. In the comet of 1843, P. Smythe observed a forked tail 25d long on March 3d, and from the end of the forked tail, and from its _north_ side, a streamer diverged at an angle of 6d or 7d to the _north_. As this was contrary to the _direction_ of the curvature, if the tail had been curved, it could only arise from a portion being driven off by the radial stream, or bent towards the plane of the ecliptic. The curvature observed by others at a later date, was concave to the south. Towards the middle and close of March, the tail became straight, and with the above exception, might be considered to move in the plane of the orbit.

The celebrated comet of Halley, as observed by Dr. Bessel in 1835, showed that a more or less well-defined tuft of rays emanated from that part of the nucleus which was turned towards the sun; and the rays being _bent backward_ formed a part of the tail. The nucleus, with its emanations, presented the appearance of a burning rocket, the end of which was turned sideways by the force of the wind. And, Bessel concludes: "That the cone of light issuing from the comet deviated considerably both to the right and left of the true direction of the sun, but that it always returned to that direction, and passed over to the opposite side; so that the cone of light, and the body of the comet from whence it emanated, experienced a rotatory, or, rather, a vibrating motion _in the plane of the orbit_." It is impossible that Bessel should here mean that this motion was certainly in the plane of the orbit; for the orbit was then viewed sideways, and he had no means of ascertaining the fact. His meaning must be that it was apparently in the plane of the orbit. If a plane be made to pass through the earth, the comet, and the sun, the tail might be placed in any position in that plane, and yet appear to be at the intersection of the two; that is, in the plane of the comet's orbit. The vibration of the tail, in this case, is another strong proof of the correctness of our theory. To make it more intelligible, we shall resort to a diagram.

In the following diagram, the comet's orbit, represented by the dotted line, is drawn on the plane of the ecliptic; it is, therefore, necessary to bear in mind, that it is tilted up from the line of nodes SN, at an angle of 17d 45'. The position of the comet, October 9th, is at C, approaching its perihelion; that of the earth at the same time at T; while S represents the sun, and SQ the line of equinoxes. Now, from a cause already explained, the tail always tends to lay behind the comet, in the direction indicated by the lower tail in the diagram at 1, and, if produced, would pass to the left of the sun, as seen from the earth: the force of the radial stream, however, will not allow this lagging of the tail, and it is straightened out by this force; but, being directed to the axis of the vortex, and not to the sun, it is not really in the plane of the orbit, but is seen in the direction of the upper tail depicted in the diagram at 3, and, if produced, would pass to the right of the sun, as seen from T. Now, there is an intermediate position of the tail, in which it will appear in the prolongation of the radius vector SC; this position is represented by the middle or central tail of the comet at 2, yet this is not in the plane of the orbit, it only appears to be, as may be readily understood by remembering that the earth at this time is under this plane, and the comet is seen at a considerable elevation above the plane of the ecliptic. When the comet's tail becomes directed to the axis of the vortex, or in the _apparent_ position of No. 3, the comet, rapidly careering on its way to the sun, again leaves the tail behind, and again it is strengthened out by the radial stream oscillating about the mean position at 2, as observed by Bessel. From this, it appears, that there is no necessity to make confusion worse confounded, by resorting to polar forces, which are about as intelligible as the foundations of the pillars of Atlas.

It may be objected that the continued action of the radial stream with that velocity we have contended for, ought to keep the tail invariably directed from the axis of the vortex; but, where there are two forces or tendencies, as in this case, analogy would teach us that a certain degree of oscillation is a necessary result. There may, also, be slight and transient changes in the direction of the radial stream. In the hurricane there are short and fitful blasts inclined to the general direction of the wind, which must arise from the inertia of the moving mass of atmosphere, causing temporary condensations and rarefactions. Be this as it may, we have assigned a cause which satisfies the phenomenon, without coming into collision with a single principle of celestial mechanics.

Prof. Struve compared the tail of this comet to a flame, or "ray of fire shot out from the nucleus, as from some engine of artillery, and driven on one side by the wind." At the same time, he saw a second emanation nearly in the opposite direction. This last might arise from a momentary fluctuation in the relative intensities of the electric radiation of the comet, and of the radial stream, owing to the probable irregularities just alluded to. Such and kindred phenomena are utterly inexplicable, without we adopt the theory we are advocating. One other feature, and we will leave the subject.

From our explanation of the solar spots, we inferred the existence of another large planet in the system. Might not the same effect be produced by a comet? Or may there not be so many comets, whose great elongation, combined with even a moderate mass, may render it impossible to calculate the position of the sun with respect to the central axis of the vortex,--always considering this last as the axis of equilibrium? In a general way, we might say that the very number of comets in all directions and all distances, would tend to neutralize each other's effects; but we are not under this necessity. A comet, moving in a parabola, does not belong to the system or to the rotating vortex; and the periodic comets, if of gaseous elements, (as seems so probable,) must, from the size of their nuclei, which the theory considers the only part constituting their mass, have far less mass than the very smallest of the asteroids, and consequently could have very little effect on the mechanical balance of the vortex, even if elongated as far as the orbit of Neptune. Did we know the influence of cold in limiting the expansibility of the elementary gases, we might approximately determine the mass of a comet, from the size of its nucleus; but this is a problem that has never yet been solved; and astronomers ought to avail themselves of every indication which promises to realize this great desideratum. The grand comet of 1556 is now probably approaching, and, from recent investigations, it appears that it will arrive at its perihelion in 1858,--subject to an error either way of about two years. An opportunity may thus be presented of determining the mass of one of the largest comets on record, which may not again occur. This arises from the possible appulse of the comet to the planet Pallas, whose mass, being so small, would more sensibly be disturbed by such an appulse than the earth. As the inclinations and ascending nodes of the two orbits approximately coincide, and as Pallas will be near the comet's path, on the approach of the latter to the sun, at the beginning of the year 1857, should the comet become visible about that time, a very close appulse is possible. It is not unlikely, also, that if the elements of Pallas were so far perfected as to afford reliable indications, that the near approach of the comet might thus be heralded in advance, and lead to an earlier detection of its presence. Would it not be a worthy contribution to science, for some one possessing the necessary leisure, to give an ephemeris of the planet for that epoch; as a very slight change in Mr. Hind's elements of the comet, would cause an actual intersection of the two orbits in about heliocentric longitude 153d? The subsequent nodal passage of Pallas will take place near opposition, and be very favorably situated for determining the instant of its passage; and, of all the elements, this would be more likely to be affected than any other.[47]

THE ZODIAL LIGHT.

A phenomenon, akin to that which we have just been considering, is presented by that great cone of diffused light which accompanies the sun, and which in tropical climes displays a brilliancy seldom witnessed in high latitudes, on account of its greater deviation from the perpendicular. Sir John Herschel conjectures that it may be "no other than the denser part of that medium, which, as we have reason to believe, resists the motion, of comets,--loaded, perhaps, with the actual materials of the tails of millions of those bodies, of which they have been stripped in their successive perihelion passages, and which may be slowly subsiding into the sun." If these materials have been stripped, it is due to some force; and the same force would scarcely permit them to subside into the sun. Once stripped, these portions must be borne outwards, by the radial stream, to the outer verge of the system. Still, there are, no doubt, denser particles of matter, of the average atomic density of Mercury and Venus, which can maintain their ground against the radial stream, and continue to circulate near the central plane of the vortex, in all that space between the earth and the sun. But if the zodial light be the denser part of that medium, which astronomers now generally recognize as a resisting medium, how happens it that it should be confined to the plane of the ecliptic? Why should it not be a globular atmosphere? Here, again, our theory steps in with a triumphant explanation; for while it permits the accumulation of such particles around the equatorial plane of the sun, it allows no resting-place very far removed from this plane. The zodial light, therefore, is not the resisting medium, but the passage of the radial stream through a diffuse nebula of atoms, brought down the poles of the vortex by the polar current, and held in check along the central plane by gravitation.

If these atoms partook of the velocity of the ether, they would not be luminous; but being held back by gravitation, they are opposed to the radial stream, and hence the light.

Many stars are also nebulous. In some cases we see the nebulosity edgewise, or along the equatorial planes of the stellar vortices; in others we look down the poles, and the nebulosities are circular, and there is an endless variety in the shape and intensity of this light. But the universe seems full of motion, and we are not justified in supposing, because a star shows no such light, that it is without rotation. The parallax of the nearest star is only one second, the whole lenticular mass of light which surrounds our sun would therefore only subtend an angle of a single second at the nearest fixed star. Seeing its extreme faintness, therefore, the effulgence of the star would render it totally invisible, provided that it _could_ traverse the vast immensity of intervening space, without feeling the influence of that extinction, which Struve has proved does actually diminish the number of visible stars.

Corruscations and flickerings have also been noticed in the zodial light, and as usual, the learned have suggested atmospheric conditions as the cause, instead of trusting to the evidence of their own senses. How prone is philosophy to cling to that which is enveloped in the mist of uncertainty, rather than embrace the _too simple_ indications of nature. As if God had only intended her glories to be revealed to a favored few, and not to mankind at large. Blessed will be the day when _all_ will appreciate their own powers and privileges, and no longer regard the oracles which emanate from a professional priesthood, whose dicta have so often tended to darken the simple counsels of truth! To set the question of pulsations in the zodial light, as well as in the tails of comets, at rest, only requires previously concerted observations, in places not very widely apart; for it is scarcely possible, that atmospheric conditions should produce simultaneous pulsations in two distant places. If the pulsations are found to be simultaneous, they are real; if not simultaneous, they may depend on such conditions; but from the nature of the cause, we should look for them as much in the zodial light, as in the aurora borealis, regarding the different intensities.

There is also reason to suspect that the northern side is always the brightest, both in spring and autumn. On the morning of October 4th, 1853, the light was very vivid and well defined, its northern margin grazing Regulus and terminating at Mars, which was also to the north of it. Now, although the _northern side_ was the brightest, the great mass of light was to the south of the ecliptic, as far down as the cone shape was preserved; but at 10d from the horizon, a still brighter mass protruded from the cone towards the north, which was all _north_ of the ecliptic, and of an irregular form, extending along the horizon. The time was 4 A.M., and consequently was not due to any crepuscular light. An explanation of the general fact of the brightest light being _always_ on the north side, is given in the present section, in connection with another phenomenon. If, as some suppose, the light does not reach to the sun, the annulus must at least fill all the space between Venus and the earth, but it is far more in accordance with facts as well as with our theory, to suppose it increases in density to the body of the sun.

Observations made at the observatory of the British Association, detected, in 1850, sudden brightenings of the light, altogether different from pulsations. The theory would refer these to that fitful irregularity in the momentary intensity of the radial stream, which gives the flickering and tremulous motion to comets' tails. But, the steady variations in the intensity of this light must be due to other causes. The longitude of the sun will here come in as a modifying cause; for the obstruction caused by the body of the sun, when displaced from the axis of the vortex, must necessarily exercise an influence on the force and direction of the radial stream. A sudden influx of cometary matter down the poles of the vortex, in more than usual quantities, will also tend to brighten and enlarge the zodial light; and, in this last cause, we have an explanation not only of ancient obscurations of the solar light, but, also, of those phosphorescent mists, such as occurred in 1743 and 1831, rendering moonless nights so light that the smallest print could be read at midnight.

In total eclipses of the sun, the denser portion of the zodial light is visible as a brilliant corona; but, on such occasions, the brightest stars only are to be seen, and, consequently, the fainter portions of the light must be invisible. Hind mentions as many as ten stars visible in the total eclipse of 1842. According to the same authority, the color of the corona was like tarnished silver, and rays of light diverged in every direction, and appeared shining through the light of the corona in the total eclipse of 1851. In this year on the day of the eclipse (July 28th), the longitude of the sun was about 340d, and, therefore, the body of the sun obstructed the radial stream as seen from the earth on the right side; but, in 1842, the longitude of the sun was, according to our table, about 116d, the sun's centre then being 700,000 miles from the axis of the vortex, and on the opposite side with respect to the earth; the position was, therefore, not so favorable for the appearance of these rays which, in many cases, have given the appearance of a whirling motion to the corona.

At this date, July 7th, 1842, the corona, according to Prof. Airy, "possibly had a somewhat radial appearance, but not sufficiently marked to interfere with the general annular structure." Mr. Baily, on the contrary, says, the corona had the appearance of brilliant rays; and, at Milan, long jets of light were particularly noticed. There can be no doubt but that the passage of the radial stream past the outer margin of the moon must also give rise to the same phenomena as when passing the sun, and in this we have an explanation of the fact, that, previous to the moment of first contact, an appearance resembling a faintly-illuminated limb of the moon, has been perceived near the body of the sun; as well as of those flashes of light which have been observed in the lunar disc as the eclipse advances. One important fact, worthy of note, is, that these luminous streaks are more nearly parallel than is due to a radiation from the centre. These streaks have, also, been seen bent at right angles at the middle of their height, as a flame is by means of a blowpipe, precisely analogous to cometary rays being driven backwards to form the tail, as already described, thus indicating a common origin. If the moon had an atmosphere, we should, no doubt, see a greater display; but, having no rotating vortex to protect her from the radial stream, her atmosphere must have been long since stripped off, leaving her exposed to the withering winter blast of the great stream of the solar vortex. In this connection, we may also allude to the appearance of the moon when totally eclipsed. Instead of disappearing at these times, she sometimes shines bright enough to reveal her smallest spots. This has been generally referred to the refraction of the earth's atmosphere bending inwards the solar rays. May it not be owing to the brilliancy of the solar corona, which, in 1842, was described as so intense that the eye was scarcely able to support it? This is a far more palpable cause for the production of this phenomenon, but of which astronomers cannot avail themselves, as long as they are uncertain of the origin of this corona.

SHOOTING STARS.

The continual influx of cosmical matter into the heart of the vortex in ever-varying quantities, and speedily dispersed along the central plane, according to its density, must necessarily give rise to another phenomenon to which we have not yet alluded. Scarcely a night passes without exhibiting this phenomena in some degree, and it is generally supposed that the hourly average of shooting stars is from five to ten, taking the whole year round. The matter composing these meteors we regard as identical with that mass of diffused atoms which forms a stratum conforming to the central plane of the vortex, and whose partial resistance to the radial stream occasions that luminosity which we call the zodial light. These atoms may coalesce into spherical aggregations, either as elastic gas, or as planetary dust, and, passing outward on the radial stream, will occasionally become involved in the vortex of our own globe; and being drawn inwards by the polar current, and acted on by the earth's gravity, be impelled with great velocity through the rarefied air of the upper atmosphere. That meteors are more abundant about the time of meridian passage of a vortex (or, perhaps, more correctly speaking, from six to twelve hours afterwards, when the current of restoration penetrates the atmosphere), well accords with the author's observations. It is about this time that high winds may be looked for, according to the theory; and it has ever been a popular opinion, that these meteors are a sign of windy weather. Even in Virgil's time, the same belief prevailed, as a passage in his Georgics would seem to indicate.

"Sape etiam stellas, vento impendente, videbis
Praecipites coelo labi; noctisque per umbram
Flammarum longos a tergo albescere tractus;"

Virgil was a close observer of nature, and commences a storm with the wind at south, "Quo signo caderent Austri;" just as we have represented the usual course when these vortices pass near the observer's latitude. It is also a well-known fact, that after a display of meteors, (and we are now speaking of ordinary displays, and not of the great showers,) the temperature falls considerably. It is not uncommon also, that meteors are more abundant during an auroral display, as they ought to be by the theory. We must, however, exempt from this influence those solid meteors which sometimes come into collision with the earth, and afterwards grace the cabinets of the curious. These bodies may be considered microscopic planets, moving in stated orbits with planetary velocity, and bear strongly on the explosive theory of Olbers, as fully detailed by Sir David Brewster.

It is a very remarkable fact, first noticed by Olbers, that no fossil meteoric stones have yet been discovered. If this fact be coupled with the hypothesis advanced by Olbers, in reference to the origin of the asteroidal group, we should have to date that tremendous catastrophe since the deposition of our tertiary formations, and therefore it might possibly be subsequent to the introduction of the present race into the world. May not some of the legendary myths of the ancient world as mystified by the Greeks, have for a foundation the disappearance of a former great planet from the system? The idea of the existence of seven planets is one of the oldest records of antiquity; but the earth of course would not be counted one, and therefore in after times, the sun was included to make up the number; just as the signs of the Zodiac have been explained in accordance with the seasons of far later times than we can possibly assign for the invention of this division of the heavens. Let those who have the leisure, try how far the contraction and dilation of the asteroidal orbits, to some average mean distance, will restore them to a common intersection or node, as the point of divergence of the different fragments. The question is interesting in many of its aspects, and may yet be satisfactorily answered.

The composition of aerolites may also be taken as indications of the common origin and elementary texture of the planets, whether they are independently formed or have originally pertained to a former planet; for no hypothesis of telluric or selenic origin yet advanced, can stand against the weight of evidence against it. Their fragmentary character rather favors the views of Sir David Brewster, and when we consider that they have been revolving for thousands of years with planetary velocity, and in very eccentric orbits, through the ether of space, continually scathed by the electric blast of the radial stream, their rounded angles, and black glossy crust of an apparently fused envelope, may be accounted for, without difficulty, from the non-vitrified appearance of the interior. The composition of aerolites as far as known, embrace nearly one-third of all known simple substances according to Humboldt, and are as follows: iron, nickel, cobalt, manganese, chromium, copper, arsenic, zinc, potash, soda, sulphur, phosphorus, and carbon.

The theory we have thus given of the common occurrence of shooting stars, will render a satisfactory general account of their sporadic appearance; but there are other phenomena of greater interest, viz.: the occasional recurrence of swarms of such meteors, which defy all numerical estimates, being more like a fiery rain than anything they can be compared to. The most interesting feature of this phenomena, is the _apparent_ periodicity of their return. In the following table we have set down the most remarkable epochs mentioned by Humboldt, (and no man has devoted more attention to the subject,) as worthy of notice:

About April 22 to 25
" July 17 to 26
" August 9 to 11
" November 12 to 14
" November 27 to 29
" December 6 to 12

Besides these, he mentions two showers, from Arabian authority, in October; one in October, observed in Bohemia; one observed by himself, in the Pacific, on March 15; one February 4, just preceding the terrible earthquake of Riobamba, in 1797. The Chinese annals also contain many showers of stars, before the present era commenced. Some were in March, more in July, and others in different months. How, then, in view of these numerous dates, can we attach so much importance to the periodicity of these showers? The great shower of 1833, in the United States, on the 12th and 13th of November, brought to mind the great shower at Cumana, observed by Humboldt and Bonpland just thirty-three years before, to a day; and it must be confessed that more than ordinary displays have been seen on this date. Yet, on the strength of this, every meteoric shower is supposed to be periodical, and has resulted in a theory which becomes more complicated as the phenomenon is more observed, and can never lead to any useful and practical results. To cite the numerous instances of discrepant results, would only encumber this brief notice with facts neither interesting to the general reader, nor convincing to those who hold a contrary opinion. The author of these pages has watched for many years, and, in view of all the facts, has concluded that the doctrine of periodicity (as held by present meteorologists) is not tenable. The celebrated August shower failed, also, this year, at least in this place, as for four hours each night, on the 9th, 10th, and 11th, there were fewer bright meteors than at the close of July.

Professor Olmsted, who has paid considerable attention to the subject, has indeed attempted to connect the great November shower with the zodial light, which last he considers a nebulous body, of an elongated form, whose external portions, at this time of the year, lie across the earth's path. (See Silliman's Journal for 1837, vol. xxxiii. No. 2, p. 392.) He even gives its periods, (about six months,) the aphelion of the orbit being near the earth's orbit, and the perihelion within Mercury's. In this way he attempts to explain both phenomena; but as the zodial light is seen unchanged all the year round in tropical latitudes, it is not the kind of body supposed by Olmsted, and the theory adds nothing to our knowledge. Others have imagined rings of nebulous matter, in which all the separate parts are moving in the same orbit around the sun, with a retrograde motion, and this, with some modifications, is the current theory of the day. The principal arguments rested on, for the support of this view, are derived from the great shower of 1833, in which a common radiant point was observed, and confirmed subsequently by the radiant of other years, in the same month of November. As this point is almost tangential to the earth's orbit at this season, the earth meets the nebulous ring moving in the contrary direction, and thus confers on these meteors the necessary velocity that is thought to be demanded by observation.

Now, our theory gives a totally different explanation of the phenomenon. We contend that a retrograde motion of such a nebulous mass, is subversive of our whole theory; and we must be permitted to examine certain points, hitherto disregarded by those entertaining antagonist views. It is supposed that the meteors in 1833 fell for eight or nine hours. The orbital velocity of the earth is more than 1,000 miles per minute, and the orbital velocity of the nebulous zone must have had a similar velocity. During the nine hours of meteoric display, therefore, the earth traversed 500,000 miles of her orbit, which would give 1,000,000 miles for the depth of the nebulous stratum. But if of such vast extent, how happened it that the only part of the earth in which these were visible in great density, was the United States, or a space embraced between the latitudes of 50d and 20d north, and the longitudes 60d and 100d west, (and these are the widest limits,) comprising only 1/40 of the surface of the globe? To a calm inquirer, this difficulty seems insurmountable. The author was then in the Mediterranean, on deck the greatest part of the night,--the weather fine, and nothing unusual visible in the heavens; from other sources he has also derived similar information. Yet, were the earth then passing through a stratum of meteors 1,000,000 miles in extent, it is utterly inconceivable that other portions of the earth escaped. Much stress is also laid on the fact that these meteors in 1833, passed from east to west generally, as they ought to do, if tangential to the earth in her orbit; but on the same phenomenon occurring in 1799, when the earth was in precisely the same part of her orbit, Humboldt says distinctly, "the direction (of the meteors) was very regular from north to south." How could this possibly happen, and at the same time be moving tangentially to the orbit?

There is also another fact of importance not duly weighed in forming such a theory. In 1833 the meteors evidently differed in velocity; one class, consisting of luminous points, passed like a shower of fire with great velocity to the westward, another class were like large fire-balls with luminous trains moving with less rapidity, while a third class consisted of nebulous patches which remained stationary for a long time, and frequently emitting large streams of light. These last, at least, do not deport themselves as planetary bodies moving 2,000 miles per minute. But the fact still remains, that unusual displays have occurred about the 12th and 14th of November; and also as a general thing when there are no unusual displays, the meteors are more abundant about this time. Let us try if we can reconcile these facts with the theory of vortices.

We will first confine our remarks to the increased number of meteors about November 12th and 14th. The cosmical matter composing the zodial light, or at least the lighter parts of it, is continually driven outwards by the radial stream, just as the matter of a comet's tail is stripped from the nucleus. This matter becomes involved in the terral vortex by descending the poles, and is again passed out along the equatorial plane. The form of the zodial light, as seen edgewise, gives a lenticular form for the stratum of planetary particles composing it, and its central plane has been considered as coinciding with the plane of the sun's equator. At the orbit of the earth, this lenticular space is narrowed to a very thin stratum, but undoubtedly reaches beyond the earth's orbit with a rapidly diminishing density. As the axis of the sun is inclined about 7d to the ecliptic, and the ascending node is in the 20th degree of Gemini, the earth can only pass through the plane of the sun's equator about the 12th of December and the 12th of June. If, therefore, the central plane of the vortex coincides with the plane of the sun's equator, meteors ought to be more numerous about the dates above mentioned. But the observed times are on November 12th and 13th. Now, from actual measurements, a computation has been made by M. Houzeau, that the elements of the zodial light are materially different from those of the sun's equator. He fixes the node of the light (according to Mr. Hind) in 2d heliocentric longitude, subject to an uncertainty of 12d or 13d, and its inclination to the plane of the ecliptic, 3d 35', subject to an uncertainty of about 2d. The truth is, astronomers have argued the coincidence of the two planes from considerations connecting the zodial light with the sun's equator, as if it were a solar atmosphere; but such an atmosphere is impossible, and it is high time such measures should be taken as will lead to some certain conclusion. If in the present state of the question, we were to take the mean, we should find the node in about longitude 40d, which is the position of the earth on November 2d. But in the absence of measurements, we will assume, for the sake of argument, that the ascending node of the central plane of the vortex was, in 1833, in 50d heliocentric longitude, and consequently the earth was passing through the meteoric stratum or central plane of the zodial light, on the night of November 12th. The opposite period of the year is May 12th--a date, it is true, on which no great shower of stars is recorded, but sporadic meteors are very plentiful at that time, and what is more important to observe is, that the 11th, 12th, and 13th of May, are the three noted _cold days_ which we have before mentioned. Thus truly indicating that the earth is then in or near the central plane of the vortex along which the radial stream is at its maximum of power at any given distance from the axis.