Part 9

The matter raised up in these envelopes seems to have undergone a certain change of character, causing it no longer to obey the sun’s attractive influence, but to experience a strong repulsive action from him, whereby it is apparently swept away with great rapidity to form the tail. “It flows past the nucleus,” says Dr. Huggins, “on all sides, still ever expanding and shooting backward until a tail is formed in a direction opposite to the sun. This tail is usually curved, though sometimes rays or extra tails sensibly straight are also seen.” The description is, however, incomplete in one important respect. The matter raised from the nucleus to form the envelopes may be, and probably is, carried past the nucleus on all sides; but the appearance presented by the tail just behind the nucleus is not exactly in accordance with our ideas as to what should result from the flowing past “on all sides.” There is a dark space immediately behind the nucleus, that is, where the nucleus, if solid, would throw its shadow, if there were matter to receive the light all around so that the shadow could be seen. Now it may be thought at first that this corresponds exactly with what should be seen: when we look just behind the nucleus there is no light, or very little; when we look on either side of that dark space there is the luminous matter which has been driven back from the envelopes in front of the nucleus. But if the luminous matter flows past the nucleus _on all sides_, it must flow past the nucleus on the side nearest to the observer, and also on the side farthest away; and it is just where the line of the sight passes through these two regions of brightness that a dark streak is seen just behind the nucleus. Let the reader draw two concentric circles—one an inch in diameter, the other two inches—and let him then draw two parallel tangents to the inner circle on opposite sides of it. Supposing now the space between the two circles to represent in section the luminous matter which flows all round the nucleus, while the surface of the inner circle represents the unilluminated part behind the nucleus, the two tangent lines will represent the lines of sight on either side of the dark region, where as we might expect, we get plenty of light; and we can also understand very well why outside of that the line of sight through the luminous matter (or the chords to our outer circle), getting shorter and shorter, the light of the luminous streaks bounding this part of the tail gets fainter and fainter; but if just inside either of the two tangents, chords are drawn parallel to them, crossing the inner circle, the parts of these chords which lie between the two circles are very nearly equal in length to the tangent lines themselves; and even a common diameter to both circles has, lying between them, two portions together equal to the radius of the outer. Hence, since the line of sight even across the middle of the space behind the nucleus, passes through a considerable range of luminous matter, while a line within but near the outskirts of that space passes through nearly as great a range of luminous matter as one just outside that space, there should be plenty of light where yet to the eye there seems to be something like absolute darkness. Either then the eye is greatly deceived, or else we must find some explanation of darkness existing where considerable brightness might be expected.[D]

The matter which forms the tail, seems, as I have said, to be swept off from the envelopes raised by the sun’s action on the nucleus. It seems as though the matter thus raised had undergone in some way a change of character, which caused it no longer to obey the law of gravity as it had done when forming part of the nucleus, but instead of yielding to the sun’s attraction, to submit rather to an intense repulsive action, carrying it at a much greater rate from the sun than, under the action of gravity—starting from rest and free from all perturbing influences—it could have been drawn toward him. Dr. Huggins thus words his account of what seems to happen: “Now is seen to take place a change which is most puzzling—namely, these envelopes of light appear to give up their substance under the influence of a strong repulsive force exerted from the sun, and to be forced backwards.” Sir John Herschel, after his long and careful study of the comet of 1830 (Halley’s at its second return) came to the conclusion that repulsive action exerted by the sun on the matter raised in these envelopes had been distinctly proved.

Yet here, where we seem to have our first firm ground for hypothesis respecting these mysterious objects—comets’ tails—we meet with stupendous difficulties. Consider, for instance, the phenomena presented by Newton’s comet. That comet had traversed the last ninety millions of miles of its approach toward the sun in four weeks. At the end of that time it passed out of view for a few days, having then a tail ninety millions of miles, at least, in length. Four days passed, and it reappeared on the other side of the sun—having in the interval traversed nearly a semi-circle—in reality, of course, the perihelion end of its long oval path. At its reappearance, it had a tail still ninety millions of miles in length, but the tail with which it reappeared had, of course, a direction entirely different from that of the tail which had been seen before—the two directions were inclined about one hundred and sixty degrees to each other. Now, as Sir John Herschel remarks, we can not look on the tail of a comet as something whirled round like a stick, as the comet circles round its perihelion sweep. The tail with which the comet reappeared must have been an entirely new formation. Nor can we doubt that if the comet had been watched as it swept around the sun, the changes in the tail’s position which had been observed to the time of disappearance, would have been observed to progress continuously, the tail passing by a uniform motion from the position it then had to that which it was observed to have at the time of reappearance. So that we may fairly suppose the tail with which the comet reappeared to have been formed in much less than the time during which the comet had been out of sight. Probably its farthest part had been formed in much less than a day, the part near the head being, of course, formed later. But if the matter repelled from the head was thus driven over a distance of ninety million miles in twenty-four hours, at the outside, the average velocity of its motion was about a thousand miles per second, or nearly three times as great as the greatest velocity which the sun _can_ communicate by his attractive energy to matter approaching him from without, even though such matter come to him from an almost infinite distance, and in a perfectly straight line—the conditions most favorable for giving a high rate of final velocity. Such velocity as the sun can thus give by his attractive energy is only given to matter which has been exposed a long time to his influence; but here, in the tail of the great comet of 1680, matter seems to have acquired almost instantaneously a velocity sufficing to carry it over ninety million miles with an average speed three times as great as the sun can thus, after long effort, communicate by means of his attractive power!

The difficulty is so great that many efforts—some bold and daring, others positively wild in the unscientific absurdity of their nature—have been made to overcome it.

Among the most ingenious of these is (or rather was, for I think it is no longer maintained even by its eminent author), Prof. Tyndall’s theory of a comet’s tail as an actinic cloud, generated by the passage of the solar rays through exceedingly tenuous matter after those rays had been in part deprived of their heating power, during their passage through the comet’s head. According to this theory the actinic cloud can not be formed under the heating rays, but so soon as the actinic rays fall on the tenuous matter alone, the cloud is formed,—so that all round the region in which would be the comet’s shadow, there is no luminous cloud, while along that region the cloud exists. The rapidity with which light travels would of course make this explanation absolutely perfect in explaining cometic tails lying always exactly in a straight line directed from the sun, or with their axis so situated. But unfortunately this exceedingly rapid formation of the tail (a tail of ninety million miles in length would be formed in about eight minutes) is more than observation requires or can explain. Prof. Tyndall made a slight oversight in dealing with this part of his theory. Noticing that the actinic cloud, as he called it, is not formed instantly, but after a delay of a few seconds, in his experiments, he reasoned as though it would follow from this that the formation of the actinic cloud behind a comet’s head in space might be a process extending its action in distance from the head at a rate considerably less than that at which light travels, yet still fast enough to account for the exceedingly rapid formation of the tail of Newton’s comet, and of other similar tails. But a little consideration will show that the few seconds following the fall of light on the vapors dealt with by Tyndall, before the luminous cloud appeared, would produce no such effect as he imagined. The rate of formation of the tail would still be that at which light travels. Imagine the head at A, for the sake of argument, and the sun’s light after reaching A, passing on to B, C, D, E, etc., to Z, a distance say of one hundred million miles, in nine minutes:

A ... B ... C ... D ... E ... ... Z.

Suppose that, when the light has reached the vaporous matter lying at B, an interval of one full minute (much greater than any noticed in Tyndall’s experiments), occurs before the actinic cloud comes into view, a similar interval after the light has passed C before the cloud is seen there, and so on, up to the time of the arrival of the light at Z. Professor Tyndall’s reasoning implied that all the time intervals thus occurring at B, C, D, E, etc., up to Z, had to be added together, to give the total time of the formation of the tail from A to Z, and hence naturally a long time might elapse, and the head having at the end of this time reached a different position from that which it had occupied at the beginning, the divergence of the tail from the direction exactly opposite to the sun, and the curvature of the tail, would be alike readily accounted for. But what are the actual facts of the case. The part of the tail formed latest by the supposed solar actinic action, namely, the part at Z, would be formed just nine minutes after the light had left A, and ten minutes after the part nearest to A had been formed (by the same light waves), for, nine minutes after leaving A, the light would be at Z, and a minute after each epoch (according to our supposition) the actinic cloud would be formed respectively at A and at Z. We get just the same interval—nine minutes—whether the actinic cloud appears immediately after light has traversed the vapour which is to form the cloud, or a minute after, or an hour after. In every case the tail would be formed outwards from A, at the rate at which light travels. This does not accord with the phenomena—in fact, the supposition that a tail could be formed at the rate at which light travels, will be found, on examination, to lead to many most manifest absurdities, which Professor Tyndall doubtless recognized when he sought escape from the supposition of such rapid tail formation, through the effects he attributed to the delayed appearance of the actinic cloud.

Another theory in explanation of the rapid formation of such a tail as that of Newton’s comet is worthy of far less notice. Professor Tyndall’s theory was based on an interesting physical fact, which he had himself discovered, and which was also manifestly akin in character to the formation of a comet’s tail. The one to be now noticed was suggested to a mathematician by a rather familiar phenomenon, the effects of which on his imagination he seems to have been never able to entirely overcome—at any rate no amount of evidence against the theory seems to counterbalance in his mind the notion once conceived that the theory might be true. (It is a way some theorists have.)

Professor Tait was once looking at a part of the sky which seemed clear. As he looked, a long streak rapidly formed, which presently disappeared (if I remember his original description aright) almost as rapidly as it had formed. At any rate, the appearance of the streak was rapid enough to remind him of what astronomers said about the rapid (apparent) development of comets’ tails. The phenomenon itself was easily explained. There had been a flight of seabirds, traveling after their wont in a widely extended layer, which when he began his observations had been looked at somewhat aslant, so that—the distance being too great for the birds to be seen individually—nothing of the flight could be discerned at all. But it is evident that in such a case a very slight movement on the part of each bird would suffice so to shift the position of the layer in which they were traveling, that it would be seen edgewise, and then the birds, being so situated that the range of sight toward any part of the layer passed athwart a great number of them, would of course be seen, not individually but as a cloud, or long straight streak, a side view in fact of the layer in which they were traveling. _Eureka!_ shouted Professor Tait; and presently announced to the world the marvelous theory that the rapid formation of comets’ tails may be accounted for on the same general principle. Astronomers have found that along the tracks of some comets (where the tails never lie, by the way, but that is a detail) are countless millions of meteoric bodies separately undiscernable (and never yet discerned as a cloud—another detail); therefore it follows that the tails of all comets are formed by movements of “brickbats and paving stones” in them (Professor Tait’s own description of meteors), after the manner of the seabirds he saw from Arthur’s Seat. Professor Thomson at the Edinburgh meeting of the British Association endorsed this theory with special reference to the value of the “seabird analogy” in explaining the phenomena of Newton’s comet. Dr. Huggins, who, as he does not claim to be a mathematician (or to speak more correctly, as his labors in physical research have not given him time for profound mathematical research), may be more readily excused, also speaks of the seabird theory as if it had some legitimate standing. “The tail, he conceives,” he says, referring to Dr. Tait, “to be a portion of the less dense part of the train illuminated by sunlight, and visible or invisible to us, according, not only to circumstances of density, illumination, and nearness, but also of tactic arrangement, as of a flock of birds under different conditions of perspective.” Of course, the theory is utterly untenable—by astronomers who know something of the actual facts, and have enough mathematics to consider simple geometrical relations. Bodies moving in a plane surface like birds, if they individually travel in the same plane, keep its position unchanged. But if they move individually at an angle to that plane (as they occasionally do), they change its position—the surface, however, in which they collectively are at any moment, still remaining plane. In such a case only could such a phenomenon as was observed by Professor Tait be seen. But in such a case the visibility of the streak formed by the flight of birds would last but a few minutes, for the same motion which had in a few minutes brought the streak into view would in the next few minutes take it out of view. During the short time that a flight is visible in this way, it has an unchanging position, or a scarcely changing one. If the tail of Newton’s comet had rapidly formed and as rapidly vanished, remaining, while visible, in an almost unchanging position, the “seabird analogy” might explain that particular phenomenon, however inadequate to explain multitudes of others. But the phenomena to be explained are entirely different. Leaving out of the question the varying position and length of the tail as it approached the sun, and after it left the sun’s neighborhood, all of which were entirely inconsistent with the seabird analogy, what we are called upon to explain is that a visible tail ninety millions of miles in length, seen in position 1A on one day, was seen three days later in position 3A (having manifestly in the meanwhile passed through all the intermediate positions, including 2A). If Professor Tait, profound mathematician though he be, though he may “differentiate and integrate like Harlequin,” can show how any flight of bodies, like or unlike seabirds, can accomplish such a feat as the above, appearing first to form a thin streak A1, and in less than four days a thin streak A3, each ninety millions of miles long, without _some_ of them having had to travel a distance nearly equal to the line 1 to 3—or some one hundred and fifty millions of miles long, instead of the trifling journeys he assigned them, he should take a rank above Newton and Laplace as a mathematician. But there is another feat, apparently equally difficult to him, which he might achieve very readily with great advantage to those non-mathematicians among astronomers whom his name—well deserved, too—as a mathematician has hitherto misled, and with not less advantage to his own reputation: he might frankly admit that the idea which occurred to him while watching those unfortunate seabirds, had not quite the value which at the moment he mistakenly attached to it, and has since _seemed_ to do.

1 2 3 \ | / \ | / \ | / \ | / . A

But apart from the consideration of theories such as those, either demonstrably untenable, though ingenious, like Professor Tyndall’s, or altogether and obviously untenable like Professor Tait’s, there are certain phenomena of comets’ tails which force upon us the belief that they are phenomena of repulsion, though the repulsive action is of a kind not yet known to physicists.

1. The curvature of all the cometic tails when not seen from a point in or near the place of their motion.

2. The existence of more tails than one to the same comet, the different tails being differently curved.

3. The phenomena of striations athwart the tail.

It is evident that all these phenomena are such as we might fairly expect if a comet’s tail is caused by the sun’s repulsive action on molecules, raised by his heating action on the head. The matter thus swept away would resemble smoke, driven upwards from the funnel of a moving steamer, and then swept in any given direction by a steady wind; we should see a curved train of such matter just as we see a curved streak of smoke. If the matter raised from the head is not all of one kind (and it is antecedently unlikely that it should be), there would be more than one trail of matter, if the sun’s repulsive action were different on these different kinds of matter. Lastly, the striations seen athwart the tail, as in the well known case of Donati’s great comet, would be explained, either as due to the observed pulsational manner in which the envelopes are raised (if matter were raised uniformly from the head there could be no formation of successive envelopes), or else as due to the carrying off into the main tail, where alone such striations are seen, of matter which, had it freed itself at the beginning, would have been swept off into the smaller tails, but being as it were entangled in the great outflow of matter forming the large tail, escapes later, and when it does, gets swept off at its own more rapid rate, and there forms a streak lying at an angle with the direction of the principal tail.

Bredichin has shown that where there are three tails to a comet, their forms correspond with the theory that the envelopes raised from the head are principally formed of hydrogen, carbon and iron, but this, which, if established, would be the most important physical discovery yet made respecting comets, seems open at present to considerable doubt, though confirmations seem to be given to it, in some respects, by the results of spectroscopic analysis.

To spectroscopic analysis we must in all probability look for such information respecting comets, as may hereafter enable us to understand their nature. On this point let us consider what is said by one who, if not the greatest living astronomical spectroscopist, is _facile princeps_ in this country—Dr. W. Huggins. First, however, we must consider the past of this method of research as applied to comets.

The first successful application of the spectroscope to comets was made by Donati in 1864—the light of the comet being then divided into three bright bands, whose position, however, was not exactly determined. In 1866 Dr. Huggins obtained two kinds of light from a telescopic comet, part of the comet’s light giving a continuous spectrum, probably reflected sunlight, the other a spectrum of three bands. In 1868 a comet was observed (Brorsen’s) with more success. Three bands were seen in the spectrum of the light from the comet’s head, and a comparison of these with measures of similar bright bands belonging to the spectra of various combinations of carbon, showed, or rather seemed to suggest, that “combinations of carbon might be present in the comet.”

“In conjunction with my friend, the late Dr. W. Allen Miller,” says Dr. Huggins, “I confronted directly with the spectroscope attached to the telescope, the comet’s light with that from inductive sparks passing in olefiant gas. The sensible identity of the two spectra left no doubt of the essential oneness of the cometary stuff with the gas composed of carbon and hydrogen that was employed for comparison.” “Since that time,” proceeds Dr. Huggins, “the light from some twenty comets has been examined by different observers. The general close agreement in all cases, notwithstanding some small divergences, of the bright bands in the cometary light with those seen in the spectra of hydrocarbons, justifies us fully in ascribing the original light of these comets to matter which contains carbon in combination with hydrogen.”

Last year photography was applied to this spectroscopic work. The spectrum of the brightest comet of that year was partly continuous, and on this continuous spectrum many of the well known Fraunhofer lines could be traced. This made it certain that part of the comet’s light was reflected sunlight; though Dr. Huggins considers also that a part of the continuous spectrum of every comet is due to inherent light. On this point some doubts may be permitted. It is one thing for special bands to show themselves, for some substances may become self-luminous under special conditions at very moderate temperatures; it is quite another thing that the solid parts of a comet’s substance should become incandescent. I venture to express my own belief that this can scarcely happen except in the case of comets which approach very near to the sun. Besides the continuous spectrum with dark lines, the photograph showed also a spectrum of bright lines.