Part 13

That the mass of comets is small, is proved by the fact that they have sometimes passed near the planets without disturbing them in any sensible manner. Lexell's comet, which in 1770 remained four months very near Jupiter, did not affect in the least the orbits, or the motions of its satellites. The same comet also came within less than 1,500,000 miles from the Earth, and on this occasion it was calculated that its mass could not have been the ¹⁄₅₀₀₀ part of that of our globe, since otherwise the perturbations which it would have caused in the elements of the Earth's orbit would have been sensible. There was, however, no change. If this comet's mass had been equal to that of our globe, the length of our year would have been increased by 2h. 47m. The comet of 1837 remained four days within 3,500,000 miles of the Earth, with no sensible effect.

It seems quite difficult to admit that the denser part of a comet forming the nucleus is solid, as supposed by some physicists, since it is so rapidly contracted and dilated by the solar forces, while the comet is yet at a too great distance from the Sun to allow these effects to be attributed to solar heat alone. This part of a comet, as indeed the other parts, seems rather to be in the gaseous than in the solid state; the changes observed in the intensity of its light and in its structure may be conceived as due to some solar action partaking of the nature of electricity.

It has been a question whether comets are self-luminous, or whether they simply reflect the solar light. When their light is analyzed by the spectroscope, it is found that the nucleus of a comet generally gives a continuous spectrum, while the coma and tail give a spectrum consisting of several bright diffused bands. The spectrum given by the nucleus is rarely bright enough to allow the dark lines of the solar spectrum to be discerned upon it; but such lines were reported in the spectrum of comet 1881, III., a fact proving that this nucleus at least reflected some solar light. The nucleus of a comet may be partly self-luminous, and either solid, liquid, or composed of incandescent gases submitted to a great pressure. As to the coma and tail, they are evidently gaseous, and partly, if not entirely, self-luminous, as is proved by the band spectrum which they give. The position of these bands, moreover, indicates that the luminous gases of which they are composed contain carbon. The phenomena of polarization, however, seem to prove that these parts of comets also reflect some solar light.

No theory so far proposed, to explain comets and the strange phenomena they exhibit, seems to have been successful in its attempts, and the mystery in which these bodies have been involved from the beginning of their apparition, seems to be now nearly as great as ever. It has been supposed that their tails have no real existence, but are due to an optical illusion. Prof. Tyndall has endeavored to explain cometary phenomena by supposing these bodies to be composed of vapors subject to decomposition by the solar radiations, and thus made visible, the head and tail being an actinic cloud due to such decompositions. According to this view, the tails of comets would not consist of matter projected into spacer but simply of matter precipitated by the solar rays in traversing the cometary nebulosity. The endeavor has also been made to explain the various phenomena presented by comets by an electrical action of the Sun on the gases composing these objects. Theories taking this as a base seem to us to be more likely to lead to valuable results. M. Faye, who has devoted much time and learning to this subject, assumes a real repulsive force of the Sun, acting inversely to the square of the distance and proportionally to the surface, and not to the mass as attraction does. He supposes, however, that this repulsive force is generated by the solar heat, and not by electricity. Prof. Wm. Harkness says that many circumstances seem to indicate that the comets' tails are due, in a great measure, to electrical phenomena.

The fact that the tails of comets are better defined and brighter on the forward side, associated with the other fact that they curve the most when their motion is most rapid, sufficiently indicates that these appendages are material, and that they either encounter some resistance from the medium in which they move, or from a solar repulsion. The phenomena of condensation and extension, which I have observed in the comets of 1874 and 1881, added to the curious behavior exhibited by the jets issuing from the nucleus, seem to indicate the action of electrical forces rather than of heat. The main difficulty encountered in the framing of a theory of comets consists in explaining how so delicate and extended objects as their tails seem to be, can be transported and whirled around the Sun at their perihelion with such an enormous velocity, always keeping opposite to the Sun, and, as expressed by Sir John Herschel, "in defiance of the law of gravitation, nay, even of the received laws of motion."

To consider the direction of the comets' tails as an indirect effect of attraction, seems out of the question; the phenomenon of repulsion so plainly exhibited by these objects seems to point to a positive solar repulsion, as alone competent to produce these great changes. The repulsive action of the Sun on comets' tails might be conceived, for instance, as acting in a manner similar to that of a powerful current of wind starting from the Sun, and constantly changing in direction, but always keeping on a line with the comet. Such a current, acting on a comet's tail as if it were a pennant, would drive it behind the nucleus just as observed. If it could once be ascertained that the great disturbances on comets correspond with the magnetic disturbances on our globe and with the display of the auroral light, the electric nature of the forces acting so strangely on the comets would be substantially demonstrated. I have shown that some of the great disturbances observed in the comets of 1874 and 1881 have coincided with auroral displays, and it will be shown hereafter that similar displays have also coincided with the passage of meteoric showers through our atmosphere. Whether these simultaneous phenomena were simple coincidences having no connection, or whether they are the result of a common cause, can only be ascertained by long continued future observations.

SHOOTING-STARS AND METEORS

PLATE XII

While contemplating the heavens on a clear moonless night, we occasionally witness the sudden blazing forth of a star-like meteor, which glides swiftly and silently across some of the constellations, and as suddenly disappears, leaving sometimes along its track a phosphorescent trail, which remains visible for a while and gradually vanishes. These strange apparitions of the night are called _Falling_ or _Shooting-stars_.

There is certainly no clear night throughout the year during which some of these meteors do not make their appearance, but their number is quite variable. In ordinary nights only four or five will be observed by a single person in the course of an hour; but on others they are so numerous that it becomes impossible to count them. When the falling stars are only a few in number, and appear scattered in the sky, they are called _Sporadic Meteors_, and when they appear in great numbers they constitute _Meteoric Showers_ or _Swarms_.

Probably there is no celestial phenomenon more impressive than are these wonderful pyrotechnic displays, during which the heavens seem to break open and give passage to fiery showers, whose luminous drops describe fantastic hieroglyphics in the sky. While observing them, one can fully realize the terror with which they have sometimes filled beholders, to whom it seemed that the stability of the universe had come to an end, and that all the stars of the firmament were pouring down upon the Earth in deluges of fire.

The ancients have left record of many great meteoric displays, and the manner in which they describe them sufficiently indicates the fear caused by these mysterious objects. Among the many meteoric showers recorded by ancient historians may be mentioned one observed in Constantinople, in the month of November, 472, when all the sky appeared as if on fire with meteors. In the year 599, meteors were seen on a certain night flying in all directions like fiery grasshoppers, and giving much alarm to the people. In March, 763, "the stars fell suddenly, and in such crowded number that people were much frightened, and believed the end of the world had come." On April 10th, 1095, the stars fell in such enormous quantity from midnight till morning that they were as crowded as are the hail stones during a severe storm.

In modern times the fall of the shooting-stars in great number has been frequently recorded. One of the most remarkable meteoric showers of the eighteenth century occurred on the night of November 13th, 1799, and was observed throughout North and South America and Europe. On this memorable night thousands of falling stars were seen traversing the sky between midnight and morning. Humboldt and Boupland, then traveling in South America, observed the phenomena at Cumana, between two and five o'clock in the morning. They saw an innumerable number of shooting-stars going from north to south, appearing like brilliant fire-works. Several of these meteors left long phosphorescent trails in the sky, and had nuclei whose apparent diameter, in some cases, surpassed that of the Moon.

The shower of November 13th, 1833, was still more remarkable for the great number of meteors which traversed the heavens, and was visible over the whole of North and South America. On that occasion the falling stars were far too numerous to be counted, and they fell so thickly that Prof. Olmsted, of New Haven, who observed them carefully, compared their number at the moment of their maximum fall to half that of the flakes of snow falling during a heavy storm. This observer estimated at 240,000 the number of meteors which must have traversed the heavens above the horizon during the seven hours while the display was visible.

[PLATE XII.--THE NOVEMBER METEORS.

As observed between midnight and 5 o'clock A.M. on the night of November 13-14 1868.]

In the years 1866, 1867 and 1868, there were also extraordinary meteoric displays on the night of November 13th. It was on the last mentioned date that I had the opportunity to observe the remarkable shower of shooting-stars of which I have attempted to represent all the characteristic points in Plate XII. My observations were begun a little after midnight, and continued without interruption till sunrise. Over three thousand meteors were observed during this interval of time in the part of the sky visible from a northern window of my house. The maximum fall occurred between four and five o'clock, when they appeared at a mean rate of 15 in a minute.

In general, the falling stars were quite large, many being superior to Jupiter in brightness and apparent size, while a few even surpassed Venus, and were so brilliant that opaque objects cast a strong shadow during their flight. A great many left behind them a luminous train, which remained visible for more or less time after the nucleus had vanished. In general, these meteors appeared to move either in straight or slightly curved orbits; but quite a number among them exhibited very extraordinary motions, and followed very complicated paths, some of which were quite incomprehensible.

While some moved either in wavy or zig-zag lines, strongly accentuated, others, after moving for a time in a straight line, gradually changed their course, curving upward or downward, thus moving in a new direction. Several among them, which were apparently moving in a straight line with great rapidity, suddenly altered their course, starting at an abrupt angle in another direction, with no apparent slackening in their motion. One of them, which was a very conspicuous object, was moving slowly in a straight course, when of a sudden it made a sharp turn and continued to travel in a straight line, at an acute angle with the first, retreating, and almost going back towards the regions from which it originally came. As nearly all the meteors which exhibited these extraordinary motions left the trace of their passage in the sky by a luminous trail, it was easily ascertained that these appearances were not deceptive. On one occasion I noticed that the change of direction in the orbit corresponded with the brightening up of the meteor thus disturbed in its progress.

Among these meteors, some traveled very slowly, and a few seemed to advance as if by jerks, but in general they moved very rapidly. One of the meteors thus appearing to move by jerks left a luminous trail, upon which the various jerks seemed to be left impressed by a succession of bright and faint spaces along the train. Some of the largest meteors appeared to rotate upon an axis as they advanced, and most of these revolving meteors, as also a great number of the others, seemed to explode just before they disappeared, sending bright fiery sparks of different colors in all directions, although no sound was at any time heard. The largest and most brilliant meteor observed on that night appeared at 5h. 30m., a little before sunrise. It was very bright, and appeared considerably larger than Venus, having quite a distinct disk. This meteor moved very slowly, leaving behind a large phosphorescent trail, which seemed to issue from the inside of the nucleus as it advanced. For a moment the train increased in size and brightness close to the nucleus, which then appeared as an empty transparent sphere, sprinkled all over with minute fiery sparks; the nucleus then suddenly burst out into luminous particles, which immediately vanished, only the luminous trail of considerable dimensions being left.

Many of the trails thus left by the meteors retained their luminosity for several minutes, and sometimes for over a quarter of an hour. These trails slowly changed their form and position; but it is perhaps remarkable that almost all those which I observed on that night assumed the same general form--that of an open, irregular ring, or horse-shoe, somewhat resembling the letter C. This ring form was subsequently transformed into an irregular, roundish cumulus-like cloud. The trail left by a very large meteor, which I observed on the evening of September 5th, 1880, also exhibited the same general character of transformation.

While I was observing a long brilliant trail left by a meteor on the night of November 13th, 1868, it was suddenly crossed by another bright shooting-star. The latter apparently went through the luminous substance forming the trail, which was suddenly altered in form, and considerably diminished in brightness simultaneously with this passage, although electrical action at some distance might perhaps as well explain the sudden change observed.

In the majority of cases the meteors appeared white; but many, especially the largest, exhibited a variety of brilliant colors, among which the red, blue, green, yellow and purple were the most common. In general the trails exhibited about the same color as the nucleus, but much fainter, and they were usually pervaded by a greenish tint. In some instances the trails were of quite a different color from the nucleus.

The luminous cloud observed at 5h. 30m. on the morning of November 14th, 1868, after having passed through the series of transformations above described, remained visible for a long while after sunrise, appearing then as a small cirrus cloud, exactly similar in appearance to the hundreds of small cirrus clouds then visible in the sky, which had probably the same meteoric origin. For over three hours after sunrise, these cirrus clouds remained visible in the sky, moving all together with the wind in the high regions of the atmosphere.

Although Plate XII. is intended to represent all the characteristics exhibited by the meteors observed on that night, every form represented having been obtained by direct observation, yet the number is much greater than it was at any single moment during the particular shower of 1868. As regards number, the intention was to give an idea of a great meteoric shower, such as that of 1833, for instance. Although many of the falling stars seem to be close to the Earth's surface, yet this is only an effect of perspective due to their great distance, very few of these meteors ever coming into the lower regions of our atmosphere at all.

The phenomena exhibited during other great meteoric showers have been similar to those presented by the shower just described, the only differences consisting in variations of size and brightness in the meteors, and also in the trails, which sometimes are not so numerous as they were in 1868.

While some shooting-stars move so rapidly that they can hardly be followed in their orbits, others move so slowly that the sight can easily follow them, and even remark the peculiarities of their movements, some remaining visible for half a minute. Some of the falling stars move at the rapid rate of 100 miles a second, but others only 10 miles a second, and even less. In general, they move about half as fast again as the Earth in its orbit. The arcs described by the meteors in the sky are variable. While some extend 80° and even 100°, others are hardly half a degree in length. While some shooting-stars are so faint that they can hardly be seen through the largest telescopes, others are so large and brilliant that they can be seen in the day-time. In general, a shooting-star of average brightness resembles a star of the third or fourth magnitude.

Whatever may be the origin of the shooting-stars, they are, when we see them, not in the celestial spaces, like the planets, the comets, or the stars, but in our atmosphere, through which they travel as long as they remain visible. The height at which they appear and disappear is variable, but in general they are about 80 miles above the surface of our globe when they are first seen, and at about 55 miles when they disappear. In many cases, however, they have been observed at greater elevations, as also at smaller. A meteor simultaneously observed at two different stations first appeared at the height of 285 miles, and was last seen at 192 miles above the Earth's surface; but in rare cases the falling stars have been seen below a layer of clouds completely covering the sky. I myself saw one such shooting-star a few years since. The fact that the meteors are visible at so great elevations, proves that our atmosphere extends much farther than was formerly supposed, although at these great heights it must be extremely rarefied, and very different from what it is in its lower regions.

There is a remarkable difference between the sporadic meteors seen in the sky on every night, and the meteoric showers observed only at comparatively rare intervals. While the first appear from different points in the sky and travel in all directions, being perfectly independent, the meteors of a shower all come from the same point of the heavens, from which they apparently diverge in all directions. This point of divergence of the meteors is called the _radiant point_ of the shower. Although the meteors seem to diverge in all directions from the radiant point, yet they all move in approximately parallel lines, the divergence being an effect of perspective.

Whatever may be the position of the radiant point in the constellations, it remains as fixed in the sky as the stars themselves, and participates with them in the apparent motion which they undergo by the effect of the diurnal motion, and thus rises and sets with the constellation to which it belongs. This fact is sufficient to prove that the orbits of these meteors are independent of the Earth's motion, and that consequently they do not originate in our atmosphere. It has been shown by Encke that the radiant point of the meteoric shower of November 13th is precisely the point towards which our globe moves in space on November 13th; a tangent to the Earth's orbit would pass through this radiant point.

The meteoric showers are particularly remarkable, not merely because of the large number of meteors which are visible and the fact that they all follow a common orbit, but chiefly because they have a periodic return, either after an interval of a year, or after a lapse of several years. At the beginning of the present century only two meteoric showers were known, those of August 10th and of November 13th, and their periodicity had not yet been recognized, although it had begun to be suspected. It was only in 1836 that Quetelet and Olbers ventured to predict the reappearance of the November meteors in the year 1867. Having made further investigations, Prof. Newton, of Yale College, announced their return in the year 1866. In both of these years, as also in 1868, the meteors were very numerous, and were observed in Europe and in America on the night of November 13th. The predictions having thus been fulfilled, the periodicity of the meteors was established. Since then, other periodic showers have been recognized, although they are much less important in regard to number than those of August and November, except that of November 27th, which exhibited so brilliant a display in Europe in 1872. These successive appearances have established the main fact that meteoric showers are more or less visible every year when the Earth occupies certain positions in its orbit.

The meteoric shower of the 10th of August has its radiant point situated in the vicinity of the variable star Algol, in the constellation Perseus, from which its meteors have received the name of Perseids. Although varying in splendor, this meteoric swarm never fails to make its appearance every year. The Perseids move through our atmosphere at the rate of 37 miles per second. The shower usually lasts about six hours.

The meteoric shower of November 13th has its radiant point situated in the vicinity of the star Gamma, in the constellation Leo, from which its meteors have been called Leonids. But while the August meteors recur regularly every year, with slight variations, the shower of November does not occur with the same regularity. During several years it is hardly noticeable, and is even totally absent, while in other years it is very remarkable. Every 33 years an extraordinary meteoric shower occurs on the 13th of November, and the phenomenon is repeated on the two succeeding years at the same date, but with a diminution in its splendor at each successive return. The Leonids move in an opposite direction to that of the Earth, and travel in our atmosphere with an apparent velocity of 45 miles per second, this being about the maximum velocity observed in falling stars. But when the motion of our globe is taken into account, and a deduction is made of the 18 miles which it travels per second, it is found that these meteors move at an actual mean rate of 27 miles a second.

In a meteoric shower the stars do not fall uniformly throughout the night, there being a time when they appear in greater numbers. Usually it is towards morning, between 4 and 6 o'clock, that the maximum occurs. The probable cause of this phenomenon will be explained in its place hereafter.

The orbits of the meteoric showers are not all approximately in the same plane, like those of the planets, but rather resemble those of comets, and have all possible inclinations to the ecliptic. Like the comets, too, the different meteoric showers have either direct or retrograde motion.