Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/121

Rh M E T E R 111 of November. Or, happening to have these two elements in common, it might have passed the earth s orbit nearer the sun or farther away from it than the earth is. Or, having these three things in common, it might, by a slight difference in velocity, have had a periodic time much more or much less than thirty-three years. Or, with all these in common, it might have crossed the earth s orbit at a far different angle than the meteors. These several independ ent elements for the comet and the meteors are substan tially identical, and this identity proves almost beyond doubt that between the two either there is now an actual or else there has been in the past a causal connexion. That there is now any physical connexion is thoroughly disproved by the immense magnitude of the stream, and by the isolation and distances from each other of the individual components. It seems difficult to find any cause that should bring into such a strangely shaped group bodies that had originally orbits distributed at random. Hence we are apparently forced to conclude that these meteoroids have something common in their past history. In fact they seem to have been once parts of a single body, and these common elements are essentially those of the parent mass. By some process not yet entirely explained they have become separated from the comet, thrown out of the control of its attractive power, and so left to travel each one in its own orbit. If the cause of separation was not too violent, each new orbit would necessarily be but slightly different from that of the comet. Very small variations in velocity, and hence in periodic time, would in the course of ages scatter the several individuals along the orbit even to the length of many hundreds of millions of miles. The Meteor Group is not the Comet s Tail. These meteoroids must be carefully distinguished from the comet s tails. The former follow or precede the comet exactly in the comet s path ; the particles that compose the latter are driven off by the sun s repulsion directly away from the comet s path. The meteoroids and the comet have orbits with nearly common elements ; the orbits of the particles of the tail have elements that are unlike each other, and unlike those of the comet. The meteoroids are undoubtedly solid masses ; the tails are pulverulent or gaseous. Twin Comets of 1366. The comet 1866 I. is probably not the only one that has been connected with the November meteors. In 1366, a few days after the earth went through the meteor stream, a comet appeared in the northern heavens, and, passing directly in the line of the stream so close to the earth as to describe an arc of 90 in a single day, disappeared in the constellation Aquarius. Immedi ately upon its disappearance a second comet was seen in the north, which followed nearly in the same path. The Chinese accounts are not sufficiently exact to furnish independent orbits for them, but both comets were undoubtedly members of the Leonid stream. The comet 1866 I. may be identical with one of them. The Andromcds and Biela sComct. Mention has been made of the star shower of November 27, 1872. The periodical comet known as Biela s, which makes three revolutions in twenty years, passes very near the earth s orbit at a longitude corresponding to November 27, but by reason of its direct motion the node has had considerable motion in longitude as the result of perturbations. Meteors having the same orbits as Biela s comet would have a radiant in the constellation Andromeda, that is, would cross the sky in lines diverging from a point in that constellation. They might, however, be at dates after or even before November 27. Unusual numbers of meteors were seen December 7, 1798, by Brandes. A like abundance was seen December 7, 1838 ; and, as they had been expected, and radiation was now looked for, they were found to diverge from a point in Andromeda. Hence they have been called Andromeds. Since 1852 Biela s comet itself has been entirely lost. The star shower of November 27, 1872, previously referred to, had a radiant in Andromeda, and in every way appeared as though its meteors had once been parts of Biela s comet. A sprinkle three days earlier, on the night of November 24, had the same radiant, and came from a less dense outlying parallel stream. A small comet was seen in the southern sky by Pogson in the direction opposite to the radiant shortly after the shower. Biela s comet had been found in 1845-46 to be in two- parts, which at its next return to perihelion in 1852 had separated to eight times their former distance. But the meteor streams of 1872 could hardly have been separated from the comet so recently, and the Pogson comet if of the same origin must also have left the parent mass at an earlier date than 1845. No ordinary perturba tions would in a short period have so changed the orbits. The parts of the small stream traversed by the earth, December 1838 and December 1798, were far from the comet, and these fragments must have been thrown off much earlier. TJie Fcrseids and the Comet 1862 III. There is a third epoch when meteors appear in unusual numbers, viz., the 9th to llth of August. This &quot;sprinkle,&quot; as it may be called, has been seen con stantly at the time nr.med for nearly fifty years, and there are on record accounts of similar appearances in the earlier years before its annual character had been discovered. Some observers have thought that there were evidences of a variation having a long period, but the proof seems as yet unsatisfactory, and the display may be regarded as tolerably constant from year to year. On every 10th of August we may confidently expect a display of meteors that shall be at least four or five times as numerous as those of ordinary nights. The radiant is in the constellation Perseus, and hence the name Perseids. The comet 1862 III., which has a period of more than a hundred years, passes close to the earth s orbit, nearly cutting it at the place of this shower, and has a velocity and direction corresponding to this radiant. Hence a connexion of the Perseid meteors with this comet is presumed, like that which the Leonids and Andromeda, have with the comet 1866 I. and Biela. The meteors are distri buted along this orbit more regularly than along either of the other two, and at the same time the breadth of this group is a hundred times greater than that of the Leonids. We must for the present regard it rather as a meteor ring, the meteoroids being scattered along the entire conic section which the comet describes. This ring has an inclination of 113 with the ecliptic. Meteors of April 20-21 Lyraids. About the 20th of April there have been several quite brilliant star showers, the earliest on record having been in the year 687 B.C. On that day meteors have been observed which radiated from Lyra, and to these the name Lyraids has been given. The comet 1861 I. passes near the earth s orbit in that longitude, and any meteors having such a connexion with it as is proved for the Leonids with comet 18U6 I. would also radiate from Lyra. Again, at several other periods of the year, meteors have been seen in unusual numbers which seem to be connected with certain comets. Meteor Radiants. We have thus definite proof that the earth at certain epochs plunges through meteor streams, and that these streams travel along the same track as cer tain comets. The question is at once asked Do not the sporadic meteors, those which are seen on any and all nights of the year, belong to similar streams ? An immense amount of labour has been spent in observing the paths of meteors, and classifying them, so as to detect and prove the existence of radiant points. As many as a thousand such radiants have been suggested by the different investigators. Some of these are duplicates, some will prove to be acci dental coincidences ; but a goodly number may reasonably be expected to endure the test of future observations. Such will show the existence of meteor streams, and per haps will be connected with comets that are now known, or that may hereafter be discovered. The radiants have been spoken of as if they were points in the heavens. This is so nearly true as to justify all the conclusions that have been deduced above. But in fact a radiant, even in the star showers in which it is most sharply defined, must be regarded as a small area. The apparent meteor paths when produced backward do not exactly meet in a point. If they be treated as proceeding from a small area, it does not appear that this is a long narrow one. Hence it may be shown that the paths of the meteors in the air are not exactly parallel either to a line or to a plane. This can hardly be due to a want of parallelism of the paths before the meteoroids meet the earth, but is rather due to their glancing as they strike the