Now in applying this to comets' tails it is only necessary to remark that they are composed of either gaseous or dusty particles, or both, rising from the nucleus, probably under the influence of the heat or the electrical action of the sun, and these particles, being below the critical size, are driven away from the sun, and appear in the form of a tail following the comet. It may be added that the same principle has been evoked to explain the corona of the sun, which may be composed of clouds of gas or dust kept in suspension by the pressure of light.
The nuclei of comets contain nearly their whole mass. The actual mass of no comet is known, but it can in no case be very great. Moreover, it is probable that the nucleus of a comet does not consist of a single body, either solid or liquid, but is composed of a large number of separate small bodies, like a flock of meteors, crowded together and constantly impinging upon one another. As the comet approaches the sun the nucleus becomes violently agitated, and then the tail begins to make its appearance.
The possibility exists of an encounter between the earth and the head of a comet, but no such occurrence is known. Two or three times, however, the earth is believed to have gone through the tail of a comet, the last time in 1910, when Halley's comet passed between the earth and the sun, but no certain effects have been observed from such encounters. The spectroscope shows that comets contain various hydrocarbons, sodium, nitrogen, magnesium, and possibly iron, but we know, as yet, very little about their composition. The presence of cyanogen gas was reported in Halley's comet at its last appearance. We are still more ignorant of the origin of comets. We do know, however, that they tend to go to pieces, especially those which approach very close to the sun. The great comet of 1882, which almost grazed the sun, was afterward seen retreating into space scattered into several parts, each provided with a tail. In at least one case, several comets have been found travelling in the same track, an indication that one large original comet has been separated into three or four smaller ones. This appears to be true of the comets of 1843, 1880, and 1882,—and perhaps the comet of 1576 should be added. But the most remarkable case of disruption is that of Biela's Comet, which first divided into two parts in 1846 and then apparently became scattered into a swarm of meteors which was encountered by the earth in 1872, when it passed near the old track of the comet. This leads us to our next subject.
8. Meteors. Everybody must, at some time, have beheld the phenomenon known as a falling, or shooting, star. A few of these objects can be seen darting across the sky on almost any clear night in the course of an hour or two of watching. Sometimes they appear more numerously, and at intervals they are seen in “showers.” They are called meteors, and it is believed that they are minute solid bodies, perhaps averaging but a small fraction of an ounce in weight, which plunge into the atmosphere with velocities varying from twenty to thirty or more miles per second, and are set afire and consumed by the heat of friction developed by their rush through the air. Anybody who has seen a bullet melted by the heat suddenly developed when it strikes a steel target has had a graphic illustration of the transformation of motion into heat. But if we could make the bullet move fast enough it would melt in the air, the heat being developed by the constant friction.
The connection of meteors with comets is very interesting. In the year 1833, a magnificent and imposing display of meteors, which, for hours, on the night between the 13th and 14th of November, filled the sky with fire-balls and flaming streaks, astonished all beholders and filled many with terror. It was found that these meteors travelled in an orbit intersecting that of the earth at the point where the latter arrived in the middle of November, and also that they had a period of revolution about the sun of 33¼ years, and were so far scattered along their orbit that they required nearly three years to pass the point of intersection with the orbit of the earth. Thus it was concluded that for three years in succession, in mid-November, there should be a display of the meteors plunging into the earth's atmosphere. But only in the year when the thickest part of the swarm was encountered by the earth would the display be very imposing. Upon this it was predicted that there would be a recurrence of the phenomenon of 1833 in the year 1866. It happened as predicted, except that the number of meteors was not quite so great as before. In the meantime, it had been discovered that these meteors followed in the track of a comet known as Temple's Comet, and also that certain other meteors, which appear every year in considerable numbers about the 10th of August, followed the track of another comet called Tuttle's Comet. Then in 1872 came the display, mentioned in the last section, of meteors which were evidently the debris of the vanished comet of Biela. The inference from so many similar cases was irresistible that the meteors must be fragments of destroyed or partially destroyed comets. Several other cases of identity of orbits between meteors and comets have been discovered.
It has been said that the August meteors appear every year. The explanation of this is that they have, in the course of many ages, been scattered around the whole circuit of their orbit, so that each year, about the 10th of August, when the earth crosses their track, some of the meteors are encountered. They are like an endless railroad train travelling upon a circular track. The November meteors also appear, in small numbers, every year, a fact indicating that some of them, too, have been scattered all around their orbit, although the great mass of them is still concentrated in an elongated swarm, and a notable display can only occur when this swarm is at the crossing simultaneously with the earth. These meteors were eagerly awaited in 1899, when it was hoped that the splendid displays of 1833 and 1866 might be repeated, but, unfortunately, in the meantime the planets Jupiter and Saturn, by their disturbing attractions, had so altered the position of the path of the meteors in space that the principal swarm missed the connection. There are many other periodical meteor showers, generally less brilliant than those already mentioned, and some astronomers think that all of them had their origin from comets.
It is not known that any meteor from any of these swarms has ever reached the surface of the earth. The meteors appear to be so small that they are entirely burnt up before they can get through the atmosphere, which thus acts as a shield against these little missiles from outer space. But there is another class of meteoric bodies, variously known as meteorites, aërolites, uranoliths, or bolides, which consists of larger masses, and these sometimes fall upon the earth, after a fiery passage through the air. Specimens of them may be seen in many museums. They are divided into two principal classes, according to their composition: first, stony meteorites, which are by far the most numerous; and, second, iron meteorites, which consist of almost pure iron, generally alloyed with a little nickel. The stony meteorites, which usually contain some compound of iron, consist of a great variety of substances, including between twenty and thirty different chemical elements. Although they resemble in many ways minerals of volcanic origin on the earth, they also possess certain characteristics by which they can be recognised even when they have not been seen to fall.
When a meteorite passes through the air it makes a brilliant display of light, and frequently bursts asunder, with a tremendous noise, scattering its fragments about. The largest fragment of a meteorite actually seen to fall, weighs about a quarter of a ton. Upon striking the ground the meteorite sometimes penetrates to a depth of several feet, and some have been picked up which were yet hot on the surface, although very cold within. It is not known that meteorites have any connection with comets, and their origin can only be conjectured. Among the various suggestions that have been made the following may be mentioned: (1) that they have been shot out of the sun—particularly the iron meteorites; (2) that they were cast into space by lunar volcanoes when the moon was still subject to volcanic action; (3) that they are the products of explosion in the stars. But some astronomers are disposed to think that they originated in a similar manner to other members of the solar system, although it is difficult, on this hypothesis, to account for their great density. The opinion that the iron meteorites have come from the sun, or some other star, is enforced by the fact that they contain hydrogen, carbon, and helium, in forms suggesting that these gases were absorbed while the bodies were immersed in a hot, dense atmosphere.
PART IV.