METEOR, METEORITE. The term meteor, in ac-cordance with its etymology (//.eTecupos), meant originally something high in the air. It has been applied to a large variety of phenomena, most of them of brief duration, which have place in the atmosphere. Disturbances in the air are aerial meteors, viz., winds, tornadoes, whirlwinds, typhoons, hurricanes, &c. The vapour of water in the atmosphere creates by its forms and precipitations the aqueous meteors, viz., clouds, fogs, mists, snow, rain, hail, &c. The effect of light upon the atmosphere and its con-tents causes certain luminous meteors, viz., rainbows, halos, parhelia, twilight, mirage, &c. Discussion of all these, and of like phenomena, belongs to METEOROLOGY (q.v.).
Another class of luminous meteors, known as shooting or falling stars, fireballs, bolides, &c, have their place in the upper parts of the atmosphere. But by reason of their origin from without they, and the aerolites or meteor-ites which sometimes come from them, belong properly to astronomy. The term meteor is often used in a restricted sense as meaning one of these latter phenomena. The present article will treat of them alone.
The most remarkable of the meteors (and the most instructive) are those which are followed by the falling of stones to the earth. These have since the beginning of the present century attracted so much attention, and the phenomena have been so frequently examined and described by scientific men, that they are very well understood. The circumstances accompanying the fall of stones are tolerably uniform. A ball of fire crosses the sky so bright as to be visible, if it appears in the daytime, sometimes even at hundreds of miles from the meteor; and if it appears in the night it is bright enough to light up the whole landscape. It traverses the sky, generally finishing its course in a few seconds. It suddenly goes out, either with or without an apparent bursting in pieces, and after a short period a loud detonation is heard in all the region near the place where the meteor has disappeared. Sometimes only a single stone, sometimes several are found. For some falls they are numbered by thousands. About three thousand were obtained from the fall of L'Aigle in 1803, scattered over a region about 7 miles long and of less breadth. A like number was obtained from the fall of Knyahinya on June 9, 1866. At Pultusk a still larger number were collected, scattered over a larger space, by a fall in January 1868. From the Emmet county (Iowa) fall, May 10, 1879, a similarly large number have been secured.
These meteors leave behind them in the air a cloud or train that may disappear in a few seconds, or may remain an hour. They come at all times of day, at all seasons of the year, and in all regions of the earth. They come irrespective of the phases of the weather, except as clouds conceal them from view.
Let us describe one or two of these meteors more in detail. On the evening of the 2d of December 1876, persons in or near the State of Kansas saw, about eight o'clock in the evening, a bright fireball rising from near where the moon then was in the western sky. It increased in- brilliancy as it proceeded, becoming so bright as to compel the attention of every one who was out of doors. To persons in the northern part of the State the meteor crossed the southern sky going to the east, to those in the southern part it crossed the northern heavens. To all it went down near to the horizon a little to the north of east, the whole flight as they saw it occupying not over a minute.
The same meteor was seen to pass in nearly the same way across the heavens from west-south-west to east-north-east by inhabitants of the States of Nebraska, Iowa, Missouri, Wisconsin, Illinois, Michigan, Kentucky, Indiana, Ohio, Pennsylvania, and West Virginia. But besides this there were heard near the meteor's path, four or five minutes after its passage, loud explosions like distant cannonading, or thunder, or like the rattling of empty waggons over stony roads. So loud were these that per.ple and animals were frightened. East of the Mississippi river these explosions were heard everywhere within about 60 miles of the meteor's path; and in Bloomington, Indiana, sounds were heard supposed to come from the meteor even at a distance of nearly 150 miles from it. Over central Illinois it was seen to break into fragments like a rocket, and over Indiana and Ohio it formed a flock or cluster of meteors computed to be 40 miles long and 5 miles broad. The sky in New York State was wholly overcast. Persons in Ohio and Pennsylvania, who from their situation could look over the cloud last, saw the meteor passing on eastward over New York. From many places in the State itself came accounts of rattling of houses, thundering noises, and other like phenomena, which at the time were attributed to an earthquake.
At one place in northern Indiana a farmer heard a heavy thud as of an object striking the ground near his house. The next morning he found on the snow a stone of very peculiar appearance weighing three-quarters of a pound, which from its character there is every reason to believe came from the meteor. By putting together the various accounts of observers, the meteor is shown to have become first visible when it was near the north-west corner of the Indian Territory, at an elevation of between 60 and 100 miles above the earth. From here it went nearly parallel to the earth's surface, and nearly in a right line, to a point over central New York. During the latter part of its course its height was 30 or 40 miles. It thus traversed the upper regions of the air through 25° of longitude and 5° of latitude in a period of time not easily determined, but probably about two minutes. A part of the body may have passed on out of the atmosphere, but probably the remnants came somewhere to the ground in New York, or farther east.
A somewdiat similar meteor was seen in the evening of July 20, 1860, by persons in New York, Pennsylvania, New England, &c, which first appeared over Michigan, at a height of about 90 miles. The light was so brilliant as to call thousands from their houses. It passed east-south-east, and over New York State, at a height of about 50 miles, broke into three parts which chased each other across the sky. At New York city it was seen in the north, while at New Haven it was in the south. At both places the apparent altitude was well observed, and its true height proved to be about 42 miles above the earth's surface between the two cities. It finally disappeared far out over the Atlantic Ocean. It is doubtful whether any one heard any sound of explosion that came from this meteor, and no part of it is known to have reached the ground. The velocity was at least 10 or 12 miles per second, or fifty times the velocity of sound. These two meteors were evidently of the same nature as those which have furnished so many stones for our museums, except that the one was so friable that it has given us but one known fragment, while the other was only seen to break in two, not even a sound of explosion being known to have come from the meteor.
Next to the stone-producing meteor is the fireball, or bolide, which gives generally a less brilliant light than the former, but in essential appearances is like it. The meteor of July 20, 1860, above described, though unusually brilliant, was one of this class, and represents thousands of bolides which have been seen to break in pieces. The bolides leave trains of light behind them just as the stone meteors do ; they travel with similar velocities both apparent and actual, and in all respects exhibit only such differences of phenomena as would be fully explained by differences in size, cohesion, and chemical constitution of stones causing them.
Next to the bolide is a smaller meteor which appears as if one of the stars were to leave its place in the heavens, shoot across the sky, and disappearall within the fraction of a second. Some meteors of this class are as bright as Venus or Jupiter. Some are so small mat though you look directly at the meteor, vou doubt whether you see one or not. In the telescope still smaller ones are seen that are invisible to the naked eye. Meteors comparable in bright-ness to the planets and the fixed stars are usually called shooting stars.
These various kinds of meteors differ from all other luminous phenomena so as to stand in a group entirely alone. Though they have been sometimes regarded as separable among themselves into three or four different species, and for purposes of description may still be so divided, yet they all seem to have a like astronomical character, and the differences are only those of bigness, chemical constitution, velocity, &c. There appears to be no clear line of distinction between the stone-producing and the detonating meteors, nor between those heard to explode and those seen to break in pieces, nor between these and the simple fireballs, nor between the fireball and the faintest shooting star.
Altitudes of Meteors.The first important fact about the meteors is the region in which they become visible to us. In hundreds of instances observations have been made upon the luminous path of a meteor at two or more stations many miles apart. When such stations and the path are properly situated relatively to each other, observa-tions carefully made will show a parallax by which the height of the meteor above the earth, the length and di-rection of the path, and other like quantities may be com-puted. The general result from several hundred instances is that the region of meteor paths may be in general regarded as between 40 and 80 miles above the earth's surface. Some first appear above 80 miles, and some descend below 40 miles. But an altitude greater than 100 miles, or one below 25, except in the case of a stone-furnishing meteor, must be regarded as very doubtful. Thus the meteor paths are far above the usual meteoro-logical phenomena, which (except auroras and twilight) have not one-tenth of the height of the meteors. But with reference to all other astronomical phenomena they are very close to us. The comets, for example, are well-nigh a millionfold, and even the moon is a thousandfold, more distant from us.
Velocities of Meteors.When the length of a luminous path is known, and the time of describing it has been observed, it is easy to compute the velocity in miles. Unfortunately the large meteors, describing long paths, come at rare intervals, and unexpectedly, and it is a happy accident when one is observed by a person accustomed to estimate correctly short intervals of time. On the other hand, the total time of visibility of the shooting stars, which come so frequently that they may be watched for, is usually less than a second. It is not easy to estimate correctly such an interval, where the beginning and ending are not marked by something like a sharp click. Hence all estimates and computations of velocities of meteors are to be received with due regard to their uncertainty. We may only say in general that the velocities computed from good observations are rarely if ever under 8 or 10 miles a second, or over 40 or 50 miles, and that some have far greater velocities than others. The average velocity seems to be nearly 30 miles.
What makes the Luminous Meteor.The cause of a meteor is now universally admitted to be something that enters the earth's atmosphere from without, with a velocity relative to the earth that is comparable with the earth's velocity in its orbit, which is 19 miles per second. By the resistance it meets in penetrating the air the light and other phenomena of the luminous train are produced. Under favourable circumstances, portions of these bodies reach the earth's surface as meteorites.
Meteoroids.A body which is travelling in space, and which on coming into the air would under favourable circumstances become a meteor, may be called a meteor-oid.
The meteoroids are all solid bodies. It would hardly be possible for a small quantity of gas out in space to retain such a density as would enable it on coming into the air to go 10 or 100 miles through even the rare upper atmosphere, and give us the clear line which a shooting star describes. Even if a liquid or gaseous mass can travel as such in space, it would be instantly scattered on striking the air, and would appear very unlike a shooting star or bolide.
Numbers of Meteors.Of the larger meteors there are ir the mean six or eight per annum which in the last fifty years have furnished stones for our collections. A much larger number have doubtless sent down stones which have never been found. Thus Daubree estimates for the whole earth an annual number of six or seven hundred stone-falls.
But of the small meteors or shooting stars the number is very much larger. Any person who should in a clear moonless night watch carefully a portion of the heavens would, in the mean, see at least as many as eight or ten shooting stars per hour. A clear-sighted and practised observer will detect somewhat more than this number. Dr Schmidt of Athens, from observations made during seventeen years, obtained fourteen as the mean hourly num-ber on a clear moonless night for one observer during the hour from midnight to 1 A.M. A large group of observers, as has been shown by trial, would see at least six times as many as a single person. By a proper consideration of the distribution of meteor paths over the sky, and in actual altitude in miles, so as to allow for mists near the horizon, it appears that the number over the whole globe is a little more than ten thousand times as many as can be seen in one place. This implies that there come into the air not less than twenty millions of bodies daily, each of which, under very favourable conditions of absence of sunlight, moon-light, clouds, and mists, would furnish a shooting star visible to the naked eye. Shooting stars invisible to the naked eye are often seen in the telescope. The numbers of meteors, if these are included, would be increased at least twentyfold.
How densely Space is filled with Meteoroids.By assuming that the absolute velocity of the meteors in space is equal to that of comets moving in parabolic orbits (we have good reason to believe that this is nearly their true velocity), we may prove from the above numbers that the average number of meteoroids in the space that the earth traverses is, in each volume equal to that of the earth, about thirty thousand. In other words, there is in the average to every portion of space equal to a cube whose edge is about 210 miles one meteoroid large enough to make a shooting star bright enough to be visible to the naked eye. Such meteoroids would, upon an equable distribution, be each in round numbers 250 miles from its near neighbours. All these num-bers rest upon Dr Schmidt's horary number fourteen, and for a less practised observer and a less clear sky they would be correspondingly changed. How much they would need to be altered to represent other parts of space than those near the earth's orbit is a subject of inference rather than of observation.
Motion in Space.The meteoroids, whatever be their size, must by the law of gravitation have motions about the sun in the same way as the planets and comets, that is, in conic sections of which the sun is always at one focus. The apparent motions of the meteors across the sky imply that these motions of the meteoroids relative to the sun cannot as a rule be in or near the plane of the ecliptic. For if they were there, since the motion of the earth is also in the ecliptic, the motion of the meteoroids relative to the earth would be in the same plane. This would involve that all the meteor paths as seen on the sky would if pro-duced backward cross the ecliptic above the horizon. In fact there is no tendency of this kind. Hence the meteor-oids do not move in orbits that are near the ecliptic as the planets do, but like the comets they may and usually do have orbits of considerable inclinations.
Numbers through the Night.There are more meteors seen in the morning hours than in the evening. If the meteors had no motion of their own in space, the earth would by its motion receive the meteors only on the hemi-sphere that was in front. There would be no meteors seen in the other hemisphere. On the other hand, if the meteors had such large velocities of their own as that the earth's velocity might be neglected in comparison, and if the directions of the meteors' motions were towards all points indiscriminately, then as many would be seen in one part of the night as another. In fact there are about three times as many seen in the morning hours as in the evening. The law of change from evening to morning gives a means of proving that the mean velocity of meteors is so great that they must in general be moving in long orbits about the sun. In this respect also the meteoroids resemble comets, and are unlike planets, in their motions. Of the stone-furnishing meteors more are seen in the day than in the night, and more in the earlier hours of the night than in the later. This is probably due to the fact that more persons are in a position to see the stone-falls at the periods of greater abundance.
Star Showers.While the average number of shooting stars for a single observer at midnight may be regarded as tolerably constant, there have been special epochs when many more have been seen. In certain instances the sky has been filled with the luminous trains, just as it is filled by descending snowflakes in a snowstorm, making a veritable shower of fire. One of the best-observed, though by no means the most brilliant, of these showers occurred on the evening of the 27th of November 1872. Some of the observers of that shower, counting singly, saw at the rate of eight or ten thousand shooting stars in the course of two hours. The distances of the meteoroids in the middle of the swarm which the earth then passed through, each from its nearer neighbours, would be 30 or 40 miles.
The following quotations show the impression made by star showers in times past :
"In the year 286 [of the Hegira] there happened in Egypt an earthquake on Wednesday the 7th of Dhu-l-Ka'dah, lasting from the middle of the night until morning ; and so-called flaming stars struck one against another violently while being borne eastward and westward, northward and southward, and no one could bear to look toward the heavens on account of this phenomenon."
" In the year 599 [of the Hegira], on the night of Saturday, on the last day of Muharram, stars shot hither and thither in the heavens, eastward and westward, and flew against one another like a scattering swarm of locusts, to the right and left; people were thrown into consternation, and cried to God the Most High with confused clamour."
"These meteors [November 12, 1799] might be compared to the blazing sheaves shot out from a firework."
"The phenomenon was grand and awful; the whole heavens appeared as if illuminated with sky rockets."
November 13, 1833. "Thick with streams of rolling fire; scarcely a space in the firmament that was not filled at every instant."
'' Almost infinite number of meteors ; they fell like flakes of snow."
November Meteors or Leonids.These quotations all refer (except possibly the first) to a shower which has appeared in October and November of many different years since its first known occurrence on the 13th of October 902 A.D. Dates of these showers are given in the following table :
Oct. 13, 902. Oct. 17, 1101. Oct. 28, 1602. Nov. 13, 1833.
Oct. 15, 931. Oct. 19, 1202. Nov. 9, 1698. Nov. 14, 1866.
Oct. 14, 934. Oct. 23, 1366. Nov. 12, 1799. Nov. 14, 1867.
Oct. 15, 1002. Oct. 25, 1533. Nov. 13, 1832. Nov. 14, 1868.
On several years after 1833, and before and after 1866-68, there were unusual numbers of those meteors seen on the mornings of November 13, 14, and 15, though per-haps they would have been unnoticed had there not been special watching for them. It will be seen that all these showers are at intervals of a third of a century, that they are at a fixed day of the year, and that the day has moved steadily and uniformly along the calendar at the rate of about a month in a thousand years. The change of twelve days in the 17th century is due to the change from old to new style.
The only explanation of this periodical display that is now seriously urged, and the one which is universally accepted by astronomers, is that there is a long thin stream of meteoroids, each of which is travelling about the sun in a conic section. These conic sections are all nearly parallel, and have nearly the same major axis, extending out about as far as to the orbit of Uranus, and each requir-ing the common period of thirty-three .and a quarter years. The length of the stream is such that the most advanced members are six or eight years ahead of the hindermost, and they all cross the earth's orbit with a velocity of about 26 miles a second. Since the earth plunges through the group nearly in the opposite direction, the velocity with which they enter the air is 44 miles a second. One of the facts which have greatly aided us in arriving at this explanation is that these meteors in all the years and through all hours of the night cross the sky as we look at them in lines which diverge from a point near the centre of the sickle in the constellation Leo; hence the paths in the air are parallel. This implies that their velocities relative to the sun are all parallel and equal to each other. The radiation from Leo has given to them the name Leonids.
Orbit of the Leonids.This orbit, common to all the Leonid meteors, is inclined to the ecliptic at an angle of 17° (or rather 163°, since the motion is retrograde), has a major axis of 10'34, a periodic time of 33'27 years, and a peri-helion distance a little less than unity.
The above orbit, and that alone, explains the several appearances of the November meteors, the annual and the thirty-three year periods, the radiation from Leo, and the change of day of the month in the course of the centuries. This it does so completely that the result has never been questioned by astronomers. Shortly after the publication by Professor Adams in 1867 of the last link in the chain of the proof of this orbit, there was also published the definitive orbit of the comet 1866 I. That the comet was running almost exactly in the orbit of the meteors was at once recognizer!. In fact the comet is itself, in a sense, a meteoroid, and the principal member, so far as we know, of the group. Leonids had been seen in 1863, two years and two months in advance of the comet, while those of 1866 were ten months behind it. Those of later years (a few Leonids were seen even in 1870) were extended along the line of the comet's path behind it. The leaders of this long file of meteoroids had passed up beyond the orbit of Jupiter long before those which brought up the rear had crossed that planet's orbit going down toward the sun. The thickness of the stream is less than the ten-thousandth part of its length. In the densest part that we have recently passed throughnamely, that traversed in 1833 the density of the stream may be expressed by saying that each meteoroid must in the mean have been 10 or 20 miles from its nearest neighbours.
What makes this Comet and these Meteors describe the same Orbit about the Sun?Its path might have been inclined to the ecliptic at any angle instead of 163°. Or, with this inclination, its plane might have cut the earth's orbit at any other place than where the earth is on the 14th 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 Andromeds and Biela's Comet.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 Bogson 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 183S and December 1798, were far from the comet, and these fragments must have been thrown oil' much earlier.
The Perseids and the Comet 1862 III.There is a third epoch when meteors appear in unusual numbers, viz., the 9th to 11th of August. This "sprinkle," as it may be called, has been seen con-stantly at the time named 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 Andromeds 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 20-21Lyraids.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 1866 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 askedDo not the sporadic meteors, those which are seen on any and all nights of the year, belong to similar streams 1 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 air. These facts add not a little to the difficulties to be overcome by the energetic observers and investigators who are trying to deduce order out of an apparent chaos.
Meteorites.The fragments wdiich fall immediately after the disappearance of large meteors have been carefully collected and preserved in mineralogical museums, and have been studied with special interest. The largest collections in Europe are in Vienna, Paris, London, and Berlin, some of these representing over three hundred localities. In the United States there are large collections at New Haven, Amherst, and Louisville.
In several respects these fragments differ at first sight from terrestrial rocks.
They are when found almost always covered in part or entirely with a very thin black crust, generally less than of an inch in thickness. This crust may have a bright lustrous surface, or it may be of a lustreless black. It has evidently been melted, yet so rapidly as not to change in the least the parts of the stone immediately adjacent. Streaks showing the flow of the melted matter are often seen on the surface. Upon some surfaces are what appear to be deposits of the melted matter that has flowed off from the others. Some surfaces are only browned, showing an apparently recent fracture, and some cracks are found in stones which are not yet completely broken in two.
The surfaces very often have small cup-like cavities, sometimes several inches in diameter, sometimes like deep imprints in a plastic mass made by the ends of the fingers, and sometimes still smaller. These " cupules " have not only various sizes in different stones, but even in the same stone differ considerably from one surface to another. They appear in meteorites that are almost exclusively iron, as well as in those mainly destitute of that metal, and they may be regarded as a characteristic of meteorites.
The meteorites have usually metallic iron as one of their component parts. Native iron is very rare indeed among terrestrial minerals, and its presence in the meteorites is therefore characteristic. Sometimes the iron forms the principal part of the body, giving it the appearance of a mass of that metal. Sometimes it forms only a connected framework which is filled in with mineral matter. Some-times particles of iron are scattered through a stony mass; and a few meteorites are said to be destitute of metallic iron altogether. The metallic iron is always accompanied with nickel.
The stony meteors when broken or cut through have usually a greyish interior, and often exhibit a peculiar globular structure. From the small rounded grains that give it this appearance, the name chondrite (from ^ovSpos, a ball) has been applied to this kind of meteorite. Some-times the irregular fragments are compacted into a kind of breccia.
The pieces as we find them are always ajmarent frag-ments of some larger mass, and there is no structural appearance which would indicate that the mass might not be a fragment of a still larger one. In some of the falls fragments picked up at a distance of miles from each other fit together in their simply browned surfaces, showing that they were true fragments recently separated. In some cases surfaces of the stones are partially polished. In some a cross section of the stone exhibits thin black lines as though the melted matter of the surface had been forced into the crevices of the partially broken stone.
The stones when seen to fall, if at once picked up, are usually too warm to be taken in the hand. But cases are on record in which the stones were excessively cold. They sometimes, on striking the ground, j>enetrate into it from 1 to 3 feet. In extreme cases large ones have struck much deeper into soft earth. Sometimes they are broken to pieces by the impact with the hard earth.
The stones are usually not very large. Although the light of the meteor is such as sometimes to be seen over a region 1000 miles in diameter, and the detonation gives phenomena suggestive of an earthquake over many counties, yet a stone exceeding 100 S) is quite exceptional in our col-lections. The total weight secured at any fall has rarely if ever amounted to a thousand pounds. The average weight of nine hundred and fifty perfect specimens of the Pultusk fall in the Paris museum is 67 grammes, or less than 2^ oz. One of the Hessle meteorites in the Stockholm museum weighs less than 1 grain. Many of the Emmet county mete-orites (May 10, 1879) are not much larger, though the largest specimen of that fall weighs nearly 500 BJ.
Meteors traversing the Atmosphere.We can now get a very good idea of the history of that part of a meteorite's life between its entrance into the air and its arrival at the earth. It is entirely invisible until it has reached that height at which the density of the air is enough to create con-siderable resistance. Up to that time it moves almost exclusively in obedience to the sun's attraction. The earth's attraction may be neglected, especially during the passage through the air. Since the velocity is a hundred times that of sound, the elasticity of the air is impotent to remove it from in front of the meteorite, or to prevent a high degree of condensation. Perhaps the air is liquefied immediately in front of the stone. Heat is developed in it enormously, and the stone being pressed closely against the hot air is melted, with an intense light. The condensed air charged with the melted matter is pushed aside, and left behind nearly in the wake of the meteor to form the train. The brightness of the train rapidly diminishes behind the meteor, so that the light of the meteor and the train, modi-fied by irradiation, make the whole appear to a distant eye of the shape of a pear or candle-flame. The stone being a poor conductor of heat, and itself rigid, is not heated in the interior either by condensation or conduction, and may reach the ground with its surface only heated, while the interior is as cold as it had been out in space.
If the stone is a small one it will soon be used up by this intense fire. Until its front surface is rounded by the flame, the irregular resistances may cause such a stone to glance. But if the stone is larger it will lose velocity less rapidly. As it comes down into the region where the air is more dense, it will in spite of loss of velocity meet greater resistance. The air pressed hard against it burns it un-equally, forming cupules over its surface. The pressure of the air cracks the stone,perhaps scaling off small frag-ments, perhaps breaking it into pieces of more uniform size. In the latter case the condensed air in front of the meteor being suddenly relieved will expand, giving the terrific explosion which accompanies such breaking up. In either case a fragment may have still velocity enough to burn on for an instant in its new path and then come invisibly to the earth, covered with a coating, the greater part obtained after the principal explosion. In the latter part of the course the original velocity has almost all dis-appeared, so that the sound travels faster than the meteor. The air's resistance exceeds the earth's attraction, and the stones strike the ground only with the force of a spent cannon ball. It is no doubt in violent disruption that some of the fractures are made in such a way as to give the rubbed and polished surfaces.
Trains of Meteors.The smaller meteors generally have no perceptible train. Only in exceptional cases do the trains of ordinary shooting stars remain visible longer than a fraction of a second. An unusual number of the Leonids have a bluish train. But the brighter shooting stars and the larger meteors sometimes have trains that endure for minutes, and in extreme cases for an hour. Such trains are at first long narrow lines of light, though much shorter than the track of the meteor. They begin at once to broaden in the middle and to fade away at one or both ends. Presently they become curved, sometimes with two or three convolutions. The white cloud floats slowly away among the stars, coiling up more and more, and finally fades out of sight. The cause of all this seems to be as follows. The heated air charged with the debris of the meteor is by the meteor's impact driven off horizontally, causing the narrow train to spread into a cloud. The currents of air differing in direction at different altitudes twist the cloud into its varied fantastic forms. Attempts to obtain the spectrum of the trains have been made, and sodium and magnesium lines have been thought to be detected in them. The observation, however, is one that is not easy to make or confirm. The trains have often colours other than white, and in the case of the brighter meteors different colours are seen in the different parts of the train.
Magnitude.Some computations have been made of the size of the shooting star meteoroids from the mechanical equivalent of the light developed by their disintegration. If all the energy of the meteor is changed into light, then these computations would be conclusive. But a part is spent in disintegrating and burning the stone, a part in heating the air, and a part in giving direct motion to portions of air. A computation based on the light developed gives only a lower limit to the size.
It seems probable that the larger meteors might be safely regarded as weighing on entering the air only a few hundreds or at most a few thousands of pounds. The smallest visible shooting stars may be equal in size to coarse grains of sand, and still be large enough to furnish all the light exhibited by them. The largest shooting stars furnish matter enough to fill with thin trains cubic miles of space, but this need not require a very large mass.
Meteoric Irons.There have been found at various times on the surface of the earth masses of metallic iron combined with nickel. These have been so like the irons which have been known to fall, both in their structure and in composition, that they have been without hesitation classed among the meteoric irons. A mass of this character weighing 1635 lb, found in Texas, is in the Yale College Museum. The Charcas (Mexico) iron in the Paris museum is about the same size. A ring-shaped mass, somewhat smaller, from Tuczon, is in the United States National Museum in Washington. A still larger mass is in the British Museum, and many other large masses are in public collections or private possession.
Widmannstatten Figures.If in any of the meteoric irons, whether seen to fall or found on the earth, a section is cut and polished and then etched with acids, a series of peculiar lines are developed which are known as Widmannstatten figures. The lines of iron nnattacked by the acid consist of an irregular grouping of parallel rulings often lying along the faces of a regular octahedron. The exhibition of these figures and the combination of iron with nickel have been usually considered conclusive evidence of the meteoric origin of any iron mass.
Nickel Iron of Ovifak.In 1870 Baron Nordenskiold, in his voyage to Greenland, found on the shore of the island of Disco fifteen iron masses, the largest of which weighed 20 tons, all in an area of half an acre. In the basaltic rocks not far distant other iron masses were found embedded in the basalt. The presence of nickel with the iron, and the development of lines like the Widmannstatten figures, were at once accepted as proof of their meteoric origin, in spite of the combination with basalt. A more complete examina-tion has, however, established the terrestrial origin of these irons, and given reasons to hope for new discoveries of relations existing between the earth and the meteors. The additional discovery of small particles of metallic iron in certain other igneous rocks proves that the union of the Ovifak irons with basalt is not excep-tional.
Chemical Constitution of the Meteorites.No new element has been found in the meteorites. Three elements most widely distri-buted and most important among the meteoritesiron, silicon, and oxygenare also most abundant in our earth. Daubree gives the following lists of elements, arranged somewhat in the degree of their importance, in meteorites (Maskelyne adds lithium and antimony):
Iron. Titanium. Arsenic.
Magnesium. Tin. Phosphorus.
Silicon. Copper. Nitrogen.
Oxygen. Aluminium. Sulphur.
Nickel. Potassium. Chlorine.
Cobalt. Sodium. Carbon.
Chromium. Calcium. Hydrogen.
Minerals in Meteorites.Among the minerals in the meteorites there are several which occur in the rocks on the earth. Among these are cited by Daubree peridote, pyroxene, enstatite, triclinic felspar, chromite, magnetic pyrites, iron oxide, graphite, and probably water. Several minerals, however, are found which, so far as now known, are peculiar to the meteorites :metallic nickel-iron, phosphide of iron and nickel (schreibersite), sesquisulphide of chromium and iron (daubreelite), sulphide of calcium (oldhamite), and chloride of iron (lawrencite).
Meteorites of different falls are in general unlike ; but there are many instances in which the stones of two falls are so similarly constituted that it is not easy to distinguish them.
In four falls (Alais, Cold Bokkeweld, Kaba, and Orgeuil) the stones contain little or no iron. In these carbon appears not as graphite but in union with hydrogen and oxygen, and also with soluble and even deliquescent saline matters. The combinations arc such as to suggest the existence of humus and organic remains. But after careful search nothing of this kind has been detected in them. In general the meteorites show no resemblance in their mechanical or mineralogical structure to the granitic and surface rocks on the earth. One condition was certainly necessary in their formation, viz., the absence of free oxygen and of enough water to oxidize the iron and other elements. Perhaps it is to this fact that are due the resemblances between these minerals and those of the deep-seated rocks of the earth in the formation of which free oxygen and water were also not present.
Gases in Meteorites.The meteoric stones and irons when reduced to fine particles and placed in the vacuum of a Sprengel air-pump give off small quantities of gases which may be reasonably pre-sumed to have been occluded by the irons at some time in their earlier history. Professor Graham found hydrogen in meteoric irons. Professor Wright has shown that a moderate heat drives off from the stony meteorites carbonic acid and carbonic oxide with a small amount of hydrogen. As the heat increases the proportion of hydrogen (and even some nitrogen) increases till at a full red heat the hydrogen given off is by far the largest portion. From the irons similar gases are given off, but the carbon compounds are not so large a component as hydrogen. The spectra seen in the tails of comets are not strikingly like those of any of these gases. But it is impossible to reproduce in the laboratory the conditions under which the matter of comet's tails is giving off its light. We cannot therefore say that these gases may not be the important parts of the cometic coma and tails.
Meteoroid as Part of a Comet.Assuming that the meteorite and meteoroid once formed an integral part of a comet, not a little information is given us of the nature of this mysterious body. There is room also for speculation.
First, the comet may be a single hard body which comes from the cold of space into the heat of the sun, and there has frag-ments broken off, just as a stone is shattered in a hot fire. The nucleus of some of the comets must be very small because invisible in the telescope, and an impulse that would raise a stone on the earth only a few inches would send it permanently away from such a comet. The exposure of new surfaces to the heat of the sun might give occasion for the development of gas to form the comet's tail.
Or, secondly, the comet may be a tolerably compact aggregation of small bodies not in contact, each one being of the size of a meteoroid, and kept near to the rest, not by cohesion, but by their combined attraction. The total mass being small, some members of the group near the comet's perihelion passage can be by the sun's perturbing action thrown out into orbits quite independent of the comet itself, and yet such as relative to the sun shall resemble that of the main group. Perturbations resembling tidal waves might be preparing other members to be cast off at the next perihelion passage of the comet.
In either case, if we suppose," as seems probable, that the comet came from outside the solar system, and that a disturbance by a large planet changed the original hyperbolic orbit into an ellipse, the comet must have passed that planet as a very compact group, if not in a single mass, else the disturbance that changed the orbit would have scattered the group beyond the power of a future recog-nition of the common origin of the fragments.
Meteoroids as Fuel of the Sun.The idea has been held by distinguished physicists that the meteoroids in falling into the sun furnish by their concussion a supply for the heat which the sun is constantly sending off into space,that they are in fact the fuel of the sun. Such a view, however, receives but little support from facts which we know about meteors. The meteoroids of the August and two November periods are evidently permanent members of the solar system moving in closed orbits. The same is by inference highly probable for most of the other meteoroids, and may be true of all of them. Permanent members of the solar system, however, if they ever fall into the sun, do so only after a long period of perturbation. If any meteoroids come from stellar spaces and have any uniform or random distribution of velocities or direc-tions, only a very small portion of these would hit the sun's surface. The far greater portion would go on in hyperbolic orbits. But the earth receives the impact of its portion of these foreign meteoroids, both in their inward and outward course, and in addition encounters a full share of the permanent members of the solar system, of which the sun receives very few or none. It is not hard to show that a supply of meteoroids to the sun sufficient to make good its daily loss of heat would require that the twenty million meteoroids which the earth daily encounters, even if all were from stellar space, should have an average weight of hundreds of tons. The facts do not warrant the admission of any such magni-tude even for the large meteors, much less for the ordinary and small shooting stars. Whatever be the source of the sun's heat, all the meteoroids of which we know anything are totally inade-quate to supply the waste.
The literature of meteors and meteoroids is very much scattered. It is mainly contained in the scientific journals and in transac-tions of learned societies. The series of valuable Reports of the Luminous Meteor Committee of the British Association contains not only the record of an immense amount of original observations, but also year by year a digest of most of the important memoirs.
Meteoric science is a structure built stone by stone by many builders. In this article no attempt has been made to assign to each builder the credit for his contribution. (H. A. N.)