The Story of the Heavens Part 24

The diagram is also designed to indicate a remarkable speculation which was put forward on the high authority of Le Verrier, with the view of explaining how the shoal came to be introduced into the solar system.

The orbit in which the meteors revolve does not intersect the paths of Jupiter, Saturn, or Mars, but it does intersect the orbit of Ura.n.u.s. It must sometimes happen that Ura.n.u.s is pa.s.sing through this point of its path just as the shoal arrives there. Le Verrier has demonstrated that such an event took place in the year A.D. 126, but that it has not happened since. We thus seem to have a clue to a very wonderful history by which the meteors are shown to have come into our system in the year named. The expectations or a repet.i.tion of the great shower in 1899 which had been widely entertained, and on good grounds, were not realised. Hardly more than a few meteors of the ordinary type were observed.

a.s.suming that the orbit of the August meteors was a parabola, Schiaparelli computed the dimensions and position in s.p.a.ce of this...o...b..t, and when he had worked this out, he noticed that the orbit corresponded in every particular with the orbit of a fine comet which had appeared in the summer of 1862. This could not be a mere matter of accident. The plane in which the comet moved coincided exactly with that in which the meteors moved; so did the directions of the axes of their orbits, while the direction of the motion is the same, and the shortest distance from the sun to the orbit is also in the two cases identical.

This proved to demonstration that there must be some profound physical connection between comets and swarms of meteors. And a further proof of this was shortly afterwards furnished, when Le Verrier had computed the orbit of the November meteors, for this was at once noticed to be precisely the same as the orbit of a comet which had pa.s.sed its perihelion in January, 1866, and for which the period of revolution had been found to be thirty-three years and two months.

Among the Leonids we see occasionally fireb.a.l.l.s brighter than Venus, and even half the apparent size of the moon, bursting out with lightning-like flashes, and leaving streaks which last from a minute to an hour or more. But the great majority are only as bright as stars of the second, third, or fourth magnitude. As the amount of light given by a meteor depends on its ma.s.s and velocity, we can form some idea as to the actual weight of one of these meteors, and it appears that most of them do not weigh nearly as much as a quarter of an ounce; indeed, it is probable that many do not weigh a single grain. But we have seen that a comet in all probability is nothing but a very loose swarm of small particles surrounded by gas of very slight density, and we have also seen that the material of a comet must by degrees be more or less dissipated through s.p.a.ce. We have still to tell a wonderful story of the breaking up of a comet and what appears to have become of the particles thereof.

A copious meteoric shower took place on the night of the 27th November, 1872. On this occasion the shooting stars diverged from a radiant point in the constellation of Andromeda. As a spectacle, it was unquestionably inferior to the magnificent display of 1866, but it is difficult to say which of the two showers has been of greater scientific importance.

It surely is a remarkable coincidence that the earth should encounter the Andromedes (for so this shower is called) at the very moment when it is crossing the track of Biela's comet. We have observed the direction from which the Andromedes come when they plunge into the atmosphere; we can ascertain also the direction in which Biela's comet is moving when it pa.s.ses the earth's track, and we find that the direction in which the comet moves and the direction in which the meteors move are identical.

This is, in itself, a strong and almost overwhelming presumption that the comet and the shooting stars are connected; but it is not all. We have observations of this swarm dating back to the eighteenth century, and we find that the date of its appearance has changed from the 6th or 7th of December to the end of November in perfect accordance with the retrograde motion of the crossing-point of the earth's...o...b..t and the orbit of Biela's comet. This comet was observed in 1772, and again in 1805-6, before its periodic return every seven years was discovered. It was discovered by Biela in 1826, and was observed again in 1832. In 1846 the astronomical world was startled to find that there were now two comets in place of one, and the two fragments were again perceived at the return in 1852. In 1859 Biela's comet could not be seen, owing to its unfavourable situation with regard to the earth. No trace of Biela's comet was seen in 1865-66, when its return was also due, nor has it ever been seen since. It therefore appears that in the autumn of 1872 the time had arrived for the return of Biela's comet, and thus the occurrence of the great shower of the Andromedes took place about the time when Biela's comet was actually due. The inference is irresistible that the shooting stars, if not actually a part of the comet itself, are at all events most intimately connected therewith. This shower is also memorable for the telegram sent from Professor Klinkerfues to Mr. Pogson at Madras. The telegram ran as follows:--"Biela touched earth on 27th.

Search near Theta Centauri." Pogson did search and did find a comet, but, unfortunately, owing to bad weather he only secured observations of it on two nights. As we require three observations to determine the orbit of a planet or comet, it is not possible to compute the orbit of Pogson's, but it seems almost certain that the latter cannot be identical with either of the two components of Biela's comet. It is, however, likely that it really was a comet moving along the same track as Biela and the meteors.

Another display of the Biela meteors took place in 1885, just giving time for two complete revolutions of the swarm since 1872. The display on the 27th November, 1885, was magnificent; Professor Newton estimated that at the time of maximum the meteors came on at the rate of 75,000 per hour. In 1892 the comet ought again to have returned to perihelion, but in that year no meteors were seen on the 27th November, while many were seen on the 23rd from the same radiant. The change in the point of intersection between the orbit of the meteors and the orbit of the earth indicated by this difference of four days was found by Bredichin to be due to the perturbing action of Jupiter on the motion of the swarm.

It is a noticeable circ.u.mstance that the great meteoric showers seem never yet to have projected a missile which has reached the earth's surface. Out of the myriads of Leonids, of Perseids, or of Andromedes, not one particle has ever been seized and identified.[35] Those bodies which fall from the sky to the earth, and which we call meteorites, do not seem to come from the great showers, so far as we know. They may, indeed, have quite a different origin from that of the periodic meteors.

It is somewhat curious that the belief in the celestial origin of meteorites is of modern growth. In ancient times there were, no doubt, rumours of wonderful stones which had fallen down from the heavens to the earth, but these reports seem to have obtained but little credit.

They were a century ago regarded as perfectly fabulous, though there was abundant testimony on the subject. Eye-witnesses averred that they had seen the stones fall. The bodies themselves were unlike other objects in the neighbourhood, and cases were even authenticated where men had been killed by these celestial visitors.

No doubt the observations were generally made by ignorant and illiterate persons. The true parts of the record were so mixed up with imaginary additions, that cautious men refused to credit the statements that such objects really fell from the sky. Even at the present day it is often extremely difficult to obtain accurate testimony on such matters. For instance, the fall of a meteorite was observed by a Hindoo in the jungle. The stone was there, its meteoric character was undoubted, and the witness was duly examined as to the details of the occurrence; but he was so frightened by the noise and by the danger he believed himself to have narrowly escaped, that he could tell little or nothing. He felt certain, however, that the meteorite had hunted him for two hours through the jungle before it fell to the earth!

In the year 1794 Chladni published an account of the remarkable ma.s.s of iron which the traveller Pallas had discovered in Siberia. It was then for the first time recognised that this object and others similar to it must have had a celestial origin. But even Chladni's reputation and the arguments he brought forward failed to procure universal a.s.sent. Shortly afterwards a stone of fifty-six pounds was exhibited in London, which several witnesses declared they had seen fall at Wold Cottage, in Yorkshire, in 1795. This body was subsequently deposited in our national collection, and is now to be seen in the Natural History Museum at South Kensington. The evidence then began to pour in from other quarters; portions of stone from Italy and from Benares were found to be of identical composition with the Yorkshire stone. The incredulity of those who had doubted the celestial origin of these objects began to give way.

A careful memoir on the Benares meteorite, by Howard, was published in the "Philosophical Transactions" for 1802, while, as if to complete the demonstration, a great shower of stones took place in the following year at L'Aigle, in Normandy. The French Academy deputed the physicist Biot to visit the locality and make a detailed examination of the circ.u.mstances attending this memorable shower. His enquiry removed every trace of doubt, and the meteoric stones have accordingly been transferred from the dominions of geology to those of astronomy. It may be noted that the recognition of the celestial origin of meteorites happens to be simultaneous with the discovery of the first of the minor planets. In each case our knowledge of the solar system has been extended by the addition of numerous minute bodies, which, notwithstanding their insignificant dimensions, are pregnant with information.

When the possibility of stone-falls has been admitted, we can turn to the ancient records, and a.s.sign to them the credit they merit, which was withheld for so many centuries. Perhaps the earliest of all these stone-falls which can be said to have much pretension to historical accuracy is that of the shower which Livy describes as having fallen, about the year 654 B.C., on the Alban Mount, near Rome. Among the more modern instances, we may mention one which was authenticated in a very emphatic manner. It occurred in the year 1492 at Ensisheim, in Alsace.

The Emperor Maximilian ordered a minute narrative of the circ.u.mstances to be drawn up and deposited with the stone in the church. The stone was suspended in the church for three centuries, until in the French Revolution it was carried off to Colmar, and pieces were broken from it, one of which is now in our national collection. Fortunately, this interesting object has been restored to its ancient position in the church at Ensisheim, where it remains an attraction to sight-seers at this day. The account is as follows:--"In the year of the Lord 1492, on the Wednesday before St. Martin's Day, November 7th, a singular miracle occurred, for between eleven o'clock and noon there was a loud clap of thunder and a prolonged confused noise, which was heard at a great distance, and a stone fell from the air in the jurisdiction of Ensisheim which weighed 260 pounds, and the confused noise was at other places much louder than here. Then a boy saw it strike on ploughed ground in the upper field towards the Rhine and the Ill, near the district of Gisgang, which was sown with wheat, and it did no harm, except that it made a hole there; and then they conveyed it from the spot, and many pieces were broken from it, which the Land Vogt forbade. They therefore caused it to be placed in the church, with the intention of suspending it as a miracle, and there came here many people to see this stone, so there were many remarkable conversations about this stone; the learned said they knew not what it was, for it was beyond the ordinary course of nature that such a large stone should smite from the height of the air, but that it was really a miracle from G.o.d, for before that time never was anything heard like it, nor seen, nor written. When they found that stone, it had entered into the earth to half the depth of a man's stature, which everybody explained to be the will of G.o.d that it should be found, and the noise of it was heard at Lucerne, at Villingen, and at many other places, so loud that the people thought that the houses had been overturned; and as the King Maximilian was here, the Monday after St. Catherine's Day of the same year, his Royal Excellency ordered the stone which had fallen to be brought to the castle, and after having conversed a long time about it with the n.o.blemen, he said that the people of Ensisheim should take it and order it to be hung up in the church, and not to allow anybody to take anything from it. His Excellency, however, took two pieces of it, of which he kept one, and sent the other to Duke Sigismund of Austria, and there was a great deal of talk about the stone, which was suspended in the choir, where it still is, and a great many people came to see it."

Admitting the celestial origin of the meteorites, they surely claim our closest attention. They afford the only direct method we possess of obtaining a knowledge of the materials of bodies exterior to our planet.

We can take a meteorite in our hands, we can a.n.a.lyse it, and find the elements of which it is composed. We shall not attempt to enter into any very detailed account of the structure of meteorites; it is rather a matter for the consideration of chemists and mineralogists than for astronomers. A few of the more obvious features will be all that we require. They will serve as a preliminary to the discussion of the probable origin of these bodies.

In the Natural History Museum at South Kensington we may examine a superb collection of meteorites. They have been brought together from all parts of the earth, and vary in size from bodies not much larger than a pin's head up to vast ma.s.ses weighing many hundredweights. There are also models of celebrated meteorites, of which the originals are dispersed through various other museums.

Many meteorites have nothing very remarkable in their external appearance. If they were met with on the sea beach, they would be pa.s.sed by without more notice than would be given to any other stone. Yet, what a history a meteorite might tell us if we could only manage to obtain it! It fell; it was seen to fall from the sky; but what was its course anterior to that movement? Where was it 100 years ago, 1,000 years ago?

Through what regions of s.p.a.ce has it wandered? Why did it never fall before? Why has it actually now fallen? Such are some of the questions which crowd upon us as we ponder over these most interesting bodies.

Some of these objects are composed of very characteristic materials; take, for example, one of the more recent arrivals, known as the Rowton siderite. This body differs very much from the more ordinary kind of stony meteorite. It is an object which even a casual pa.s.ser-by would hardly pa.s.s without notice. Its great weight would also attract attention, while if it be scratched or rubbed with a file, it would appear to be a ma.s.s of nearly pure iron. We know the circ.u.mstances in which that piece of iron fell to the earth. It was on the 20th of April, 1876, about 3.40 p.m., that a strange rumbling noise, followed by a startling explosion, was heard over an area of several miles in extent among the villages in Shropshire, eight or ten miles north of the Wrekin. About an hour after this occurrence a farmer noticed that the ground in one of his gra.s.s-fields had been disturbed, and he probed the hole which the meteorite had made, and found it, still warm, about eighteen inches below the surface. Some men working at no great distance had heard the noise made in its descent. This remarkable object, weighs 7-3/4 lbs. It is an irregular angular ma.s.s of iron, though all its edges seem to have been rounded by fusion in its transit through the air. It is covered with a thick black pellicle of the magnetic oxide of iron, except at the point where it first struck the ground. The Duke of Cleveland, on whose property it fell, afterwards presented it to our national inst.i.tution already referred to, where, as the Rowton siderite, it attracts the attention of everyone who is interested in these wonderful bodies.

This siderite is specially interesting on account of its distinctly metallic character. Falls of objects of this particular type are not so frequent as are those of the stony meteorites; in fact, there are only a few known instances of meteoric irons having been actually seen to fall, while the observed falls of stony meteorites are to be counted in scores or in hundreds. The inference is that the iron meteorites are much less frequent than the stony ones. This is, however, not the impression that the visitor to the Museum would be likely to receive. In that extensive collection the meteoric irons are by far the most striking objects. The explanation is not difficult. Those gigantic ma.s.ses of iron are unquestionably meteoric: no one doubts that this is the case. Yet the vast majority of them have never been seen to fall; they have simply been found, in circ.u.mstances which point unmistakably to their meteoric nature. Suppose, for instance, that a traveller on one of the plains of Siberia or of Central America finds a ma.s.s of metallic iron lying on the surface of the ground, what explanation can be rendered of such an occurrence? No one has brought the iron there, and there is no iron within hundreds of miles. Man never fashioned that object, and the iron is found to be alloyed with nickel in a manner that is always observed in known meteorites, and is generally regarded as a sure indication of a meteoric origin. Observe also, that as iron perishes by corrosion in our atmosphere, that great ma.s.s of iron cannot have lain where it is for indefinite ages; it must have been placed there at some finite time. Only one source for such an object is conceivable; it must have fallen from the sky. On the same plains the stony meteorites have also fallen in hundreds and in thousands, but they crumble away in the course of time, and in any case would not arrest the attention of the traveller as the irons are likely to do. Hence it follows, that although the stony meteorites seem to fall much more frequently, yet, unless they are actually observed at the moment of descent, they are much more liable to be overlooked than the meteoric irons. Hence it is that the more prominent objects of the British collection are the meteoric irons.

We have said that a noise accompanied the descent of the Rowton siderite, and it is on record that a loud explosion took place when the meteorite fell at Ensisheim. In this we have a characteristic feature of the phenomenon. Nearly all the descents of meteorites that have been observed seem to have been ushered in by a detonation. We do not, however, a.s.sert that this is quite an invariable feature; and it is also the case that meteors often detonate without throwing down any solid fragments that have been collected. The violence a.s.sociated with the phenomenon is forcibly ill.u.s.trated by the Butsura meteorite. This object fell in India in 1861. A loud explosion was heard, several fragments of stone were collected from distances three or four miles apart; and when brought together, they were found to fit, so as to enable the primitive form of the meteorite to be reconstructed. A few of the pieces are wanting (they were, no doubt, lost by falling un.o.bserved into localities from which they could not be recovered), but we have obtained pieces quite numerous enough to permit us to form a good idea of the irregular shape of the object before the explosion occurred which shattered it into fragments. This is one of the ordinary stony meteorites, and is thus contrasted with the Rowton siderite which we have just been considering. There are also other types of meteorites. The Breitenbach iron, as it is called, is a good representative of a cla.s.s of these bodies which lie intermediate between the meteoric irons and the stones.

It consists of a coa.r.s.ely cellular ma.s.s of iron, the cavities being filled with mineral substances. In the Museum, sections of intermediate forms are shown in which this structure is exhibited.

Look first at the most obvious characteristic of these meteorites. We do not now allude to their chemical composition, but to their external appearance. What is the most remarkable feature in the shape of these objects?--surely it is that they are fragments. They are evidently pieces that are _broken_ from some larger object. This is apparent by merely looking at their form; it is still more manifest when we examine their mechanical structure. It is often found that meteorites are themselves composed of smaller fragments. Such a structure may be ill.u.s.trated by a section of an aerolite found on the Sierra of Chaco, weighing about 30 lbs. (Fig. 79).

The section here represented shows the composite structure of this object, which belongs to the cla.s.s of stony meteorites. Its shape shows that it was really a fragment with angular edges and corners. No doubt it may have been much more considerable when it first dashed into the atmosphere. The angular edges now seen on the exterior may be due to an explosion which then occurred; but this will not account for the structure of the interior. We there see irregular pieces of varied form and material agglomerated into a single ma.s.s. If we would seek for a.n.a.logous objects on the earth, we must look to some of the volcanic rocks, where we have mult.i.tudes of irregular angular fragments cemented together by a matrix in which they are imbedded. The evidence presented by this meteorite is conclusive as to one circ.u.mstance with regard to the origin of these objects. They must have come as fragments, from some body of considerable, if not of vast, dimensions. In this meteorite there are numerous small grains of iron mingled with mineral substances.

The iron in many meteorites has, indeed, characters resembling those produced by the actual blasting of iron by dynamite. Thus, a large meteoric iron from Brazil has been found to have been actually shivered into fragments at some time anterior to its fall on the earth. These fragments have been cemented together again by irregular veins of mineral substances.

[Ill.u.s.tration: Fig. 79.--Section of the Chaco Meteorite.]

For an aerolite of a very different type we may refer to the carbonaceous meteorite of Orgueil, which fell in France on the 14th May, 1864. On the occasion of its descent a splendid meteor was seen, rivalling the full moon in size. The actual diameter of this globe of fire must have been some hundreds of yards. Nearly a hundred fragments of the body were found scattered over a tract of country fifteen miles long. This object is of particular interest, inasmuch as it belongs to a rare group of aerolites, from which metallic iron is absent. It contains many of the same minerals which are met with in other meteorites, but in these fragments they are _a.s.sociated with carbon_, and with substances of a white or yellowish crystallisable material, soluble in ether, and resembling some of the hydrocarbons. Such a substance, if it had not been seen falling to the earth, would probably be deemed a product resulting from animal or vegetable life!

We have pointed out how a body moving with great velocity and impinging upon the air may become red-hot and white-hot, or even be driven off into vapour. How, then, does it happen that meteorites escape this fiery ordeal, and fall down to the earth, with a great velocity, no doubt, but still, with very much less than that which would have sufficed to drive them off into vapour? Had the Rowton siderite, for instance, struck our atmosphere with a velocity of twenty miles a second, it seems unquestionable that it would have been dissipated by heat, though, no doubt, the particles would ultimately coalesce so as to descend slowly to the earth in microscopic beads of iron. How has the meteorite escaped this fate? It must be remembered that our earth is also moving with a velocity of about eighteen miles per second, and that the _relative_ velocity with which the meteorite plunges into the air is that which will determine the degree to which friction is operating. If the meteorite come into direct collision with the earth, the velocity of the collision will be extremely great; but it may happen that though the actual velocities of the two bodies are both enormous, yet the relative velocity may be comparatively small. This is, at all events, one conceivable explanation of the arrival of a meteorite on the surface of the earth.

We have shown in the earlier parts of the chapter that the well-known star showers are intimately connected with comets. In fact, each star shower revolves in the path pursued by a comet, and the shooting star particles have, in all probability, been themselves derived from the comet. Showers of shooting stars have, therefore, an intimate connection with comets, but it is doubtful whether meteorites have any connection with comets. It has already been remarked that meteorites have never been known to fall in the great star showers. No particle of a meteorite is known to have dropped from the countless host of the Leonids or of the Perseids; as far as we know, the Lyrids never dropped a meteorite, nor did the Quadrantids, the Geminids, or the many other showers with which every astronomer is familiar. There is no reason to connect meteorites with these showers, and it is, therefore, doubtful whether we should connect meteorites with comets.

With reference to the origin of meteorites it is difficult to speak with any great degree of confidence. Every theory of meteorites presents difficulties, so it seems that the only course open to us is to choose that view of their origin which seems least improbable. It appears to me that this condition is fulfilled in the theory entertained by the Austrian mineralogist, Tschermak. He has made a study of the meteorites in the rich collection at Vienna, and he has come to the conclusion that the "meteorites have had a volcanic source on some celestial body." Let us attempt to pursue this reasoning and discuss the problem, which may be thus stated:--a.s.suming that at least some of the meteorites have been ejected from volcanoes, on what body or bodies in the universe must these volcanoes be situated? This is really a question for astronomers and mathematicians. Once the mineralogists a.s.sure us that these bodies are volcanic, the question becomes one of calculation and of the balance of probabilities.

The first step in the enquiry is to realise distinctly the dynamical conditions of the problem. Conceive a volcano to be located on a planet.

The volcano is supposed to be in a state of eruption, and in one of its mighty throes projects a missile aloft: this missile will ascend, it will stop, and fall down again. Such is the case at present in the eruptions of terrestrial volcanoes. Cotopaxi has been known to hurl prodigious stones to a vast height, but these stones a.s.suredly return to earth. The gravitation of the earth has gradually overcome the velocity produced by the explosion, and down the body falls. But let us suppose that the eruption is still more violent, and that the stones are projected from the planet to a still greater height above its surface.

Suppose, for instance, that the stone should be shot up to a height equal to the planet's radius, the attraction of gravitation will then be reduced to one-fourth of what it was at the surface, and hence the planet will find greater difficulty in pulling back the stone. Not only is the distance through which the stone has to be pulled back increased as the height increases, but the efficiency of gravitation is weakened, so that in a twofold way the difficulty of recalling the stone is increased. We have already more than once alluded to this subject, and we have shown that there is a certain critical velocity appropriate to each planet, and depending on its ma.s.s and its radius. If the missile be projected upwards with a velocity equal to or greater than this, then it will ascend never to return. We all recollect Jules Verne's voyage to the moon, in which he described the Columbiad, an imaginary cannon, capable of shooting out a projectile with a velocity of six or seven miles a second. This is the critical velocity for the earth. If we could imagine the air removed, then a cannon of seven-mile power would project a body upwards which would never fall down.

The great difficulty about Tschermak's view of the volcanic origin of the meteorites lies in the tremendous initial velocity which is required. The Columbiad is a myth, and we know no agent, natural or artificial, at the present time on the earth, adequate to the production of a velocity so appalling. The thunders of Krakatoa were heard thousands of miles away, but in its mightiest throes it discharged no missiles with a velocity of six miles a second. We are therefore led to enquire whether any of the other celestial bodies are ent.i.tled to the parentage of the meteorites. We cannot see volcanoes on any other body except the moon; all the other bodies are too remote for an inspection so minute. Does it seem likely that volcanoes on the moon can ever launch forth missiles which fall upon the earth?

This belief was once sustained by eminent authority. The ma.s.s of the moon is about one-eightieth of the ma.s.s of the earth. It would not be true to a.s.sert that the critical velocity of projection varies directly as the ma.s.s of the planet. The correct law is, that it varies directly as the square root of the ma.s.s, and inversely as the square root of the radius. It is hence shown that the velocity required to project a missile away from the moon is only about one-sixth of that which would be required to project a missile away from the earth. If the moon had on its surface volcanoes of one-mile power, it is quite conceivable that these might be the source of meteorites. We have seen how the whole surface of the moon shows traces of intense volcanic activity. A missile thus projected from the moon could undoubtedly fall on the earth, and it is not impossible that some of the meteorites may really have come from this source. There is, however, one great difficulty about the volcanoes on the moon. Suppose an object were so projected, it would, under the attraction of the earth, in accordance with Kepler's laws, move around the earth as a focus. If we set aside the disturbances produced by all other bodies, as well as the disturbance produced by the moon itself, we see that the meteorite if it once misses the earth can never fall thereon. It would be necessary that the shortest distance of the earth's centre from the orbit of the projectile should be less than the radius of the earth, so that if a lunar meteorite is to fall on the earth, it must do so the first time it goes round. The journey of a meteorite from the moon to the earth is only a matter of days, and therefore, as meteorites are still falling, it would follow that they must still be constantly ejected from the moon. The volcanoes on the moon are, however, not now active; observers have long studied its surface, and they find no reliable traces of volcanic activity at the present day.

It is utterly out of the question, whatever the moon may once have been able to do, that at the present date she could still continue to launch forth meteorites. It is just possible that a meteorite expelled from the moon in remote antiquity, when its volcanoes were active, may, under the influence of the disturbances of the other bodies of the system, have its...o...b..t so altered, that at length it comes within reach of the atmosphere and falls to the earth, but in no circ.u.mstances could the moon send us a meteorite at present. It is therefore reasonable to look elsewhere in our search for volcanoes fulfilling the conditions of the problem.

Let us now direct our attention to the planets, and examine the circ.u.mstances in which volcanoes located thereon could eject a meteorite which should ultimately tumble on the earth. We cannot see the planets well enough to tell whether they have or ever had any volcanoes; but the almost universal presence of heat in the large celestial ma.s.ses seems to leave us in little doubt that some form of volcanic action might be found in the planets. We may at once dismiss the giant planets, such as Jupiter or Saturn: their appearance is very unlike a volcanic surface; while their great ma.s.s would render it necessary to suppose that the meteorites were expelled with terrific velocity if they should succeed in escaping from the gravitation of the planet. Applying the rule already given, a volcano on Jupiter would have to be five or six times as powerful as the volcano on the earth. To avoid this difficulty, we naturally turn to the smaller planets of the system; take, for instance, one of that innumerable host of minor planets, and let us enquire how far this body is likely to have ejected a missile which should fall upon the earth. Some of these globes are only a few miles in diameter. There are bodies in the solar system so small that a very moderate velocity would be sufficient to project a missile away from them altogether. We have, indeed, already ill.u.s.trated this point in discussing the minor planets. It has been suggested that a volcano placed on one of the minor planets might be quite powerful enough to start the meteorites on a long ramble through s.p.a.ce until the chapter of accidents brought them into collision with the earth. There is but little difficulty in granting that there might be such volcanoes, and that they might be sufficiently powerful to drive bodies from the surface of the planet; but we must remember that the missiles are to fall on the earth, and dynamical considerations are involved which merit our close attention.

To concentrate our ideas, we shall consider one of the minor planets, and for this purpose let us take Ceres. If a meteorite is to fall upon the earth, it must pa.s.s through the narrow ring, some 8,000 miles wide, which marks the earth's path; it will not suffice for the missile to pa.s.s through the ecliptic on the inside or on the outside of the ring, it must be actually through this narrow strip, and then if the earth happens to be there at the same moment the meteorite will fall. The first condition to be secured is, therefore, that the path of the meteorite shall traverse this narrow ring. This is to be effected by projection from some point in the orbit of Ceres. But it can be shown on purely dynamical grounds that although the volcanic energy sufficient to remove the projectile from Ceres may be of no great account, yet if that projectile is to cross the earth's track, the dynamical requirements of the case demand a volcano on Ceres at the very least of three-mile power. We have thus gained but little by the suggestion of a minor planet, for we have not found that a moderate volcanic power would be adequate. But there is another difficulty in the case of Ceres, inasmuch as the ring on the ecliptic is very narrow in comparison with the other dimensions of the problem. Ceres is a long way off, and it would require very great accuracy in volcanic practice on Ceres to project a missile so that it should just traverse this ring and fall neither inside nor outside, neither above nor below. There must be a great many misses for every hit. We have attempted to make the calculation by the aid of the theory of probabilities, and we find that the chances against this occurrence are about 50,000 to 1, so that out of every 50,000 projectiles hurled from a point in the orbit of Ceres only a single one can be expected to satisfy even the first of the conditions necessary if it is ever to tumble on our globe. It is thus evident that there are two objections to Ceres (and the same may be said of the other minor planets) as a possible source of the meteorites. Firstly, that notwithstanding the small ma.s.s of the planet a very powerful volcano would still be required; and secondly, that we are obliged to a.s.sume that for every one which ever reached the earth at least 50,000 must have been ejected. It is thus plain that if the meteorites have really been driven from some planet of the solar system, large or small, the volcano must, from one cause or another, have been a very powerful one.

We are thus led to enquire which planet possesses on other grounds the greatest probability in its favour.

We admit of course that at the present time the volcanoes on the earth are utterly devoid of the necessary power; but were the terrestrial volcanoes always so feeble as they are in these later days? Grounds are not wanting for the belief that in the very early days of geological time the volcanic energy on the earth was much greater than at present.

We admit fully the difficulties of the view that the meteorites have really come from the earth; but they must have some origin, and it is reasonable to indicate the source which seems to have most probability in its favour. Grant for a moment that in the primaeval days of volcanic activity there were some mighty throes which hurled forth missiles with the adequate velocity: these missiles would ascend, they would pa.s.s from the gravitation of the earth, they would be seized by the gravitation of the sun, and they would be compelled to revolve around the sun for ever after. No doubt the resistance of the air would be a very great difficulty, but this resistance would be greatly lessened were the crater at a very high elevation above the sea level, while, if a vast volume of ejected gases or vapours accompanied the more solid material, the effect of the resistance of the air would be still further reduced.

Some of these objects might perhaps revolve in hyperbolic orbits, and retreat never to return; while others would be driven into elliptic paths. Round the sun these objects would revolve for ages, but at each revolution--and here is the important point--they would traverse the point from which they were originally launched. In other words, every object so projected from the earth would at each revolution cross the track of the earth. We have in this fact an enormous probability in favour of the earth as contrasted with Ceres. Only one Ceres-ejected meteorite out of every 50,000 would probably cross the earth's track, while every earth-projected meteorite would necessarily do so.

If this view be true, then there must be hosts of meteorites traversing s.p.a.ce in elliptic orbits around the sun. These orbits have one feature in common: they all intersect the track of the earth. It will sometimes happen that the earth is found at this point at the moment the meteorite is crossing; when this is the case the long travels of the little body are at an end, and it tumbles back on the earth from which it parted so many ages ago.

It is well to emphasise the contrast between the lunar theory of meteorites (which we think improbable) and the terrestrial theory (which appears to be probable). For the lunar theory it would, as we have seen, be necessary that some of the lunar volcanoes should be still active. In the terrestrial theory it is only necessary to suppose that the volcanoes on the earth once possessed sufficient explosive power. No one supposes that the volcanoes at present on the earth eject now the fragments which are to form future meteorites; but it seems possible that the earth may be now slowly gathering back, in these quiet times, the fragments she ejected in an early stage of her history. a.s.suming, therefore, with Tschermak, that many meteorites have had a volcanic origin on some considerable celestial body, we are led to agree with those who think that most probably that body is the earth.

It is interesting to notice a few circ.u.mstances which seem to corroborate the view that many meteorites are of ancient terrestrial origin. The most characteristic const.i.tuent of these bodies is the alloy of iron and nickel, which is almost universally present. Sometimes, as in the Rowton siderite, the whole object consists of little else; sometimes this alloy is in grains distributed through the ma.s.s. When Nordenskjold discovered in Greenland a ma.s.s of native iron containing nickel, this was at once regarded as a celestial visitor. It was called the Ovifak meteorite, and large pieces of the iron were conveyed to our museums. There is, for instance, in the national collection a most interesting exhibit of the Ovifak substance. Close examination shows that this so-called meteorite lies in a bed of basalt which has been vomited from the interior of the earth. Those who believe in the meteoric origin of the Ovifak iron are constrained to admit that shortly after the eruption of the basalt, and while it was still soft, this stupendous iron meteorite of gigantic ma.s.s and bulk happened to fall into this particular soft bed. The view is, however, steadily gaining ground that this great iron ma.s.s was no celestial visitor at all, but that it simply came forth from the interior of the earth with the basalt itself. The beautiful specimens in the British Museum show how the iron graduates into the basalt in such a way as to make it highly probable that the source of the iron is really to be sought in the earth and not external thereto. Should further research establish this, as now seems probable, a most important step will have been taken in proving the terrestrial origin of meteorites. If the Ovifak iron be really a.s.sociated with the basalt, we have a proof that the iron-nickel alloy is indeed a terrestrial substance, found deep in the interior of the earth, and a.s.sociated with volcanic phenomena. This being so, it will be no longer difficult to account for the iron in undoubted meteorites.

When the vast volcanoes were in activity they ejected ma.s.ses of this iron-alloy, which, having circulated round the sun for ages, have at last come back again. As if to confirm this view, Professor Andrews discovered particles of native iron in the basalt of the Giant's Causeway, while the probability that large ma.s.ses of iron are there a.s.sociated with the basaltic formation was proved by the researches on magnetism of the late Provost Lloyd.

Besides the more solid meteorites there can be no doubt that the _debris_ of the ordinary shooting stars must rain down upon the earth in gentle showers of celestial dust. The snow in the Arctic regions has often been found stained with traces of dust which contains particles of iron. Similar particles have been found on the towers of cathedrals and in many other situations where it could only have been deposited from the air. There can be hardly a doubt that some of the motes in the sunbeam, and many of the particles which good housekeepers abhor as dust, have indeed a cosmical origin. In the famous cruise of the _Challenger_ the dredges brought up from the depths of the Atlantic no "wedges of gold, great anchors, heaps of pearl," but among the mud which they raised are to be found numerous magnetic particles which there is every reason to believe fell from the sky, and thence subsided to the depths of the ocean. Sand from the deserts of Africa, when examined under the microscope, yield traces of minute iron particles which bear the marks of having experienced a high temperature.

The earth draws in this cosmic dust continuously, but the earth now never parts with a particle of its ma.s.s. The consequence is inevitable; the ma.s.s of the earth must be growing, and though the change may be a small one, yet to those who have studied Darwin's treatise on "Earth-worms," or to those who are acquainted with the modern theory of evolution, it will be manifest that stupendous results can be achieved by slight causes which tend in one direction. It is quite probable that an appreciable part of the solid substance of our globe may have been derived from meteoric matter which descends in perennial showers upon its surface.

CHAPTER XVIII.

THE STARRY HEAVENS.

The Constellations--The Great Bear and the Pointers--The Pole Star--Ca.s.siopeia--Andromeda, Pegasus, and Perseus--The Pleiades: Auriga, Capella, Aldebaran--Taurus, Orion, Sirius; Castor and Pollux--The Lion--Bootes, Corona, and Hercules--Virgo and Spica--Vega and Lyra--The Swan.

The student of astronomy should make himself acquainted with the princ.i.p.al constellations in the heavens. This is a pleasing acquirement, and might well form a part of the education of every child in the kingdom. We shall commence our discussion of the sidereal system with a brief account of the princ.i.p.al constellations visible in the northern hemisphere, and we accompany our description with such outline maps of the stars as will enable the beginner to identify the chief features of the starry heavens.

In an earlier chapter we directed the attention of the student to the remarkable constellation of stars which is known to astronomers as Ursa Major, or the Great Bear. It forms the most conspicuous group in the northern skies, and in northern lat.i.tudes it never sets. At eleven p.m.