In the High Heavens/Chapter 13

by Robert Stawell Ball

CHAPTER XIII.

VISITORS FROM THE SKY.

ETEORITES have quite a peculiar claim on the attention of the astronomer. No doubt the spectroscope provides him with the means of demonstrating inferentially the existence of iron, and of many other terrestrial elements both in the sun and in many of the stars. But he is actually able to handle something which has come from the outside to the earth when he is in possession of a meteorite. Then, too, the movements of meteorites have a certain significance both for the astronomer and the mathematician. It is true that only the last stage of its wild career can be observed, for the flight of the meteorite for millions of preceding years is quite unknown, so far as any direct observations are concerned. The observer never beholds one of these bodies until that supreme moment when at the conclusion of its incalculable journey it flashes from the sky and strikes the earth beneath. The history of these objects before they are plunged into our atmosphere can only be ascertained by conjecture. I do not mean to say that we have no definite information by which to guide such conjectures. The laws of dynamics are fortunately available, and though they do not, it is to be regretted, convey all that we should like to know, yet still they teach us something with regard to the movements of the meteorites, and thus conduce to the solution of the great problem of their origin.

I am fully aware that there is considerable diversity of opinion as to the origin of meteorites. To me, however, it appears that the source of these bodies ought not to be a matter of much uncertainty. At the commencement of the inquiry it will be well to remove what is, I believe, a not uncommon misconception with regard to the character of meteorites. There can be no doubt that one of these bodies is often accompanied in its descent to the earth by a flash of fire, and not unfrequently a loud detonation announces at the same time that some violent disruption of a mass of matter has taken place. To this extent the fall of a meteorite presents phenomena resembling those which often accompany the apparition of a great fire-ball. In fact I suppose it can hardly be doubted that many of the fire-balls not recognised as meteoritic might have let fall mineral masses under somewhat different circumstances of impact upon the earth's atmosphere. But now comes one of the difficulties in the subject.

In general great fire-balls are little more than conspicuous shooting - stars, and there are shooting - stars of every dimension from those which illumine a country side with splendour down to the little streak of light which can only just be discerned as it flashes through the field of a powerful telescope. A noteworthy circumstance which interests the student of shooting-stars is their periodical exhibition in splendid displays. I allude to such a great shower as that of the Leonids which took place in 1866, and to other showers from the same system which have appeared in such a way as to indicate their recurrence in a period of thirty-three years. It appears perfectly certain that a distinguishing line has to be drawn between those bodies which dash into the atmosphere to be hurled to the earth as meteorites, and those shoals of little bodies which constitute the great meteoric showers.

Fig. 34.—The Orbit of the Leonids which produced the Great Shower in 1866.

This is, I know, a point about which there is much disagreement among astronomers. Excellent authorities have maintained that there is perfect continuity between these different classes of objects. Those who hold this view urge that a shower of shooting- stars is merely the aggregation on a grand scale of the isolated or sporadic shooting- stars which are always more or less to be seen, and they affirm that between these occasional shooting stars and the mightiest fire-ball there is a perfect continuity, exhibited by the fact that shooting-stars of every gradation of lustre are from time to time observed. They are thus led to regard meteorites as congenerous with the objects which appear in shooting-star showers. In this inference I am convinced that a serious mistake has been made.

One of the most important results of the great shower of 1866 was the demonstration that the swarm of little bodies to which that shower owed its origin was connected with a comet. The swarm was found in fact to follow the exact track which the comet pursued round the sun. So remarkable a coincidence could not reasonably be accounted for on any other supposition than that the meteors, if not themselves actual fragments of the comet, were at all events so closely connected with it, that they could not have come from any source very different from that in which the comet itself took its rise. This remarkable discovery made with regard to the Leonids may be illustrated and confirmed by similar discoveries of a cometary association in the case of other notable meteoric displays. Probably one of the most remarkable episodes in the whole of this branch of astronomy is connected with Biela's comet. It was known that the body so named revolved in a certain orbit, and a highly dramatic proof was rendered that a shoal of meteors were its fellow-travellers along the same path. As, however, I have discussed this episode in my work entitled "In Starry Realms," I need not now do more than refer to what has been there said.

Of this connection between cometary orbits and revolving swarms of meteors, many other instances could be cited. I may refer to the remarkable lists published by the British Association, in which, beside the name of the comet or the designation which astronomers had affixed to it, the meteoric swarm with which the comet is associated is also given. The day of the year on which the earth crosses the track pursued by the comet is the day on which the shower of meteors appropriate to that comet is to be expected when the proper interval of entire years has elapsed. The position of the "radiant," or point on the heavens from which the meteors appear to diverge, is exactly that point in the sky where the comet would be seen as it approached the point of crossing over the earth's track by an observer stationed at the crossing. When a meteoric shower appears on the day that has been foretold from the attitude of the cometary orbit, and when the radiant from which the shooting-stars of that shower are directed also occupies the precise position on the sky which has been indicated by the comet, it is then impossible to refuse assent to the belief that the meteors and the comet are in direct association, even if we cannot distinguish that one is the cause, and the other the effect.

On these grounds it appears to be perfectly certain that the origin of the shooting stars which appear in swarms cannot be dissociated from the origin of the comets by which those swarms are accompanied. This fact seems to lead to a demonstration of the important truth that meteorites have no affinity whatever with the ordinary shooting-stars. Whatever may be the nature of the belief entertained with regard to comets in other respects, it seems quite impossible to regard these bodies as composed of materials in anything like the same forms as those which we find in a meteorite. No doubt the mere elements present in comets may be much v the same as those from which meteorites are constituted. In illustration of this I need not do more than refer to the single element iron. It seems to have been demonstrated that iron is a constituent of certain great comets. This same element is of course a leading component of meteorites, and it might therefore be contended that to this extent there was an affinity between the two different classes of bodies. It may also be added that sodium has been found both in comets and in meteorites, while a still more striking instance is presented in the case of the element carbon. This remarkable substance often seems to be one of the principal constituents in a comet, in so far at least as the spectra of those bodies may be regarded as indications of the proportions in which the different elements have united to form it. A curious discovery with regard to the composition of meteorites has been the detection of graphite, as well as of carbon in other forms. Indeed, it may be noted as an interesting circumstance that M. Moisson, in his recent investigations of a meteorite from the Canon Diablo, detected the presence of minute particles of carbon which possessed the hardness of diamonds.

But while we admit all this it seems that the evidence on the other side is far too strong to permit us to regard comets, and meteorites, as derived from the same source, or as standing to each other in any particular relation. It would be easy to over-estimate the significance of whatever argument may be derived from the similarity in ultimate chemical composition. It must be remembered that one of the most remarkable results of spectroscopic analysis has been to suggest the practical identity as to ultimate composition of the different bodies of the solar system. There is no good reason to believe, so far as spectroscopic evidence is concerned, that there are any considerable number of elements present in the sun, in addition to those elementary bodies with which terrestrial chemistry has made us acquainted. There is no doubt a mysterious indication of some possible element of an extremely light description in the solar corona, for which the name of "coronium" has been suggested; and there is also some element known as "helium" which is believed to be found in the prominences. But the existence of the bodies so designated is still undemonstrated; and even if it were, the presence of the bodies of such a character would not affect our present argument.

It is therefore not surprising that the elementary bodies which have been discovered in the meteorites should resemble those already known on the earth. Nor need it be a matter for astonishment if the materials found in comets should resemble those found elsewhere throughout the solar system. I therefore think that we are warranted in refusing to draw any inference from the fact that iron is present, both in some meteorites and in some comets, with reference to the presumed relationship between these two classes of celestial objects. They both have iron and carbon simply because they both belong to the solar system, where iron and carbon are elements which appear to be widely distributed. No one will be likely to doubt that iron and carbon are both present in the planet Mars, although it is true that, from the nature of the case, direct spectroscopic evidence cannot be cited on the matter.

Fig. 35.—The large Youndegin Meteorite. 4 feet 2 inches long, 2 feet 3 inches wide, 20 inches thick,
Weight 2,044 lbs. (181/4 cwt).

Nor will it be questioned that iron and carbon are both presumable constituents in the mighty mass of Jupiter, and yet no one will, on that account, claim that meteorites have been derived from that planet.

The identity of the actual materials in the varied bodies of the solar system is so striking, that it would now seem wonderful if any object which undoubtedly belonged to that system contained materials to any considerable extent not otherwise known to us from their presence in the earth. It appears that there is, therefore, no force in the argument which would connect meteorites with comets, merely because iron and some other substances have been found common to both. On the other hand, the very form in which the iron is found in meteorites, and the condition in which iron must exist in bodies possessing the nature of comets, seems to afford conclusive evidence that the origin of meteorites, and the origin of comets, must be sought for in widely different directions.

Everything we know with regard to the structure or texture of a comet, seems to demonstrate that it is not in the least likely to contain solid masses of which meteorites might be fragments. Take, for instance, the ordinary telescopic comet of which some half-dozen or so come to visit our system every year. Such a body is a light volume of gas or vapour far less dense than the lightest cloud that ever floated in a summer sky. Indeed, it is well known that, as such a comet in its progress across the heavens passes between us and the stars, those stars are often seen twinkling brilliantly right through the many thousands of miles of cometary matter, which their rays have to traverse, The lightest haze in our atmosphere would suffice to extinguish the faint gleam of these small stars; indeed a few feet of mist would have more power of obstructing the stellar light than cometary material scores of thousands of miles thick.

It is true that the central portions of many of these comets often exhibit much greater density than is found in the exterior regions. Still, in the great majority of such objects, there is no opacity even in the densest part sufficient to put out a bright star. In the case of the more splendid bodies of this description, it may be supposed that the matter is somewhat more densely aggregated as well as more voluminous. Still, however, it will be remembered that the great comet of 1858 passed over Arcturus, and that the star was seen shining brilliantly notwithstanding the interposition of a cometary curtain millions of miles in thickness. So far as I know, no case is known in which the nucleus of a really bright and great comet has been witnessed in the act of passage over a considerable star. It would indeed be extremely interesting to ascertain whether in such a case the star experienced any considerable diminution in its lustre.

There is a delicate method of testing the quantity of matter which a comet might possess in the vicinity of its nucleus. If there were any substantial quantity of gaseous matter, it is plain that though the rays from the star might not be altogether extinguished, or might not even be largely reduced in lustre, yet the apparent direction of the star must be deflected from its course by refraction through the comet. It is well known that when a telescope is used to observe a star, it is not generally pointed exactly in the direction of the star, because the light which it is sought to observe has been bent in, its passage through the atmosphere surrounding the earth, so that the direction in which the light enters the telescope is somewhat different from the direction which the light had when it first encountered the atmosphere at an altitude of some hundreds of miles. The amount of this change in the direction of a ray by refraction is by no means inconsiderable, and the practical astronomer has always to allow for it. At sunset, for example, the light from the departing orb is bent to such an extent, that at the time when the sun has really sunk below the horizon it still appears to be above, on account of the curvature in the rays produced by atmospheric refraction. If, therefore, the light from a star had to traverse in its passage through the comet any quantity of vapour at all comparable in refractive power to the earth's atmosphere, that light would be deflected to an extent which could not be overlooked in the refined methods of modern astronomy.

Here then we have a delicate means for investigating the quantity of refractive matter in a comet. The observations are conducted in this manner. Two neighbouring stars are selected which are so placed with regard to the movements of the comet that it passes over one of the stars while leaving the other uncovered. Suppose that the apparent distance between the two stars upon the sky be measured, before the comet has come into their immediate neighbourhood. The measurement is to be repeated while one of the stars is behind the comet, and a third and concluding observation is to be made after the comet has passed on its way and left both stars behind. We have thus obtained the necessary materials for the investigation. If the body possessed any appreciable refractive power, then the apparent distance of the two stars would be different in the middle observation of the three, from that which it was both in the first and in the third. Every practical astronomer knows this is a research which admits of being made with great precision. The angular distance of the two star-like points is an element which the micrometer will indicate accurately to the fraction of a second, and if there were a displacement amounting to one hundredth part of that which corresponds to the horizontal refraction of our own atmosphere, the refractive capacity of the comet would be quite unmistakable.

Fig. 36. Crystals of Olivine, embedded in an iron meteorite. Found
in the Desert of Atacama, South America.

By observations of this class it has been shown that there is little or no appreciable refractive power in one of these vast bodies. These results demonstrate conclusively that the quantity of matter even in a comet is extremely small when compared with its bulk.

The conclusion thus arrived at is confirmed by the fact that our efforts to obtain the weight of a comet have hitherto proved unsuccessful. We have the means of measuring the weight of a planet, by the disturbances of other bodies which it can affect, and if a comet were massive enough to produce disturbances in the planetary movements there would be no difficulty in discovering within certain limits what the cometary mass might amount to. There have been several instances in which a comet has approached so close to a planet, that the attraction between the two bodies must have had significant influence on the planet, if the cometary mass had been at all comparable with that of the more robust body. The most celebrated instance is presented in the case of Lexell's comet which happened to cross the track of Jupiter. The effect upon this body was so overwhelming that it was wrenched from its original path, and started afresh along a wholly different track. The reaction of the comet upon the planet seems, however, to have been incapable of influencing by any measurable quantity the movements of the giant globe. It is, therefore, obvious that the mass of this comet, and it was a large one of its class, was inappreciable in comparison with the mass of Jupiter.

But the rencontre between the two bodies supplies us with an argument of a still more cogent type. The retinue of moons by which Jupiter is attended forms a delicately organized system. Their movements have been observed for centuries, and any derangement introduced into the group by the approach of a considerable foreign body would be rendered manifest by the disturbance of the little moons from the paths which they had so long traversed. Careful attention was directed to this point in order to see whether, after the collision between Jupiter and Lexell's comet, the satellites offered any indications of the vicissitudes through which they had gone. The evidence on this point was entirely negative. It was not possible to discover any irregularity in the movements of the bodies which could be attributed to the attraction of the comet. It has thus been demonstrated that, notwithstanding the stupendous bulk of a great comet, its mass must have been so inconsiderable as to have been insufficient to disturb even such unimportant members of the solar system as the satellites of Jupiter.

These different lines of reasoning convince us that comets contain no appreciable portion of actual solid material. But meteorites are shown from their structure to be fragments rent from some mighty mass which has cooled but slowly from a highly heated state. They resemble certain volcanic products so closely that it seems quite impossible to refuse assent to the doctrine of Tschermak that, whatever be their source, the materials must have come from gigantic masses, not greatly varying in dimensions from the earth or other solid planets of our system. But we have seen that comets are in every respect different from bodies possessing the characteristics of objects from which meteorites can have been derived. I am, therefore, forced to the conclusion that meteorites and comets can have no connection, except what may be implied in the circumstance that they all belong to the solar system.

I am quite aware that this view is very different from that which is entertained by distinguished astronomical authorities. For instance, in connection with Sir Norman Lockyer's spectroscopic work, he has been led to frame his meteoric hypothesis in which a comet is represented as containing a cloud of isolated meteorites. For the reasons already given I am unable to assent to this view. The structure of a meteorite seems to be wholly incompatible with the supposition that it had any other origin than as a fragment of some vast mass slowly cooled.

Another distinguished authority, Professor Newton, of Yale College, to whose labours we are so largely indebted for our knowledge of shooting - stars, also considers that a link of connection between meteorites and comets has been established. Indeed, I am aware that this belief is very widely entertained. A specimen of a meteorite has been exhibited in a museum, bearing a label with the words, "a bit of a comet," On the other hand, Professor Lawrence Smith may be mentioned as one distinguished student of meteoric matters who has accepted the view which I have here adopted, namely, that meteorites have no closer connection with ordinary periodic showers or with comets, than they have with Mars, or with Jupiter, or with the sun itself.

It will be remembered that in the early part of this chapter I have insisted on the connection between comets and shooting - star showers, which has, I believe, been abundantly demonstrated. But meteorites seem to be bodies of a radically different character from those meteors which arrive in periodic showers. Meteorites are to be explained on quite different principles; their origin is to be sought in quite different sources. In fact, the only common bond between objects of such widely different characters is expressed by the fact that they each come into the atmosphere from outside.

If I may say so without offence, it would seem that the logic of the reasoning which connects meteorites with comets is not wholly satisfactory. Some of the arguments which have been brought forward by those who maintain the affinity of meteorites to shooting-star showers appear to be derived from the two following premises. Shooting-star showers come into the air from outside. Meteorites come into the air from outside. But the premises, though both unquestioned, do not admit of our drawing any conclusions as to the affinity of meteorites and shooting-star showers. It is perfectly certain that periodic showers such as the Leonids, or the Perseids, or the Orionids, or the Geminids, or any of the other similar showers, are all cosmical systems possessing distinct affinities to comets. Their origin cannot be discussed separately from the origin of comets. Whence the comets have come, thence these meteors have most probably come, and where that may have been is a question into which I do not now enter. But, besides these bodies, there is another class of objects to which the meteorites belong, which come into our atmosphere, no doubt, but which seem to have no connection with comets. Doubtless there are many of the so-called shooting-stars, or so-called fire-balls, which it would be impossible, with our present knowledge, to assign with certainty to their proper classes. All I am now insisting on is that there are at least two classes of these objects, one of which includes those of cometary affinity, and the other includes the non-cometary objects. It is to the latter that the meteorites belong. This is proved emphatically by their structure, which is incompatible with the supposition that the body possessing it has originated in a comet.

I would specially commend the important researches of Tschermak to those interested in meteorites. In the museum at Vienna there is a collection of meteorites rivalling our own splendid collection in the Natural History Museum at South Kensington. The eminent Austrian Professor has made an elaborate study of the different bodies of this class in the various museums, and he has come to the conclusion that meteorites have been derived from volcanoes on some large celestial body. So far as I am aware, no other mineralogist has maintained an opinion of an opposite character to that entertained by Tschermak. We may, therefore, inquire where the volcano must have been situated so that the missiles it projects should tumble down on our earth. Placed in this aspect the problem is no longer one for geologists or of mineralogists. It has now come within the province of astronomers and mathematicians. It is for them to say where, in all probability, those volcanoes have been situated from which the meteorites have come. This is indeed a very interesting question, and I propose to undertake its solution.

The missile from a cannon discharged vertically upwards will fall back to earth with a speed nearly as great as that with which it was projected. In fact, the speed at the return would be quite as great as the speed at the departure were it not for the resistance of the atmosphere. Under the actual circumstances of this globe, and with the actual strength of our artillery, and the potency that any available explosives may possess, we are not able to project a missile with a speed sufficiently great to carry it to a height which would even be an appreciable fraction of the earth's diameter. Our globe is so massive that any velocity which we were able to impart to the upward movement of the bullet would not suffice to carry it to an altitude that bears the same relation to the diameter of the earth as the thickness of an egg shell bears to the diameter of the egg.

It is, however, interesting to consider the circumstances under which a missile would take flight if projected from a globe differing widely from our earth in bulk or mass or physical constitution. Let us first of all suppose that a piece of artillery was to be transferred to some globe much more massive than the earth. Take, for example, some globe possessing the same mass as the sun, and of like dimensions. Under these circumstances, the attraction by which the speed of the ascending missile would be gradually lessened is much more effective than the corresponding force upon the earth. It follows that, even though the missile might leave the mouth of the cannon with the same pace on the large globe that it had on the small one, yet the upward velocity would be abated much more quickly when the heavier mass was underneath, than when it was only the attraction of the smaller of the two globes that was checking the ascent. From the big globe the projectile could not ascend to anything like the altitude which it would be able to attain on the small one, and the time that would be occupied in its flight would undergo a corresponding diminution. Thus, although a projectile might be discharged by a piece of our modern artillery, with a speed sufficient to carry it to an elevation of several miles, yet a like velocity of projection from the surface of a globe as large as many of the globes in space would only suffice to carry the body to a height of one mile, or even less. It is, therefore, understood that the elevation to which the missile is capable of soaring depends not alone on the efficiency of the cannon from which it has been projected, but on the mass of the globe on which that cannon is placed.

Nor is it indeed only the mass of the globe which is concerned in the matter. It will easily be seen that the diameter of the body must enter as a significant element. Suppose that there were two globes equal in mass, but that in one the materials were of a lighter specific gravity than in the other, and that consequently the globes were of unequal dimensions. Then the attractions of the two globes exercised upon bodies on their respective surfaces would be very different. In the one case the attracted object would be further away from the centre of the globe than in the other, and though the masses of the globes may have been the same yet as the attraction varies inversely as the square of the distance, it would be less on the surface of the greater body than it is on the smaller.

To give an illustration of what I mean, let us suppose the case of two globes equal in weight, but one of which was made of platinum and the other of granite. Platinum is nearly eight times as heavy as granite, it therefore follows that as the globes are of equal weight, the granite globe must be about eight times as bulky as the metallic globe, and this being so it can easily be shown that the radius of the larger globe must be double that of the smaller. There would thus be considerable difference in the gravitation which would be experienced by the inhabitants of the granite and of the platinum globe respectively. For though the masses of those globes, that is, the quantity of matter they possess, would be the same, yet a denizen of the globe of stone would be twice as far away from the centre of his world as a denizen of the globe of metal would be from the centre of his. So that though the attractions exerted by the two globes at equal distances from their centres would be identical, yet, owing to the law of the inverse square, the inhabitant of the big globe would feel only one-fourth the attraction experienced by the occupant of the small one. It is thus plain that a piece of artillery placed on one of these globes would launch forth its missile under very different conditions from those which would determine the movement if the projection were made from the other globe. As the body left the muzzle the force striving to draw it back to the metallic globe would be no less than four times as great as that which would commence to operate when the missile left the muzzle of the gun on the stone globe. The consequence is that in the latter case the body would ascend to a far greater elevation before its speed was checked and reversed than when it was shot from the platinum world. From this illustration it will be plain that it is necessary to take into account both the mass of a globe and its dimensions when we would determine the height to which a projectile can ascend.

Let us consider the case of a great cannon placed on a globe of comparatively small mass, the density of that globe being about the same as the average density of the worlds which we find in the solar system. It is plain that as the object goes aloft a lessened attractive force will be exerted to check its upward movement, and consequently the speed will but slowly abate. It, therefore, follows that this force must be put forth for a long time before the upward movement is entirely neutralised, and during this long time the body will have attained a correspondingly high elevation.

It can, indeed, be demonstrated that there is a critical velocity for every globe, such that if a body be projected upwards with that critical velocity or any greater one, it will be permitted to escape altogether.

Fig. 37.—Critical velocities in miles per second on the sun and the several planets.

When we know the mass of the globe as well as its radius, we are enabled to calculate what this critical velocity has to be. To take, for instance, the case of the earth. It can be shown that for any globe possessing the weight of the earth, and the exact size of the earth, the velocity with which a missile would have to be shot upwards so as to effect its escape must be about seven miles a second. If the velocity with which the body started on its vertical ascent were less than seven miles a second, then after reaching a height dependent upon the speed of projection it will commence to return. If, however, the pace be as much as or more than the critical value of seven miles a second, the body will continue its journey, and the attraction of the earth will not be sufficient to recall it.

This point is of so much importance that I am inclined to dwell on it a little longer in hope of removing the paradox which it may seem to involve. For as in the ascent of the body the velocity must ever be lessened by the attractive force pulling it back, it might seem obvious that the initial velocity must be ultimately overcome if only long enough time be granted. But this is not a valid objection. It can be shown that the difficulty is something like that which arises in the well-known case of a geometric series. If we add a half to a quarter, and that to an eighth and that to a sixteenth, and that to a thirty-second, and so on indefinitely, we shall make an infinite number of additions, and yet, though the number of the quantities added together may be infinite, the total that they produce can never be more than finite. This can be more simply seen by a very elementary illustration. If you eat half a cake to-day, and a quarter of it to-morrow, and an eighth the next day, and a sixteenth the day after, and a thirty-second the next, and so on, ever consuming one day exactly half what was left the day before, the cake will never entirely disappear. It would, in fact, last for ever, notwithstanding the fact that an infinitely great number of portions had been abstracted from it. Somewhat similar is the nature of the operation by which the attraction of the earth gradually reduces the ascending velocity of the moving body. All that the earth can effect by attraction is to reduce the pace by the extent of seven miles a second. If the original velocity exceeds seven miles a second, even by the smallest amount, then the attraction of the earth will never be able to overcome it entirely. The projectile would consequently travel outwards into space, never to return to the globe from which it started, unless it should happen that other agents not hitherto contemplated should be brought into operation.

It is known that the energy possessed by a flying bullet is proportional to the square of the velocity with which it is animated. Thus if one of two equal bullets should have a speed double as great as that of the other, then the energy possessed by the more rapidly moving of the two, in consequence of its velocity, will be four times as great as that possessed by the more slowly moving body. It thus appears that a projectile launched with a velocity twelve times as great as that which can be generated by our artillery must possess a quantity of energy one hundred and forty-four times greater than would be acquired by a bullet of equal size fired from a Woolwich gun. We would, therefore, need gunpowder one hundred and forty-four times as potent as any gunpowder we now possess if we expected to shoot a cannon ball away from the earth altogether. Need it be added that the resisting power of the cannon would have to be enormously increased to withstand the stress that such an explosive would generate. In fact, it is perfectly certain that no materials known to us would be strong enough to form a weapon sufficiently tough for the purpose.

If, however, we were performing our experiments on some of the smaller globes belonging to the solar system, then a speed considerably less than that of seven miles a second would suffice to discharge the missile finally from the vicinity of the globe on which it was placed. A cannon, for instance, which was strong enough to impart to the projectiles that issued from it a velocity double as great as that which our missiles receive here, would send the body entirely away from the moon. On globes still smaller a speed correspondingly less would suffice. Thus in the case of some of the minor planets where the diameters are not expressed in thousands, but only in hundreds of miles, the necessary projectile force would be quite within the range of modern artillery. Indeed, on some of the still smaller globes like the satellites of Mars, where the diameter is a score of miles or less, it is quite likely that velocities comparable with those with which a cricket ball is thrown would suffice to transcend the critical amount. The arm of a vigorous cricketer would cause the ball to ascend with a speed sufficient to make it go on further and further until at last it gradually sank away into the heavens never more to tumble back again to the globe from which its way was sped.

Provided with these considerations, let us now seek to determine the source of meteorites from an astronomical point of view. It is plain that if these objects had been shot as bombs from volcanoes, those volcanoes must have had a potency sufficient to discharge fragments of the solid crust of the globe aloft with such speed that they did not presently return in consequence of that globe's attraction. The great difficulty which we encounter in the consideration of this question arises from the high velocity which would be necessary to set the missiles free from the obligation of immediate return to the parent globe. As the speed of seven miles a second is undoubtedly far in excess of any speed which is attained in terrestrial volcanoes nowadays, we are tempted to look at some of the smaller globes of the solar system and see whether they can possibly be the abodes of the great volcanoes by whose explosions meteorites have been emitted. We might, at all events, hope by such a supposition to get rid of the initial difficulty with regard to the high speed of projection, for as a low velocity will suffice to carry bodies quite clear of a small globe, it follows that we should not in such a case demand an exceptionally potent volcano.

It has, indeed, been maintained, by at least one distinguished astronomer, that in all probability meteorites are bodies which have taken their departure from volcanoes in the moon. No doubt, so far as the initial velocity is concerned, this supposition would obviate the fundamental difficulty. In the first place, about a fifth of that velocity would be required which would be necessary to discharge a missile with the critical speed from the earth. We know, also, that there is abundant testimony as to the former existence of volcanoes in the moon. That side of our satellite which alone is visible from the earth, is marked over with hundreds of craters, indicating the intense volcanic activity which once reigned there. There can hardly be any doubt that their efficiency may have been ample to impart to the missile a speed sufficiently great. Indeed, the circumstances which we know with regard to Krakatoa seem to show that some of the bombs launched from this volcano in the memorable eruption, issued from the mouth of the crater with a velocity which can hardly have fallen short of a mile a second. In other words, if a volcano like Krakatoa were to break out to-day on the surface of the moon, and if it were to discharge its missiles upwards with velocities comparable with those
Fig. 38.—Slab of Crystallised Iron cut from the Aerolite of Lenarto, Hungary.

actually observed in the Straits of Sunda in 1883, it is not at all unlikely that many bodies would be shot away from the moon altogether.

So far, however, as the descent of meteorites to the surface of this earth is concerned, it is not alone sufficient to inquire whether these bodies can have left the parent world; it is further necessary to examine the circumstances under which a missile projected from some other globe shall tumble down on this one. This is a point that has been sometimes not sufficiently attended to by those who have considered the matter. It has been thought that as the volcanoes on the moon have been in all probability potent enough formerly, it is therefore reasonable enough to look to them as the source of meteorites. But a little further examination will show that though missiles might undoubtedly have left the moon owing to projection from lunar volcanoes, yet, that it would be in the highest degree improbable that any of these objects should now be from time to time descending to the earth. For suppose that a meteorite is shot away from the moon, it presently comes so completely under the attraction of the earth that it must revolve about it in accordance with the laws imposed by the earth's gravitation. The question, therefore, as to whether it is to fall on the earth or not, is simply decided by whether the distance of the little body when at perigee would be less than the radius of the earth. If this were so, then of course the body would strike the earth at its first revolution. If, however, the circumstances under which the body was projected were such that its movement did not bring it into collision with the earth at the first approach, then it would remain as a permanent satellite to our globe. It could never then fall to the earth except under the possible action of such disturbing forces as are generated by the attraction of the sun or the moon. No doubt it is conceivable that under the influence of these disturbing forces it might happen, in extremely rare instances, that a missile, after wandering for ages round the earth in an elliptic orbit, should at last be so deflected from its original path as to strike against the earth's surface. But a little attention will show that such an occurrence must be of a rarity so extraordinary, that it may be dismissed from consideration as the probable source of meteorites, at all events when a more rational explanation is at hand.

The question as to the lunar origin of meteorites has thus become narrowed down to a simple issue. Are there at present any active volcanoes on our satellite? If there are, then it might be quite conceivable that the meteorites which now arrive here left the moon some few days previously. If, however, the moon's volcanoes are all extinct, it is then excessively unlikely that any of the meteorites that are now falling could have been derived from this source. The lunar origin of the meteorites must therefore he rejected, if it can be shown that the lunar volcanoes are all now extinct.

At the time when it was believed that the moon might be the source of meteorites, there was thought to be good reason for the supposition that some of the lunar volcanoes retained their igneous energy. Modern research has, however, demonstrated that the lunar volcanoes are absolutely silent and ineffective. No doubt some slight indications of change have been detected, in certain details, on the lunar surface, but I do not think that, even if we admit every case of change which has been alleged by recent observers, it could be contended that any one of the lunar volcanoes now possesses the necessary activity. We are, therefore, forced to discard the lunar theory of meteorites altogether, for the simple reason that if the moon ever did project meteoritic masses, they must have tumbled down on the earth ages ago, at the time when the lunar volcanoes were still active. We do not, therefore, look for any lunar explanation of the meteorites which fall down here in these modern days, when the volcanoes in our satellite have become extinct. Let us, therefore, go further a-field, and search for the possible origins of these bodies in the volcanoes of other worlds.

It will be convenient, at this point, to lay down the principle by which we shall be guided in determining the critical velocity which would be necessary to project bodies away from any particular globe. It can be demonstrated by mathematical principles, on which it is not necessary now to enter, that the critical velocity for each globe will be directly proportional to the square root of its mass, and inversely proportional to the square root of its radius. It can hence be easily shown that, supposing a number of globes are all made from the same materials, but are of different sizes, the critical velocity with which a body will have to be projected upwards varies simply as the radius of the globe. Of course, the condition supposed does not apply exactly to the various heavenly bodies, and, consequently, it would not be correct to assume that the law is quite so simple as that here stated. But for the purpose of our illustration we may so regard it, and this being so, let us consider a globe which has a diameter of 650 miles.

Such a body would be large enough for one of the greater of the minor planets, as large, perhaps, as the planet Ceres. As this is about one-twelfth part of the diameter of the earth it will follow, from the principle we have already laid down, that the speed with which a missile would have to be projected from Ceres, in order to carry it away from that globe altogether, would have to be one-twelfth of that which would be necessary to carry it away from the earth. As already stated, this is a speed quite comparable with that attained by our modern artillery; it therefore follows that if Ceres were 650 miles in diameter, and it must be of dimensions not very greatly differing from this amount, one of our great cannons, pointed vertically on this particular globe, would discharge its missile so that it would not return. It might therefore seem that by locating the volcano on one of the minor planets, a way is offered of getting out of the great difficulty, with regard to the tremendous volcanic power necessary to impart the acquired velocity to the missile. Quite a moderate volcano placed on such a globe would undoubtedly shoot bodies upwards that would not return. But here, again, we have to remember that before such a missile could descend to our earth as a meteorite, it is necessary for the circumstances of projection to be such that the body shall take a direction which will ultimately cause it to strike the earth. The conditions that this implies are of very great importance. It will be necessary to consider them.

If a meteorite projected from a volcano on Ceres is ever to strike the earth it must, it need hardly be said, pass through that narrow strip in the ecliptic, some eight thousand miles wide, which the earth traces out in its annual movements. I say narrow strip, for although it may seem that eight thousand miles is a considerable width, it must be remembered that on the scale on which the orbit has to be drawn the width named would only correspond to an extremely fine line. The projectile from the planet, as it quits the neighbourhood of the parent globe, becomes appreciably affected by the attraction of the sun, and as its distance from the planet increases, the attraction of that planet dwindles to evanescence, while the attraction of the sun becomes the predominating influence by which the movement is guided. The projectile accordingly pursues a track in accordance with the known laws of planetary motion. We are therefore to think of the little body as revolving around the sun in the same manner as the planet itself revolves, only possibly with an orbit considerably more eccentric, and inclined at a much larger angle to the ecliptic than that at which orbits are generally placed. If, therefore, this little body is to fall on our earth as a meteorite, it is obviously essential that its orbit shall cross the track followed by the earth. Unless this condition is fulfilled the potential meteorite may pass near the earth on one side or the other, but would not fall down thereon, and we should know nothing about it. We have, therefore, to consider the conditions under which the orbit of the missile shall possess the very fundamental character of crossing the earth's track.

It can be demonstrated by mathematical calculation, as to which there can be no uncertainty, that it would be impossible for a missile projected from the planet Ceres to cross the present track of our earth around the sun, unless at the instant of projection the missile had a velocity, of which the component perpendicular to the radius of Ceres' orbit was about eight miles a second. The actual velocity with which the little body would start on its journey depends partly on the speed with which it was projected, and partly on the speed which the planet has in its orbit. In fact, as a mathematician would say, the velocity with which the object was animated would be the resultant between the velocity of the planet and the velocity imparted by the projecting agent. The velocity with which Ceres moves round in its path is determined by Kepler's law, and it can easily be shown to be about eleven miles a second. It therefore follows from this circumstance alone that the missile will have a speed of eleven miles a second perpendicular to the radius of the orbit of Ceres. The velocity which it receives from the projective force must be compounded with that which it derives from the movement of the planet. We have already seen that it would be utterly impossible for the meteorite to reach the earth if the component of its velocity in the direction of the planet's movement differed from eight miles per second.

Fig. 39.—The Middlesborough Meteorite, showing corrugations of melted matter. March 14, 1881.

This consideration shows that the volcano on Ceres would have to be possessed of very considerable power, quite independently of whatever projective force might be necessary for the mere purpose of conveying the missile clear from the planet.

Let us suppose the most favourable case possible. In other words, let us try to conceive the circumstances under which, with the least expenditure of power, a projectile might be launched from Ceres under such conditions that it should cross the earth's track. It is obvious that this most favourable condition would be presented in the case of a volcano which was so placed on the planet as to lie exactly on the opposite side of the little globe, from that point which was foremost in its motion. For what the volcano has now to do is to abate the velocity which the missile possesses in virtue of the planetary movement, which it possesses in common with every other part of its globe. It follows that the velocity which must be imparted by the explosive power of the volcano, has to be at least three miles a second. For as the little object has a velocity of eleven miles a second in the direction perpendicular to the radius of the planet's movement, it is necessary to reduce this by three miles a second, in order to bring the actual velocity of the planet to the eight miles a second, which we have already stated to be an indispensable requirement if the object is to arrive at the earth as a meteorite. It can be easily shown that a volcano which happened to lie in any other situation than that just mentioned would have to impart an initial speed of more than three miles a second, if it were to reduce the velocity that the meteorite acquires from the planet down to the amount under which alone it would be possible for it to fall on the earth. It thus appears from the consideration of the orbit of Ceres, and of the orbit of the earth, that a velocity of three miles a second would be demanded by dynamical considerations quite independently of whatever additional speed the missile, should receive, in order to carry it free from the attraction of the globe on which the projective agent was situated. No doubt, as Ceres is small, this last might be, as we have said above, a velocity of moderate dimensions, attainable, in all probability, by ordinary artillery. But the velocity which has to be imparted on the other account is so considerable that no matter how small the mass of Ceres may be, a volcano of a projective power of at least three miles a second would be demanded. We thus see that there is no alleviation of the difficulty gained by locating the volcano on one of the minor planets.

Quite independently of this there is a line of reasoning which demonstrates that in all probability meteorites could not have come from any planet situated where Ceres is. It must not be forgotten that the track which the earth pursues in its annual progress round the sun is only an extremely fine line when viewed from the distance of Ceres. This consideration shows that it is only under exceptional circumstances that a meteorite projected from Ceres, even if it had speed enough, should ever tumble on our globe. The question is one in the theory of probabilities. In another part of this volume I have illustrated the importance which the theory of probabilities has for the astronomer. To the cases which are there given I may now add that which is connected with our present argument.

Let us imagine that Ceres was covered with volcanoes; suppose that these volcanoes were from time to time projecting clouds of missiles with sufficient vehemence to set them finally free from the globe from which they spring. It is not easy to state the question simply, but we must make the attempt. I shall suppose that the speed which the missile receives from the volcano is compounded with that derived from the orbital movement of the planet. We have already seen that if this total speed is less than eight miles a second, then no matter what the direction of the movement of the projectile may be, it must fall short of the earth's track, and can therefore never possibly reach our globe. If, on the other hand, the volcano on Ceres were so powerful that the speed it imparted, when combined with that which the missile derives from the orbital movement, exceeded sixteen miles a second, then the path in which the body starts on its voyage through space would take the form of a hyperbola. In this case, although the missile might cross the earth's track once, it would never do so again, for the attraction of the sun would not be sufficient to recall it. Should the total speed of projection lie between eight miles and sixteen miles a second, then the orbit would be elliptical, and the body would move round and round with the same regularity as a planet. But among all the different possible orbits of this kind comparatively few will actually intersect the earth's track.

To take an illustration, let us suppose the case of those missiles which start with a total speed intermediate between the two extremes we have just named. Let us imagine that they have a velocity of twelve miles per second; it can be demonstrated that projectiles launched forth at the speed just named, but in all directions, will assume all sorts of orbits, and of these orbits only one out of a very large number can intersect the track of the earth even when due allowance has been made for the effect of perturbation. It therefore follows that out of all the missiles projected from Ceres, only very few could be expected to reach the earth, even after the lapse of an indefinitely great time.

This examination of the conditions under which bodies projected from Ceres could fall to the earth as meteorites, has shown that such a source for these bodies is highly improbable. In the first place, it has been demonstrated that the immediate object sought to be gained by locating the volcano on a small planet would not be realised, for a very high velocity would be necessary on account of the circumstances of the situations of the orbits in the solar system. We should, therefore, in any case need volcanoes with tremendous power even if placed on so small a globe as Ceres. It is further shown, that even if this highly improbable condition could be fulfilled, the volcanoes on Ceres would be so badly adjusted for the work to be done that they would miss a large number of shots for every one that was successful. These improbabilities are so great that we are forced to reject the hypothesis which implies them, especially when, as we shall see in the next chapter, we can point out a locality for the volcanoes to which no such improbability attaches. I need not go into details with regard to the other planets. Setting aside all other objections the large ones would require tremendous volcanoes to drive the missiles free from the attraction of their globes; and there is besides the further circumstance, that there will be in every case an enormous preponderance of missiles which can never pass through the earth's track, over those which may happen to do so.

But before I come to discuss the real source of these bodies it may be well to consider the possibility that meteorites should have been projected from bodies in space which do not belong to the solar system. This is indeed a favourite notion with some, but here again as elsewhere through astronomy the laws of probability afford a reliable guide.

Let us briefly consider the conditions under which a meteorite projected from some volcano located in the stellar spaces would actually pass through the earth's track. No doubt there are scores of millions of stars, and though we cannot see them, there are in all probability thousands of millions of dark globes which, in so far as their non-luminous character is concerned, clearly resemble the earth. It is not improbable that thousands of these globes, or millions of them, may have volcanoes quite as potent, or far more potent than any volcanoes which have ever come within our experience. But even if there were millions of volcanoes or bodies in the stellar space, and even if those volcanoes were powerful enough to discharge missiles which would soar free from their parent globes, the probabilities against the arrival of any such objects on this earth are indeed stupendous. I find it wholly impossible to believe that such can have been the source of meteorites, at all events so long as I can discover another source to which so great a degree of improbability does not attach.

For suppose that a volcano were located on some body lying at the distance of the nearest fixed star, which is believed to be Alpha Centauri. An observer placed in that remote locality and viewing the solar system from the awful distance which intervenes, would see the earth describe a little circle, about a second and a half in diameter. This is extremely small, it is about as large as a penny piece would look if placed three miles from the observer. The number of such circles, whose collective areas would be required to cover the sky, would be about 500,000,000,000. Now what would be the chance that a rifle bullet, supposing it could carry far enough, directed perfectly at random, would strike a bull's eye the size of a penny piece at a distance of three miles?

It is obvious that the solution to this is to be found in the following manner. Suppose a sphere to be constructed with a radius of three miles, and that the whole inside of this sphere be divided into a mosaic each with an area as large as a penny piece; then as each one of those pieces would be just as likely as any other to be struck by a bullet discharged absolutely at random, the improbability that any particular piece would be hit will be expressed by unity divided by their entire number. A volcano placed at the distance of Alpha Centauri, and discharging missiles quite at random, could only hit that ring which represents the earth's track in one shot out of every five hundred thousand millions. But even these figures do not express the improbability that a meteorite should arise in this manner. For if the missile happened to pass through the interior of the earth's orbit, it would not fall on the earth any more than if it had passed outside the orbit altogether. It is, as we have already explained, an indispensable condition that the body should pierce that particular zone eight thousand miles in width which marks the track of the earth in the ecliptic. As the area of this ring is not the five- thousandth part of the whole area of its orbit, it follows that to pierce the ring by any one missile a number must on the average be projected, which is five thousand times as great as that already named.

We thus see that the improbability that a body shot from a volcano situated on a globe at the same distance as Alpha Centauri, should ever fall on the earth as a meteorite, must be twenty-five hundred of millions of millions to one. Surely this presents in a very forcible light the extreme improbability that meteorites should have been derived in the way this doctrine suggests. Of course if the volcano were so much further off as to have a distance comparable with that of most of the stars whose distance is known, then the improbability would be still more enhanced. But taking the figures as they stand, it would appear that even if there were at least two thousand million volcanoes, launching forth missiles into space, not more than one out of every million bodies thus projected could ever cross the earth's track, and thus conceivably reach the earth as a meteorite.

It will also be observed that in this calculation I have regarded the earth's track as placed squarely to the line of fire. If it were more or less edgewise, as would of course generally be the case, then the length of the projected track would be correspondingly reduced, and the improbability would be correspondingly increased. From all these considerations I have come to the conclusion that we may reject any hypothesis which would ask us to derive the meteorites from volcanic sources in the stellar spaces. The actual source from which they seem to have come will be considered in the next chapter.