EARTH-BORN METEORITES.
shooting-stars must be one which will give an account of cometic phenomena also. And this, to some degree, limits us in our speculations respecting these bodies. But the case is other- wise, or we may more correctly say, some men of science hold that the case is otherwise, with the bodies called Meteorites. Tschermak, in an interesting memoir on meteorites, rejects, in- deed, as untenable the belief that these bodies have any con- nection with shooting-stars. On his arguments respecting meteorites has been based one of the theories now to be de- scribed. He points out that meteorites are always angular fragments, even before they reach our air ; that many of them have a crystalline structure, which requires a very long period of formation, at a nearly constant temperature,—a condition only fulfilled in a large mass ; that in others, many fragments are welded together, as in breccia; and lastly, that in other cases, meteoric stones are composed of very small particles, analogous to volcanic tufas. Hence Tschermak arrives at the conclusion that meteorites have had their origin in volcanic eruptions, and he considers that they must have been expelled from planets so small that projectiles driven from them in volcanic eruptions would not be drawn back again by gravity. It is on this point that Dr. R. Ball, Astronomer-Royal for Ireland, has based his theory as to the actual body from which these meteoric masses were expelled. He does not attach any weight, it seems, to Tschermak's sug- gestion that the body expelling meteorites must be of small dimensions. He doubtless reasons, and in our opinion justly, that if a larger orb has to overcome greater gravitating energy, it has correspondingly greater energies with which to do this. So Dr. Ball does not hesitate to consider whether the Sun may not have been the source of meteorites; but he finds in some of the peculiarities above described clear evidence that many meteorites must have come from a globe already possessing solid rock-masses, a state of things which can hardly be supposed to exist in the Sun.
Dr. Ball turns next to a much smaller body than the Earth,— her companion planet, the Moon. But here another objection, equally insuperable, presents itself. The moon might readily have expelled meteoric masses from her interior, in the old times when she was instinct with fiery energies. But a meteorite thus expelled would either fall there and then upon the earth, or missing the earth, would continue to travel round her, in an orbit which might, and probably would, in the course of many centuries, restore the meteorite to the moon, but could never carry it to the earth. So that unless we assume the moon to be at this day expelling meteoric masses in. considerable number from her volcanic craters, we cannot suppose the meteorites now falling to have come from her; and it is quite clear that the moon's volcanoes are not in the state of activity this would require, even if (as the wilder selenographers imagine) she is still undergoing vocanic changes.
Next, Dr. Ball considers the minor planets. He shows that a body must be projected from Ceres—taking her as a con- venient, illustrative case—so as to have a total velocity between eight and sixteen miles per second, in order neither, on the one hand, to fall short of the earth's orbit, nor, on the other, to have a path carrying it away (after only one swoop within the .earth's distance) to the star-depths. Then he shows that even with a velocity of the right amount, the chance would be only one in many thousands that a projectile expelled from a minor planet would cross the earth's track. Thus, as he justly says, there are two objections to a minor planet, as the source of a meteorite. First, notwithstanding the planet's small mass, a very powerful volcano would be required ; and secondly, we are obliged to assume that for each meteorite which could ever fall upon the earth, at least 50,000 must have been ejected.
He turns, next, to a planet which has altogether exceptional claims to consideration. It is true, he says, that in this par- ticular orb a volcano would be required which must be capable of expelling bodies with a velocity of at least six miles per second (our own calculations make the least velocity more nearly seven miles) ; but then, everyprojectile launched from that volcano into space would, after traversing an elliptic orbit round the Sun, dash through the track of the earth, and again pass through the same point at every subsequent revolution, " It is not here a case of one solitary projectile out of 50,000 crossing the earth's track, but every one of the 50,000 possesses the same property." Where, it will be asked, is this specially favoured planet, whose meteoric projectiles thus inevitably intersect the track of the earth? We have not far to look for it ; it is the
earth itself, on which we live. The earth is certainly not now capable of expelling masses with the velocity which the theory requires, or if capable of so doing, she (fortunately, perhaps, for us) refrains from exerting her full powers in this way. But in the remote past, as we have every reason to believe, the earth possessed much greater volcanic energy than she now does. If in those remote times,—many millions of years, perhaps, before the surface of the earth was fit to be the abode of life,—there were colossal volcanoes on her surface, which had explosive energy sufficient to expel missiles with a velocity great enough to carry them away from the earth's surface, these missiles would then continue to move in orbits round the Sun, crossing at each revolution the point of the earth's track from which they were originally discharged. If so, then there would be at this moment an enormous number of these projectiles moving through the Solar System, on paths of every degree of eccentricity and perhaps of inclination (though in the majority of cases the inclination of the meteoric path to the earth's would be small), all these paths possessing this char- acteristic, that they would intersect the track of the earth. Whenever the earth in her motion around the sun chanced to reach a point where a meteoric path crossed hers, at or very near the time when the meteorite itself was there, the meteorite would be reabsorbed by its parent planet.
Now, while Dr. Ball was treating the subject of Meteorites from this side, M. Stanislas Meunier was receiving the recog- nition of the Paris Academy of Sciences for a series of re- searches which, at first sight, might seem to point in an entirely different direction. Astronomers had followed with interest the labours of Daubree, which had indicated a connection between meteorites and the lower strata of our globe. M. Meunier, Daubree's pupil and follower, has found that the analogy alluded to is not confined to mineralogical constitution, but extends to the relation which these cosmical materials, disseminated in space, present, when compared among themselves as we compare the constituent rocks of our globe. He concludes therefore, and. the Commission of the Paris Academy appointed to consider claims, considers he is justified in concluding, " that all these masses once belonged to a considerable globe like the earth, having true geological epochs, and that later it was decomposed into separate fragments, under the action of causes difficult to define exactly, but which we have seen more than once in operation in the heavens themselves " (referring, probably, to the phenomena presented by the so-called new stars). If we adopt Dr. Ball's theory, have we not an answer to the question where that globe can possibly be, whence the fragments were torn off or expelled in some way, which reach the earth as meteorites ? Meunier's theory, as it stands, is preposterous, let Commission or Academy say what they will. That some other planet (for so he presents his theory) has been torn into fragments, millions of which have in successive eras reached our earth, their constitution varying according to the depth of the strata of the planet-home from which they were successively torn, is a thing utterly inadmissible, so long as the laws of probability are to be our guide in such matters. But that the earth herself, in various past stages of her existence as an intensely volcanic orb, should have expelled immense numbers of bodies, and that the successive periods of meteoric downfall should thus come to exhibit changes corresponding to the successive stages of terrestrial stratification, seems reasonable enough. Nay, we may even say that if many meteorites really are proved by the evidence adduced by Tschermak to have had a volcanic origin, no theory but Dr. Ball's will account for those meteorites, at any rate,—while nothing could accord better than this theory with the results of M. Meunier's researches.
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