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had observed the phenomena of rain, cloud, mist, snow, &c. might be supposed to inake: (1) Every shower of rain implies the existence of a cloud; (2) every cloud implies the descent, at some time or other, of rain, greater or less in quantity and heaviness; and (3) all drops of water, from the tiniest water vesicles in a cloud to the heaviest rain drops, are of the same kind, differing only in shape or in size: snowflakes also, as formed of water particles in a changed form, must be put in the same class.
And as the insect by studying the relations which exist between clouds and rain might be led to form an opinion whence clouds come, which would tell him also (as we know) whence rain comes,' so perhaps may we by studying the relations which exist between meteor-streams and comets be led to form an opinion whence comets (which are meteor collections have originally come.
The very first suggestion ever made respecting the origin of comets came, indeed, from such considerations as I have mentioned above. Schiaparelli, to whom we owe the happy guess, and the beginning of its confirmation as a useful truth, that meteors are bodies following in the tracks of comets, threw out the idea that comets, regarded as flights of meteors, may be travelling in multitudes through the interstellar depths, and be from time to time drawn out thence by the attraction of our sun. He pictured our sun, in his swift rush onward with his train of planetary attendants, as coming into ever-fresh regions of comet-strewn space. A comet or meteor flight drawn towards him by the sun would approach the solar system on a path which may be described as casual. It might cross the general plane near which all the planets travel at any point, the chance that that point would lie near a planetary orbit being very small indeed. Supposing the point where the meteor flight crossed that important plane—the life plane of the solar system—to be on or near a planetary orbit, the chance would still be very small that the meteor flight would cross there at a time when the planet to which that orbit belonged was near that particular point. The chances would, in fact, be millions of millions, or rather of billions, to one that the meteor flight would visit our solar system without coming near any planetary body, in which case it would pass out from our solar system again, never to return to it. But, if a meteor flight did chance to come very close indeed to a planet of adequate mass, the flight might,
1 To us, who know how clouds and rain are really produced, this imagined inquiry of the insect may seem trivial. But man had advanced far in scientific research before he had learned anything about the source and nature of rain, hail, snow, cloud, mist, and fog. The whole subject was as completely mysterious, for example, to all the writers whose works were included by the Jews among their sacred books (in probably all their ancient documents), as were the phenomena of comets, which with them were veritable angels or messengers from Yahveh.
? Nerer; because, by the nature of its supposed indrawing, it possessed relative motion of its own before it began to be drawn in; and the sun could not take from
said Schiaparelli, be captured. The planet migbt abstract so much of the comet's velocity as to leave only a balance corresponding to motion in a closed or elliptic path; and on such a path would the meteor flight or comet necessarily travel thereafter-unless, perhaps, after many revolutions of each, the planet at some subsequent encounter undid the work which it had accomplished when first it approached the comet.
So far Schiaparelli reasoned soundly on the basis of his assumption. I say assumption of set purpose ; for it is altogether a mistake to regard the idea thus thrown out by Schiaparelli as if it were a theory. His idea that meteors follow in the track of comets developed into a theory when it had been tested and confirmed by observation. But the case is different with the idea, that meteor flights are travelling amid the star depths like fish in the depths of
But Schiaparelli did not even reason quite correctly. A single meteoric mass, or even a small meteor flight, might be introduced into our solar system in the way suggested by Schiaparelli ; for undoubtedly the giant planets possess the power he attributed to them, and if a body from without came near enough to any one of them, could so reduce its velocity as to change its path from the hyperbolic (or unclosed) form to an elliptic or closed orbit. And thenceforth such a body would travel around the sun systematically, on an eccentric path passing very near the orbit of the planet by whose influence it had been originally introduced into the system.
But a giant planet could do no more. It could not generate a meteor-stream in the way suggested by Schiaparelli. So soon as we test the matter by mathematical analysis, we find that very close approach would have to be made to a planet that a single body might be forced into a closed path, and it is certain that a flight of bodies large enough to produce any of the known meteor-streams would have its components very widely scattered by the planet's perturbing action, simply because the different components of the flight would be exposed to very different degrees of disturbing action.
This I have shown mathematically, and my demonstration has not been questioned-though Professor Young, of Princeton, N.J., in admitting the validity of my reasoning, suggests the possibility that some way may hereafter be found for eluding the difficulty. But then Professor Young holds the strange idea that Schiaparelli's speculation as to the origin of comets and meteor-streams is an accepted theory; and labouring under this delusion, imagines that there must be some way of meeting objections to it.
But it is worthy of notice that Schiaparelli's fancy, even if accepted,
it that relative motion. He would impart motior, and take such imparted motion away again, leaving untouched the original motion.
would prove nothing about the origin of comets and meteors. To say that they came from out the interstellar depths on hyperbolic paths, is to assert what can be disproved by mathematical demonstration. But if it could be proved, what would it amount to ? Merely to this—that comets which now travel on closed paths once travelled on endless paths. We are no whit nearer the explanation of their origin. If the interstellar depths are crowded with meteor flights, we have to ask whence the meteor flights came. To say that fish which have been drawn from the sea were originally swimming about in the sea, is surely not to add much to our knowledge about fish.
It may be urged, however, that comets and meteor-streams are simply the material left unused after the various solar systems in our galaxy had been formed, by processes of meteoric aggregation.
Unfortunately for this explanation, the comets and meteor systems we have to explain are precisely those which, had they existed from the earlier ages, when our solar system and its fellows were forming, would have been the first to be gathered up. For they are those which pass near the orbits of various planets, some near the orbit of Jupiter, some near that of Saturn, or of Uranus, or of Neptune, and about four hundred which pass near the orbit of our earth. These comets, with their associated meteor systems, would have had less chance of escape than any others, during the millions of years belonging to the formative processes of our solar system. Yet those are precisely the comets and meteor systems which we chiefly need to interpret.
Suppose that, instead of making mere guesses, we consider actual facts, and open our eyes to the views suggested by them.
I take first the millions of meteors encountered by the earth each year, and the hundreds of earth-crossing meteor systems already recognised. Taking for our guide proposition (1), we are led to the conclusion that in remote ages there were hundreds, if not thousands, of comets whose tracks crossed the track of the earth, or at any rate approached very near to it. That some of these comets thus crossed the earth’s track casually, that is through mere chance coincidence, we may well believe. Nay, this is known, as will presently be seen. But if all did, then must there have been millions of millions of comets in remote times, to account for so many chancing to cross the earth’s track ;—with this startling circumstance to be considered in addition, that ninety-nine out of a hundred of those whose paths did not cross the earth's track have entirely disappeared, while a considerable proportion of those which do cross that track (and which, therefore, have been exposed for millions of years to an extra risk of destruction) remain.
This idea we may safely reject. But if we do, then we have to account for a special earth-crossing family of comets and meteorstreams, without going outside to look for the origin of such bodies; for the moment we go outside we encou ter the difficulty which bas just driven us from any merely casual interpretation.
In other words, we must look to the earth herself to explain the great majority of these earth-crossing systems.
In this way Meunier and Tschermak were driven to look to the earth herself for the origin of meteorites. Proposition (3) above enables us to extend their reasoning, specially directed to particular classes of aerolites, to all classes of such bodies, to all meteors, down even to the tiniest falling star, only visible perhaps in the field of a powerful telescope. Not all these bodies, but a goodly proportion, must have been generated in some specially terrene manner.
We have actually no possible way of explaining the terrestrial origin of any meteors but in volcanic outbursts. Moreover, we are obliged to set the time when such outbursts took place very far back in the past, seeing that at present the volcanic forces of the earth, even as manifested at Krakatoa recently, possess nothing like the power necessary for the ejection of matter beyond the range of the earth's back-drawing power. Looking, however, at the immense extrusive power of the volcanoes of the tertiary era, when basaltic lava covering hundreds of thousands of square miles to a depth of from 1,000 to 14,000 feet were poured forth, we can conceive the still mightier energies of volcanoes in the secondary era, their still more tremendous power in the primary era, and so, passing backwards to millions of years beyond the first beginnings of life on the earth, we can even picture to ourselves volcanoes ejecting matter with velocities of ten or twelve miles per second. With such velocities flights of ejected particles would pass beyond the earth's attraction, and if she were the only body in the universe, such ejected matter would travel away from her never to return.
But, although such expelled bodies would never return to the earth, they would not escape from the solar system. To drive them for ever away from her, the earth would have to impart a much larger velocity-an average of about twenty-six miles per second. The greater number of the expelled bodies would travel thenceforth on an orbit round the sun, crossing the earth's track at or near the place where they were first sent forth from their parent planet.
One may almost say that this origin of many meteorites and meteor systems is forced upon us by the evidence. Still it would be negatived if we found that volcanoes do not eject matter at all resembling meteorites in structure. The reverse, however, is the case. Ranging the products of volcanic ejection in order according to the amount of iron they contain, and ranging meteorites in like manner, we find the two series coinciding over the greater portion of the longer—the volcanic series. We might not indeed have known how closely the most ferruginous volcanic products resemble the iron meteorites in structure but for the accident that Nordenskjold discovered a mass which he mistook for an iron meteorite, but which is found now to be really a volcanic ejection, akin in structure to the VOL. XIX.-No. 111.
field of basaltic lava (at Ovifak on the shores of Greenland), in the midst of which it had fallen while the lava was still plastic to retain this missile as it fell after its flight through many miles of air.
We may, therefore, regard the terrestrial origin of many meteorites as highly probable, if not in effect demonstrated.
Here Tschermak and Meunier pause, as also does Ball, who thus far had followed them. The last named does not even ask, in that singularly interrogative and irresponsive work the Story of the Heavens, whether we may not go further.
For my own part I find in this result the first step in a most interesting and suggestive path of inquiry.
Regarding a large proportion of the material visitants of the earth as originally earthborn, we may conclude that in the remote time when our earth was a baby world, sunlike in condition, her path was traversed by hundreds of comets, her own progeny. These comets were followed severally by their trains of meteoric attendants. They were exposed to the action of those solar forces by which, within the last balf-century, a once promising member of another comet family became dissipated until it finally lost altogether its cometic character. Millions of years ago, probably, every one of them had been thus broken up until nothing remained but the streams of meteoric bodies, travelling round the orbit which had once been that of the earthejected comet.
But this being the case with the earth, was the case also no doubt with every planet. Even our little moon, whose scarred face still shows signs of the volcanic energies she once possessed, played her part in giving birth to such comets as she was equal to. If she possessed less volcanic power than the earth (at the same stage of the life of each), she required less power to eject matter for ever from her interior. On the other hand, the giant planets required greater power; but then they also possessed it. If Jupiter, for example, required power enough to eject bodies with a velocity of forty or fifty miles per second, yet it must be remembered that he is 310 times as massive, and therefore 310 times as strong as our earth. (For matter, 'inert matter’as many choose to call it, measures in reality the strength of the orbs in space, and not only possesses power, but a power acting so swiftly across vast distances that the velocity of light is rest by comparison. Moreover, this power possessed by inert' matter is the source of every form of energy of which we know, even of life itself.) So with the other giant planets.
Jupiter, then, and each one of his giant brethren, must during its sunlike stage have possessed the comet-ejecting power. Each giant planet must have had its comet family, at that remote time in the history of the solar system. And the comets thus formed by the giant planets, while no doubt very numerous, must, many of them, have been far more important than those to which our earth gave