PART III. INFLUENCE OF COMETS AND PLANETS UPON ONE ANOTHER.
. . . . Upon the whole, then, we may be assured, that, by proximity alone, Comets are almost wholly incapable of affecting either the movements of the Planets, or the system of things upon their surface. But the case is very different, on the supposition of actual contact: for one of those circumstances, which would be the chief means of counteracting a comet's influence in approaching a planet, viz. the rapidity of its motion, would serve, by the momentum, to give great effect to a collision. . . .
But though the probability of such a collision is extremely small, we see that it is perfectly possible in itself; whilst the amount of that probability may be greatly increased by lapse of time. Let us now, therefore, shortly attend to the consequences which might ensue from such an event It is evident that much will depend on the direction of the Comet's course at the time of its encountering a Planet. If both be moving towards the same quarter of the heavens, each will glide off from the surface of the other, and no very material changes will be produced, either on their movements or on their physical constitution. But should the directions of their respective courses be exactly opposite, when the concurrence takes place, (a case, however, which it is easy to see can happen only with retrograde comets), the consequences would necessarily be far more serious and permanent. . . .
Seeing, then, that the collision of a Comet and Planet is an event lying within the verge of possibility, Have we any reason to suppose that it is one which has ever happened? This question we can answer, only by examining the movements and constitution of the Planets as they at present exist, and tracing back the circumstances now characterizing both to those causes by which they seem to have been produced. . . . .
It appears highly probable that none of the planetary bodies have sustained any alterations in their orbits by the collision of a comet. But on this account we are not to suppose that a contact has never taken place; because, though it may not have been sufficiently violent to have altered the planet's orbit, it may nevertheless have materially affected its physical organization, by impinging on its surface; nor, least of all, are we to conclude, from the experience of the past, that the collision of a comet with any of the planetary bodies, will never happen in the course of time. Even though it were demonstrated that such a catastrophe has never yet been fulfilled, this circumstance could afford no assurance that it may not occur at some future period; and therefore, it behoves us shortly to consider what would be the nature and amount of the physical changes which the collision of a comet would produce on the surface of a planet.
It is very true, as was formerly remarked, that the masses of comets are usually small; and for this reason we might be disposed to imagine, that the result of a collision would be trivial. But if a comet, moving with the prodigious velocity which it acquires near its perihelion, should chance to strike a planet, as for instance the Earth, then coming in an opposite direction, the consequences would be truly disastrous . . . . waters of the ocean, now attracted by the close approach and next driven from their ancient bed by the contact of the comet, would sweep over the face of the globe, covering even the highest mountains in their impetuous course, and involving all things in undistinguishable ruin. Whole species of plants and animals, existing in different quarters of the Earth, would, by this cataclysm, be at once overwhelmed and annihilated: whilst the few among the human race, who should happily be saved amid this shipwreck of Nature, would soon relapse into a state of pristine ignorance and barbarism. After such an event, by which all the monuments of art, and all the records of learning, would be destroyed, mankind would necessarily for many centuries be occupied with providing for their bare subsistence; and a long succession of ages would elapse, before those stores of knowledge could be retrieved, which their ancestors had been able to attain. When, however, posterity, in the progress of time, had again become so far enlightened, as to observe and speculate on the striking physical appearances, which in all parts of the world would meet their attention, they could not fail to consider them as the records of some great and sudden catastrophe, which at one period must have befallen their globe. . . .
There seems, then, to be no fact better authenticated in the physical history of our globe, than that there have taken place the most violent and extensive inundations from the ocean: The only question of doubt or difficulty, is to fix upon the causes which could thus have impelled the ocean from its natural bed; and I have been somewhat particular in detailing the various phenomena, in order that we may possess some data for estimating the character of the agent to which these striking physical convulsions must be attributed. Now, it is quite evident, that there exists no agent on the earth itself, at all capable of creating such vast effects as those which have been here described; seeing that there are no physical causes of change on the surface of our planet, but what are so local and so gradual in their operation, as to be totally inconsistent with the sudden and extensive convulsions which we seek to explain. Since, then, this deluge cannot be referred to any agent residing in the Earth itself, the only foreign cause to which it can be ascribed, is either the near approach, or the actual contact, of a Comet. But it is not difficult to see which of those two hypotheses is, in this case, the one to be adopted. For when we consider the astonishing violence by which this deluge was characterised; huge fragments of rocks rent asunder and transported over ridges and valleys; whole species of animals overwhelmed, and even the highest mountains overtopped; the surface of the globe broken into isolated or disjointed groups, and even a large portion of the materials of the southern hemisphere driven beyond the equator,--it is impossible to conceive that these tremendous effects could have been occasioned by any other agency, not wholly miraculous, than the collision of a Comet. . . . .
The same propensity which leads men to search into the history of the past, awakens into the mind a still stronger desire of knowing the secrets of futurity: And, accordingly, astronomers, not content with the endeavour to learn the physical revolutions which the earth has already sustained by the contact of a Comet, have sought to discover the period when it may be again exposed to a similar catastrophe. This they have attempted to accomplish, by computing for a multitude of successive revolutions the motions of those Comets, whose orbits are exactly computed, and ascertaining the time of their greatest proximity to the earth. But, before we detail the result of these curious investigations, it may be proper to give some account of the Comets, whose calculated orbits and periods of revolution have been verified by observation. . . . .
But such speculations, however striking the results, conduce to no practical advantage, and contribute little to the advancement of science. They afford astonishing proofs of the energy of man's intellectual power, by which he extends his vision to the horizon of the most distant futurity, and looks forward, it may be, with a feeling of complacent assurance, to those momentous events, which, from his knowledge of nature, he is enabled to foresee. But let him not rest too confidently on the verity of such anticipations. Astronomers have prophesied, it is true, the collision of a Comet with the earth; an event that will at once destroy the greater part of the human species: but any slight attraction, which, in calculating the movements of this comet, they have chanced to overlook, must invalidate all their conclusions, and render the prediction at once vain and futile; while, perhaps, some other comet, among the many thousands traversing the system, and following an orbit to us unknown, may, in the mean while, come in contact with our globe, and thus, without any warning of its approach, produce the same terrible effects, long before the expected period have arrived.
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