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250 of 1,250,000 miles; for it is to be remembered that the comet as well as the earth would have an enormously rapid motion, and the disturbing power of the earth would therefore only act for a short time. But a minor planet—even the largest of the family—would not have the twenty-thousandth part of the earth's power to disturb a passing comet. At a distance of 200,000 miles a comet would pass such an asteroid without any marked disturbance of its motions," and at a distance of 1,250,000 miles there would practically be no disturbance at all. "It is, of course, not absolutely impossible that one of the comets of the pair should have been encountered by one minor planet, and the other by another, but the probability against such a contingency is so great, that we need scarcely entertain the idea even as a bare possibility."

On the other hand, the supposition that the comet was destroyed or dissipated by meteor-streams, though not altogether untenable, seems little likely to be correct. I was disposed, when I wrote the article from which I have quoted the above passages, to think otherwise. "The comet," I said, "had been seen to divide into two parts in a portion of the solar system where certainly no bodies but meteorites can be supposed to travel. It seems reasonable to suppose that on that occasion the head of the comet had come upon some group of meteors, and so had divided, as a stream of water divides against a rock. Assuming this, we find reason for believing that the track of this comet crosses a rich meteor region. The particular group which had caused the division of the comet would of course pass away, and would not probably come again in the comet's way for many years or even centuries; but another group belonging to the same system might in its turn encounter the comet, and complete the process of dissipation which the former had commenced. On this theory the distance between the companion comets would introduce no difficulty. For not only is it quite a common circumstance for meteoric systems to have a range of several millions of miles, but—a much more important consideration—both the comets would be bound to return to the scene of the former encounter. It was there that each had been sent off on a new track, but each new track started from there, and therefore each new track must pass through there." The reasoning here is correct enough as far as it goes, but it does not duly take into consideration the extreme sparsity of meteoric distribution and the extreme tenuity of the heads and even of the nuclei of comets. As I pointed out in an essay which appeared in the "Popular Science Review" two months only after the essay from which I have quoted above had appeared in the "St. Paul's Magazine" (if I remember rightly), the meteors of even one of those comparatively rich clusters which produce an important display are strewed so sparsely that each occupies on the average a space corresponding in volume to a cube one hundred miles in length, breadth, and height. The largest meteor in