Page:Popular Science Monthly Volume 15.djvu/468

452 its inclined position it does not intersect the path of any of the intermediate planets Saturn, Jupiter, and Mars. M. Le Verrier also calculated back the epochs at which the planet and the meteors were at the point of intersection, and found that early in the year A.D. 126 they were both at that spot, but that this has not happened since. Taking this in conjunction with what Signor Schiapparelli pointed out, we seem to have a clew to a truly wonderful past history. All would be explained if we may suppose that, before the year 126, the meteors have been moving beyond the solar system; and that in that year they chanced to cross the path of the planet Uranus, traveling along some such path as that represented in the diagram. Had it not been for the planet, they would have kept on the course marked out with a dotted line, and, after having passed the sun, would have withdrawn on the other side into the depths of space, to the same measureless distance from which they had originally come. But their stumbling on the planet changed their whole destiny. Even so great a planet would not sensibly affect them until they got within a distance which would look very short indeed upon our diagram. But they seem to have almost grazed his surface, and, while they were very close to such a planet, he would be able to drag them quite out of their former course. This the planet Uranus seems to have done, and when, pursuing his own course, he again got too far off to influence them sensibly, they found themselves moving slowly backward, and slowly inward; and accordingly began the new orbit round the sun, which corresponds to the situation into which they had been brought, and the direction and moderate speed of their new motion.

They seem to have passed Uranus while they were still a small, compact cluster. Nevertheless those members of the group which happened to be next the planet as they swept past, would be attracted with somewhat more force than the rest, the farthest members of the group with the least. The result of this must inevitably have been that, when the group were soon after abandoned to themselves, they did not find themselves so closely compacted as before, nor moving with an absolutely identical motion, but with motions which differed, although perhaps very little, from one another. These are conditions which would have started them in those slightly different orbits round the sun, which, as we have seen, would cause them, as time wore on, to be drawn out into the long stream in which we now, after seventeen centuries, find them.

What is here certain is, that there was a definite time when the meteors entered upon the path they are now pursuing; that this time was the end of February or beginning of March in the year 126 is still a matter of probability only. It is, however, highly probable, because it explains all the phenomena at present known; but astronomers are not yet in a position to assert that it is ascertained, since one link in the complete chain of proof is wanting. We who live now should be