But the adventures of comets are not confined to possible encounters with unknown obstacles. We have referred to the fact that the great planets, and especially Jupiter, frequently interfere with the motions of comets. This interference is not limited to the original alteration of their orbits from possible parabolas to ellipses, but is sometimes exercised again and again, turning the bewildered comets into elliptical paths of all degrees of eccentricity. A famous example of this kind of planetary horse-play is furnished by the story of Lexell’s missing comet. This comet was first seen in 1770. Investigation showed that it was moving in an orbit which should bring it back to perihelion every five and a half years; yet it had never been seen before and, although often searched for, has never been seen since. Laplace and Leverrier proved mathematically that in 1767 it had approached so close to Jupiter as to be involved among the orbits of his satellites. What its track had been before is not known, but on that occasion the giant planet seized the interloper, threw it into a short elliptic orbit and sent it, like an arrested vagrant, to receive sentence at the bar of the sun. On this journey it passed within less than 1,500,000 miles of the earth. The form of orbit which Jupiter had impressed required, as we have said, its return in about five and a half years; but soon after 1770 it had the misfortune a second time to encounter Jupiter at close range, and he, as if dissatisfied with the leniency of the sun, or indignant at the stranger’s familiarity, seized the comet and hurled it out of the system, or at any rate so far away that it has never since been able to rejoin the family circle that basks in the immediate rays of the solar hearth. Nor is this the only instance in which Jupiter has dealt summarily with small comets that have approached him with too little deference. [Illustration: Brooks’ comet. Photographed by Barnard, October 21, 1893]