mentum acquired carries it up the opposite side, till it gradually slackens and assumes a snail's pace. The comet approaching the sun is going down hill, and when it reaches the nearest point it wheels round, and then ascends till its speed is gradually arrested. It is reined in by the sun, from which there are invisible lines of force dragging it back; and, if its momentum be not too great, it is effectually checked and brought back to pursue its former course. Most frequently, however, its course is so impetuous that all the strength of the sun, in reining back, avails nothing. It breaks loose, like a fiery steed from its master; speeds off into space, and is heard of no more. We shall, first, follow the fortunes of one of the more tractable comets, or those that remain permanent members of the solar system, performing their revolutions regularly round the sun. Of these there are six whose orbits are well determined.
Let us enter the cometary vehicle at some point beyond the confines of the solar system; and Halley's comet makes an excursion three hundred millions of miles beyond Neptune—the most distant planet in the system. Here the comet is a globular mass, lazily floating along like a filmy cloud in the heavens. It is on its way to the sun, and we shall suppose that the planets are so many stations on the line. When we near Neptune, his attraction is powerfully felt. The sun would have us go straight on, but our motion is so slow, and the sun so distant, that Neptune
