The word “comet” means “a hairy body,” the name having been given from the hairy appearance of the light around the nucleus. About seventeen hundred different comets have been seen at different times by men--some large, some small; some visible to the naked eye, but most of them only visible through telescopes. These hundreds are, there is no doubt, but a very small number out of the myriads ranging through the heavens. If you were seated in a little boat in mid-ocean, counting the number of fishes which in one hour passed near enough in the clear water for your sight to reach them, you might fairly conclude, even if you did not know the fact, that for every single fish which you could see, there were tens of thousands which you could not see. Reasoning thus about the comets, as we watch them from our earth-boat in the ocean of space, we feel little doubt that for each one which we can see, millions pass to and fro beyond reach of our vision. Indeed, so long ago as the days of Kepler, that great astronomer gave it as his belief that the comets in the Solar System, large and small, were as plentiful as the fishes in the sea. And all that modern astronomers can discover only tends to strengthen this view. Why should the comets be called “visitors?” I call them so simply because many of them _are_ visitors. Some, it is true, belong to the Solar System. But even in their case, strong doubts are felt whether they were not once visitors from a distance, caught in the first instance by the attraction of one of the larger planets, and retained thenceforward, for a time at least, by the strong attraction of the sun. [Illustration: PASSAGE OF THE EARTH AND THE MOON THROUGH THE TAIL OF A COMET.] Every comet, like every planet, has his own orbit or pathway in the heavens, though the kind of orbit varies with different comets. There are, first, those comets which travel round and round the sun in “closed orbits”--that is, in a ring with joined ends. Only the ring is always oval, not round. There are, secondly, those which travel in an orbit which _may_ be closed; but if so, the oval is so long and narrow, and the farther closed end is at so great a distance, that we can not speak certainly. There are, thirdly, those which decidedly are mere visitors. They come from the far-off star-depths, flash once with their brilliant trains of light through our busy Solar System, causing some little excitement by the way, and go off in another direction, never to return.