Interest in a possible planet or planets outside the orbit of Neptune is likely to increase rather than diminish. To the ancients seven was the perfect number, there were seven heavenly bodies already known, so there could be no use whatever in looking for an eighth. The discovery of Uranus in 1781 proved the futility of such logic, and Neptune followed in 1846 with further demonstration, if need be. The cosmogony of the present day sets no outer limit to the solar system, and some astronomers advocate the existence of many trans-Neptunian planets. CHAPTER XL COMETS--THE HAIRY STARS Comets--hairy stars, as the origin of the name would indicate--are the freaks of the heavens. Of great variety in shape, some with heads and some without, some with tails and some without, moving very slowly at one time and with exceedingly high velocity at another, in orbits at all possible angles of inclination to the general plane of the planetary paths round the sun, their antics and irregularities were the wonder and terror of the ancient world, and they are keenly dreaded by superstitious people even to the present day. Down through the Middle Ages the advent of a comet was regarded as: Threatening the world with famine, plague and war; To princes, death; to kingdoms, many curses; To all estates, inevitable losses; To herdsmen, rot; to plowmen, hapless seasons; To sailors, storms; to cities, civil treasons. Comets appeared to be marvelous objects, as well as sinister, chiefly because they bid apparent defiance to all law. Kepler had shown that the moon and the planets travel in regular paths--slightly elliptical to be sure, but nevertheless unvarying. None of the comets were known to follow regular paths till the time of Halley late in the seventeenth century, when, as we have before told, a fine comet made its appearance, and Halley calculated its orbit with much precision. Comparing this with the orbits of comets that had previously been seen, he found its path about the sun practically identical with that of at least two comets previously observed in 1531 and 1607. So Halley ventured to think all these comets were one and the same body, and that it traveled round the sun in a long ellipse in a period of about seventy-five or seventy-six years. We have seen how his prediction of its return in 1758 was verified in every particular. On the comet's return in 1910, Crowell and Crommelin of Greenwich made a thorough mathematical investigation of the orbit, indicating that the year 1986 will witness its next return to the sun.