Comets are by no means rare visitors to our skies, and very few years have elapsed in historical times without such objects making their appearance. In the Dark and Middle Ages, when Europe was split up into many small kingdoms and principalities, it was, of course, hardly possible for a comet to appear without the death of some ruler occurring near the time. Critical situations, too, were continually arising in those disturbed days. The end of Louis le Debonnaire was hastened, as the reader will, no doubt, recollect, by the great eclipse of 840; but it was firmly believed that a comet which had appeared a year or two previously presaged his death. The comet of 1556 is reported to have _influenced_ the abdication of the Emperor Charles V.; but curiously enough, this event had already taken place before the comet made its appearance! Such beliefs, no doubt, had a very real effect upon rulers of a superstitious nature, or in a weak state of health. For instance, Gian Galeazzo Visconti, Duke of Milan, was sick when the comet of 1402 appeared. After seeing it, he is said to have exclaimed: "I render thanks to God for having decreed that my death should be announced to men by this celestial sign." His malady then became worse, and he died shortly afterwards. It is indeed not improbable that such superstitious fears in monarchs were fanned by those who would profit by their deaths, and yet did not wish to stain their own hands with blood. Evil though its effects may have been, this morbid interest which past ages took in comets has proved of the greatest service to our science. Had it not been believed that the appearance of these objects was attended with far-reaching effects, it is very doubtful whether the old chroniclers would have given themselves the trouble of alluding to them at all; and thus the modern investigators of cometary orbits would have lacked a great deal of important material. We will now mention a few of the most notable comets which historians have recorded. A comet which appeared in 344 B.C. was thought to betoken the success of the expedition undertaken in that year by Timoleon of Corinth against Sicily. "The gods by an extraordinary prodigy announced his success and future greatness: a burning torch appeared in the heavens throughout the night and preceded the fleet of Timoleon until it arrived off the coast of Sicily." The comet of 43 B.C. was generally believed to be the soul of Cæsar on its way to heaven. Josephus tells us that in A.D. 69 several prodigies, and amongst them a comet in the shape of a sword, announced the destruction of Jerusalem. This comet is said to have remained over the city for the space of a year! A comet which appeared in A.D. 336 was considered to have announced the death of the Emperor Constantine.