The ancients divided comets into different classes, the chief points of distinction being derived from the shape, length, and brilliancy of the tails. Pliny distinguished twelve kinds, which he thus characterised:--"Some frighten us by their blood-coloured mane; their bristling hair rises towards the heaven. The bearded ones let their long hair fall down like a majestic beard. The javelin-shaped ones seem to be projected forwards like a dart, as they rapidly attain their shape after their first appearance; if the tail is shorter, and terminates in a point, it is called a sword; this is the palest of all the comets; it has the appearance of a bright sword without any diverging rays. The plate or disc derives its name from its shape, its colour is that of amber, it gives out some diverging rays from its sides, but not in large quantity. The cask has really the form of a cask, which one might suppose to be staved in smoke enveloped in light. The retort imitates the figure of a horn, and the lamp that of a burning flame. The horse-comet represents the mane of a horse which is violently agitated, as by a circular, or rather cylindrical, motion. Such a comet appears also of singular whiteness, with hair of a silver hue; it is so bright that one can scarcely look at it. There are bristling comets, they are like the skins of beasts with their hair on, and are surrounded by a nebulosity. Lastly, the hair of the comet sometimes takes the form of a lance." Pingre, a celebrated historian of comets, tells us that one of the first comets noticed in history is that which appeared over Rome forty years before Christ, and in which the Roman people imagined they saw the soul of Caesar endowed with divine honours. Next comes that which threw its light on Jerusalem when it was being besieged and remained for a whole year above the city, according to the account of Josephus. It was of this kind that Pliny said it "is of so great a whiteness that one can scarcely look at it, and _one may see in it the image of God in human form_." Diodorus tells us that, a little after the subversion of the towns of Helix and Bura, there were seen, for several nights in succession, a brilliant light, which was called a beam of fire, but which Aristotle says was a true comet. Plutarch, in his life of Timoleon, says a burning flame preceded the fleet of this general until his arrival at Sicily, and that during the consulate of Caius Servilius a bright shield was seen suspended in the heavens. The historians Sazoncenas and Socrates relate that in the year 400 A.D. a comet in the form of a sword shone over Constantinople, and appeared to touch the town just at the time when great misfortunes were impending through the treachery of Gainas. The same phenomenon appeared over Rome previous to the arrival of Alaric.