The stadium is equal to one hundred and twenty-five of our Roman paces, or six hundred and twenty-five feet. Posidonius supposes that there is a space of not less than forty stadia around the earth, whence mists, winds and clouds proceed; beyond this he supposes that the air is pure and liquid, consisting of uninterrupted light; from the clouded region to the moon there is a space of two million of stadia, and thence to the sun of five hundred million. It is in consequence of this space that the sun, notwithstanding his immense magnitude, does not burn the earth. Many persons have imagined that the clouds rise to the height of nine hundred stadia. These points are not completely made out, and are difficult to explain; but we have given the best account of them that has been published. CHAPTER IV. OF THE STARS WHICH APPEAR SUDDENLY, OR OF COMETS. A few things still remain to be said concerning the world; for stars are suddenly formed in the heavens themselves; of these there are various kinds. The Greeks name these stars _comets_, we name them Crinitæ, as if shaggy with bloody locks, and surrounded with bristles like hair. Some of them have a mane hanging down from their lower part, like a long beard, some vibrate like a dart with a very quick motion. It was one of this kind which the Emperor Titus described in his very excellent poem, as having been seen in his fifth consulship; and this was the last of these bodies which has been observed. Some are short and pointed, of a pale color, and shine like a sword without any rays; others of an amber color emit a few rays from their margin only. One kind exhibits the figure of a cask, appearing convex and emitting a smoky light; another has the appearance of a horn; it is like the one which was visible when the Greeks fought at Salamis. Occasionally you see one like a burning torch; and again one like a horse’s mane; the latter often has a very rapid motion, like a circle revolving on itself. There is also a white comet, with silver hair, so brilliant that it can scarcely be looked at, exhibiting, as it were, the aspect of the Deity in a human form. There are some also that are shaggy, having the appearance of a fleece, surrounded by a kind of crown. There was one, where the appearance of a mane was changed into that of a spear; it happened in the 109th olympiad, in the 398th year of the City.[16] The shortest time during which any one of them has been observed to be visible is seven days, the longest one hundred and eighty days.