To Dante and his contemporaries the supernatural world was not what it is to us to-day, a vast, unbounded space filled with star-systems like our own: the topography of Hell, Purgatory, and Paradise seemed to them as definite as that of our own planet. The Ptolemaic system of astronomy (overthrown by Copernicus, yet still forming the framework of Milton's Paradise Lost) was accepted with implicit confidence. According to this system the universe consisted of ten heavens or concentric spheres, in the center of which was our earth, immovable itself, while around it revolved the heavenly spheres. The earth was surrounded by an atmosphere of air, then one of fire, and then came in order the heavens of the moon, Mercury, Venus, the sun, Mars, Jupiter, Saturn, the fixed stars, and the Primum Mobile (the source of the motion of the spheres) beyond which stretched out to infinity the Empyrean, the heaven of light and love, the seat of God and the angels. According to Dante, hell is situated in the interior of the earth, being in shape a sort of funnel with the point downward, and reaching to the center of the earth, which is also the center of the universe. Purgatory rises in the form of a truncated cone in the surface of the southern hemisphere, having in solid form, the same shape as the hollow funnel of hell. It was formed of the earth which fled before Lucifer, and splashed up behind him like water, when, after his revolt against the Almighty he was flung headlong from heaven and became fixed in the center of the earth, as far as possible according to the Ptolemaic system from the Empyrean and God. Hell is formed of nine concentric, ever-narrowing terraces, or circles, exhibiting a great variety of landscapes, rivers, and lakes, gloomy forests and sandy deserts, all shrouded in utter darkness except where flickering flames tear the thick pall of night, or the red-hot walls of Dis gleam balefully over the waters of the Stygian marsh. Here are punished the various groups of sinners, whom Dante sees, whose suffering he describes, and with whom he converses as he makes his way downward from circle to circle.