Dante follows the classics and agrees with his contemporaries in arranging the four elements in four spheres, which are below the celestial spheres. The sphere of Fire is immediately within that of the moon, below this comes Air, then Water, and lastly the solid sphere of Earth. The contrast between Fire which constantly tends to rise towards the moon, and Earth which sinks towards the centre of the universe, is a favourite thought with him;[621] and lightning striking the earth is described as fleeing its proper place.[622] Aristotle seems to have regarded the four elements as flowing into one another, so that these lowly spheres beneath the moon were not sharply divided, but his mediæval disciples, following the Greek idea to its logical conclusion, conceived them with boundaries as definite as those of the celestial spheres. “Exhalations” could pass from one to another, but each particle of every element tended always to revert to its own place, which was distinct from the others. ------------------------------------------------------------------------ [621] _Par._ i. 115, 117; _De Mon._ I. xv. 38-41; and III. vii. 30, 31, etc. [622] _Par._ i. 92, 130-134. ------------------------------------------------------------------------ And here they encountered a serious difficulty. If the sphere of water is naturally higher than that of earth, how comes it that in one quarter of the globe land rises above the ocean? It was recognised as necessary that this should happen in some part of the world, in order for an opportunity to be given for the elements to combine and to form all those substances and all those living creatures, including man, for which this earth was prepared; it was also agreed that the celestial spheres were the agency by which the Creator had first chosen to “let the dry land appear,” and by which it was maintained above the ocean; but the exact nature of this force and the way in which it acted was a question sometimes discussed. Albertus Magnus was of opinion that the rays of the sun and the stars, which had more power in the temperate and tropic zones than in the arctic, had here dried up the ocean, and he quotes Albumassar’s statement that this proves the existence of land on the southern side of the equator also. Ristoro, on the other hand, believed that the waters had been drawn up and rolled back from the habitable earth, and he attributes the elevating force to the stars of the north, bringing in his favourite theory that in this hemisphere they are far more numerous, and the constellations, from their upright position, are far more powerful than in the south.