He compares this lifting force of the stars to the attractive power of a magnet over iron, and argues that it must be the waters which are drawn away from the earth, not the earth which is lifted above the water, because the starry influence in its downward course would meet first the sphere of water, and moreover it is lighter and easier to move than Earth. He also thinks that the presence of springs on mountain tops is to be explained by the fact that the ocean is preserved at a higher level, hence water percolating through the porous land, as through a sponge, is forced up to its utmost height. It does not seem as if Dante had paid much attention to this problem when he was writing the _Convivio_, for although he calls the habitable part of the earth “the uncovered land,” “la terra discoperta” (_Conv._ III. v. 73), he speaks of Earth and ocean as if they formed a globe together, “Questa terra ... col mare è centro del cielo,”[623] in the manner of Alfraganus, “Inter sapientes convenit, terram unà cum aqua globosam esse.”[624] But if we may believe that Dante is the author of the _Quæstio de Aqua et Terra_—a belief shared by many experts—he became greatly interested in the problem later on. According to the introduction and the final colophon, he attended a discussion on the subject and joined in it himself, when in Mantua, and afterwards in Verona less than two years before his death he wrote and read in public a Latin treatise in which his arguments and conclusions are set forth. ------------------------------------------------------------------------ [623] “This earth, together with the sea, is the centre of heaven.” (_Conv._ III. v. 63-65.) [624] “It is agreed among learned men that earth and water together form a globe.” (_El. Ast._ iii.). ------------------------------------------------------------------------ He seems to have had in mind Ristoro’s book, and while refuting some of the conclusions agrees with others. As a thirteenth century scholar he admits, as a matter of course, that the centre of the earth is the centre of the Universe and the goal towards which all heavy things tend; that earth as the heaviest element ought to be everywhere nearest this centre, while water ought to occupy the nobler place above it, that is, nearer to the most noble sphere of all which envelops all the others (the Empyrean). He also reasons, in true Dantesque and true mediæval fashion that there must be some place in the universe where all the elements may meet and mingle so that all potential forms of matter may be realized; otherwise the Mover of heaven, in whom all these forms (Plato’s Ideas) actually exist, would fail to give complete expression to his goodness, which is impossible.[625] ------------------------------------------------------------------------ [625] _Qu._ xviii. 28-54. This may be compared with _Par._ xxix. 13-18. ------------------------------------------------------------------------