Theodoric of Chartres, Adelard of Bath, and John of Holywood, on the other hand, adduced proofs of the immobility of the earth which had been derived indirectly from Aristotle. Theodoric asserted[676] that the earth does not gain its compactness either from its inherent nature, because earth is actually observed on occasions to become mingled with air; or from the weight of the overlying atmosphere and sphere of fire, because these have no weight. What, then, keeps it from flying to pieces? Here Theodoric appealed to the Peripatetic reasoning that the circular motion of the heavens necessitates the existence of a solid and immovable body in the center.[677] All heavy bodies acquire their _substantia_, or solidity, from the motion of light bodies; and conversely light bodies derive their motion from heavy bodies. The _Quaestiones naturales_ of Adelard of Bath is in the form of a dialogue between Adelard and a nephew who asks questions. The nephew was much puzzled by the fact that, whereas heavy objects like rocks need a piece of wood or other support to hold them up in the air, the earth as a whole, much the heaviest of all, requires no such support.[678] Adelard replies first that the earth does not fall because there would be no utility in its doing so; then he proceeds to show by a rational argument (_rationabiliter_) why the earth does not need a support. The principal quality of earth he says, is heaviness; heavy bodies naturally seek the lowest position (_infimum_); the lowest position of a spherical body like the universe is its center—though why this latter proposition is so, Adelard fails to make clear. At all events, the earth tends to seek the center of the universe, just as a stone thrown into an imaginary hole piercing the center of the earth would come to a halt there.[679] Since the center of the universe is one point, not several, the earth forms a single unit, not several; and for these reasons, moreover, the earth is stable and immobile. John of Holywood explained[680] the same thing more briefly than Adelard by simply stating that the immobility of the earth is due to its weight, since it is the nature of all heavy things to seek the center of the universe and since the earth is the heaviest of all elements. Both Adelard’s and John of Holywood’s arguments suggest the Aristotelian doctrine of an equilibrium of forces around the center of the globe, though this doctrine is not cited in so many words. Like most medieval writers, Adelard and John seem only partially to have understood the obscure texts from which they derived their proofs and to have left out many links in their chains of reasoning. SIZE OF THE EARTH