The recirculation mill was the solution most studied by medieval engineers who worked on perpetual motion. The idea was to build a millstone driven by a hydraulic wheel powered by the water raised through a piston pump, driven by the wheel itself. Such a technology would have had a considerable economic impact and it is for this reason that entrepreneurs and patrons funded this type of research. Francesco di Giorgio was very involved in the study of such machines, of which we find numerous examples (at least 19) in the notebook he wrote in his youth (BAV, Ms. Vat. Urb. Lat. 1757). A selection of these devices would also be included in the first draft of his Treatise of Architecture. Interestingly, recirculation mills completely disappear in the second draft of the treatise (BNCF, Cod. Magliab. II.I.141), a sign that Francesco had deepened his studies of physics in the decade that separate the two versions of the treatise, abandoning the idea of the possibility of perpetual motion, at least as working machines.