The medieval “science of weights” (i.e., mechanics) owes much of its importance to the work of Jordanus. In the "Elementa super demonstrationem ponderum", he introduces the concept of “positional gravity” and the use of component forces. Pierre Duhem (in his "Origines de la statique", 1905) thought that Jordanus also introduces infinitesimal considerations into statics in his discussion of "virtual" displacements (this being another interpretation of Duhem) of objects in equilibrium. He proves the law of the lever by means of the principle of work. The "De ratione ponderis" also proves the conditions of equilibrium of unequal weights on planes inclined at different angles – long before it was re-established by Simon Stevin (with his clootcrans -- "wreath of spheres" experiment) and later by Galileo.