In the first half of the 1490s, Leonardo abandoned the notion of recirculation mills and began to study wheels overbalanced by mechanical means (with moving or oscillating spheres) or hydraulic means (systems using Archimedes’ screws to raise and lower water). He designed several versions and probably also built models. However, he concluded his studies by rejecting the possibility of perpetual motion. Identifying gravity and attrition as the forces that made it impossible, he compared perpetual motion to the alchemists’ quest for the transmutation of metals.