Over the centuries, scholars and inventors have proposed machines that would far transcend human expectations. There is always the chance that a future technological breakthrough will facilitate the transformation of such technological dreams into reality. However, among these technological dreams, there are ideas for machines that future technological developments cannot make possible because these ideas violate fundamental scientific laws.
Perpetual-motion devices are probably the best known of such impossible machines. For more than fifteen hundred years, inventors have offered plans for, and actually built, machines that given proper construction, materials, and lubrication were supposed to operate forever. Such devices were often expected to do useful work and to generate more energy than was required merely to keep them running.
The classic version of a perpetual-motion device is a wheel that spins continuously upon its axle without the aid of an external power source. A self-moving wheel is described in the ancient South Asian treatise Siddhanta-Ciromani (A.D. 400-450), and a thirteenth-century illustration of one appears in the sketchbook of Villard de Honnecourt of France. In Villard's device an uneven number of mallets hang loosely from the rim of a vertically mounted wheel. They are spaced so that the wheel is constantly unbalanced and therefore always moving.
The European Renaissance, which first witnessed so many other manifestations of technological fantasy, was a popular time for the invention of perpetual-motion devices. Often quite elaborate in conception, these might utilize water, air, or the force of gravity, and all were designed as closed cycle operations; for example, the energy generated by a continuous stream of water flowing over a waterwheel would be used to power the pump that lifted the water up to the waterwheel, and so on endlessly.
Along with unceasing motion, some inventors also promised the production of excess energy that could be used to run the machinery in a flour mill or serve some other useful purpose. The promise of the boon to humankind of unlimited free power combined with the tremendous challenge of getting the device to work in the first place made perpetual motion an exciting venture for many technologists.