Archimedes was a Greek mathematician who lived in the second century B.C.E. He discovered several laws of physics. These included Archimedes' Principle, relating to buoyancy and specific gravity, and the principle of the lever, which was used in making machines that could move heavy loads. He also is credited with inventing the Archimedean screw, which was used to draw water from ships and prevent them from sinking. This invention demonstrated that when a screw was rotated inside a cylinder, the screw moved the water in front of it. At the same time, the water resisted and pushed back. This movement of screws through water also applied to the movement of a screw through air like a helicopter rotor does.