Engraving depicting Christiaan Huygens' cycloidal pendulum. A rod of flexible metal with bob, P, which describes a the cycloidal arc. The rod suspended between two solid cheeks in form of cycloidal arcs at tangent to the starting point. As the rod oscillates, it bends and rests on each arc by turn, so diminishing length of pendulum depending on oscillation. Christiaan Huygens (1629-1695) a Dutch physicist, mathematician, astronomer and inventor. Dated 19th century