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 01/02/1874