Philosophiæ Naturalis Principia Mathematica
Horologium Oscillatorium: Sive de Motu Pendulorum ad Horologia Aptato Demonstrationes Geometricae ("The Pendulum Clock: or Geometrical Demonstrations Concerning the Motion of Pendula as Applied to Clocks") is a book published by Christiaan Huygens in 1673 and his major work on pendulums and horology. It is regarded as one of the three most important works on mechanics in the 17th century, the other two being Galileo’s "Discourses and Mathematical Demonstrations Relating to Two New Sciences" (1638) and Newton’s "Philosophiæ Naturalis Principia Mathematica" (1687).