Sentence:  Although Galileo had designed a pendulum clock, he never actually constructed one. The first pendulum clock was constructed by the Dutch physicist Christian Huygens (1629–1695) in 1657. He also developed the <WSD>mathematical</WSD> theory of the pendulum. Newton also studied the motion of a pendulum and experimented with pendulums made of different materials and of different lengths. 
Meaning of the word marked with <WSD> tags:  Of, or relating to mathematics