English: The second pendulum clock built around 1673 by Christiaan Huygens , inventor of the pendulum clock. Drawing is from his treatise Horologium Oscillatorium , published 1673, Paris, and it records improvements to the mechanism that Huygens had illustrated in the 1658 publication of his invention, titled Horologium . It is a weight driven clock (the weight chain is removed) with a verge escapement (K,L), with the 1 second pendulum (X) suspended on a cord (V). The large metal plate (T) in front of the pendulum cord is the first illustration of Huygens' 'cycloidal cheeks', an attempt to improve accuracy by forcing the pendulum to follow a cycloidal path, making its swing isochronous. Huygens claimed it achieved an accuracy of 10 seconds per day. Each gear is labeled with the number of teeth it has. Alterations to image: Cropped out figure number, converted to 32 color PNG.

Labeled parts: (A) front plate, (B) back plate, (C) minute wheel, (D) weight chain pulley, (E) third wheel pinion, (F) third wheel, (G) third (seconds) wheel, (H) contrate wheel, (I) crown wheel pinion, (K) crown wheel, (L) pallets, (M) verge, (N,P) verge supports, (Q,R) crown wheel shaft supports, (S) crutch, (T) cycloidal cheeks, (V) pendulum rod, suspended from two cords hidden behind cycloidal cheeks, (X) pendulum bob, (Y) hour hand, (Z,λ) second hand (ζ) hour wheel (Δ) rate adjustment weight.