Huygens synchronization of two pendulum clocks
  The synchronization of two pendulum clocks hanging from a wall was first observed by Huygens during the XVII century. This type of synchronization is observed in other areas, and is fundamentally different from the problem of two clocks hanging from a moveable base. We present a model explaining the phase opposition synchronization of two pendulum clocks in those conditions. The predicted behavior is observed experimentally, validating the model. 
Henrique M. Oliveira and Lu\'is V. Melo
physics.class-ph nlin.CD