The rigorous and scientific study of the properties of pendulums, however, didn't start until the 17th century, when Galileo took up the question. Undeniably, Galileo did some great and unprecedented work, but one can't help but wonder how much easier and productive his time could have been if he had thought up of this beautiful little experiment (without just becoming mesmerized by the optical illusion created by it):
And here is another beautiful version of the same basic idea.
And if you're curious about how seismometers, including Zhang Hen's, work, listen to this:
And if you want more on pendulums, I'll post an awesome entry on Foucault's pendulum soon. Stay tuned.