The law of the pendulum....

I once saw in Pisa Cathedral the low-hanging bronze lamp that, according to a popular story, Galileo was watching sway gently during Mass when the law of the pendulum occurred to him. Curious to find out how long it took the lamp to swing back and forth, he used his pulse to time large and small swings. Galileo discovered something that no one else had ever realized: the period of each swing was exactly the same. The law of the pendulum, which would eventually be used to regulate clocks, made Galileo Galilei instantly famous.

Sadly, the lamp was actually cast in 1586, a few years after Galileo's discovery, but another lamp almost certainly hung here before this one.