To time the ball’s descent he used a water clock. It worked like a stopwatch. To start the clock he would open a valve. Water would then flow steadily, at a constant rate, straight down through a thin pipe and into a container. To stop the clock, he would close the valve. By weighing how much water had accumulated during the ball’s descent, Galileo could quantify how much time had elapsed to within “one-tenth of a pulse-beat.”
