The only source reference to that experiment was from his biographer Vincenzo Viviani who became Galileo's secretary during the later day of the famous scientist. It was likely that the real experiment was done by using two balls of the same size, one of iron, and one of soft wood hanging at the ends of two pendulums. In fact, the iron ball did swing faster gradually, however, not as Aristotle's prediction, the speed was not proportional to the weight. Especially, when two balls were released, their speed was the same initially.