The phenomenon develops calmly, but it is invisible, unstoppable. One feels, one sees it born and grow steadily; and it is not in one's power to either hasten it or slow it down. Any person, brought into the presence of this fact, stops for a few moments and remains pensive and silent; and then generally leaves, carrying with him forever a sharper, keener sense of our incessant motion through space.

On his pendulum experiment, as quoted in Pendulum : Léon Foucault and the Triumph of Science (2003) by Amir D. Aczel