Chapter LXXVII: Section XII: The Vibrations Must Be Similar

But if the vibration be not similar at every impression, it can never be carried beyond the number of actual impressions; for, move any body as a pendulum, in one way, and it will continue to oscillate in an arch of the same circle, until the known causes make it rest; but if, after first putting it in motion in one direction, you push it into another, it can never reassume the first direction; because it can never move itself, and consequently it can have but the effect of that last motion; whereas, if in the same direction you act upon it several times, it will describe a greater arch, and move a longer time.