Of the oscillating motion which is always consumed in the incident motion.

The oscillating motion is a certain flexible motion which is generated in bodies constrained not to be able to follow the impetus joined to them in the direction for which it was created; but now this way, now that it moves, converting the reflected motion into incident, and the incident motion into reflected; and so successively it goes on until that impetus, driver of the body in which it is created, is consumed and dies together with the motion of that body.

The oscillating motion, though it is always born from impetus, is generated in diverse ways and in diverse bodies; and this will be seen sometimes in a heavy body suspended by one or more cords or other supports above or below that body, sometimes in the arms of the balance about its pole, sometimes in a boat set upon the water, and often in water within its vessels or in wind rebounding between the walls of buildings. The same we see of fires in their furnaces, and so we see the sea do, and in the various gulfs of rivers.

The oscillating motion is of two natures, that is straight in itself and revolving. The straight is when that motion is made between only two directions, that is, it will move between north and south, or between east and west, or the like. The revolving motion is when a cord is wound about a mounted wheel, which cord has one of its ends fixed at a height, and the other end fixed to that wheel; then, letting the wheel fall by a perpendicular line, it goes on unwinding from the cord until all of it is unwound; then, by the impetus acquired, it rewinds and climbs up again near the place from which it parted, and so it continues until the consumption of its impetus.