Of the ventilating (oscillating) motion, which always consumes itself in the incident motion.

The ventilating 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 in 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, the driver of the body in which it is created, consumes itself and dies together with the motion of that body.

The ventilating motion, although it always arises from impetus, is generated in diverse ways and in diverse bodies; and this will be seen at times in a heavy body suspended from one or more cords or other supports above or below that weight, at times in the arms of a balance about its pole, at times 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 diverse gulfs (mouths) of rivers.

The ventilating motion is of two natures, that is, straight in itself and revolvable (rotary). The straight is when this 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 revolvable 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 along a perpendicular line, it will go unwinding from that cord until all of it is unwound; then, by the impetus acquired, it will re-wind and mount up near the place whence it parted, and so it will continue until the consumption of its impetus.