The science of the blow, impetus, and rebound (ff. 80v-92r)
Dense propositions deriving rebound height from the blow and its length from impetus.
Two closely written pages develop Leonardo's theory of percussion in strings of propositions marked p. and c. He argues that a moving body flees the struck site in proportion to the impetus and rises in proportion to the blow, so that all the rebound's height comes from the blow and all its length from the impetus. The limit of the reflected motion is set as the beginning of natural motion, and four causes of percussion are named: distances, the moving body, the impetus, and the object. He closes with the general claims that motion arises from the congregation or disgregation of matter - as in winds or the muscles that move animals - and that percussion is properly an impediment to motion. Small margin diagrams of circles, a rebound fan, quadrants, and pulleys with hung weights accompany the text.
On this page
Height from the blow, length from the impetus
A body flees the struck site as far as the impetus drives it and rises as far as the blow pushes it up. Hence all the height of the rebound is born of the blow, and all its length of the impetus that caused the blow. The proportion of impetus to blow equals that of the rebound's height to its length.
The four causes of percussion
Leonardo names the distances, the moving body, the impetus, and the object as the four powers that cause percussions. Because these powers may combine in endless ways, the force of a blow is variable to infinity even when its causes are equal. Remove the causes and their effects - the height and length of the rebound - necessarily vanish.
Motion from the congregation or disgregation of matter
Motion is caused by the congregation or disgregation of any matter whatsoever, as seen in the making of winds or in the muscles that produce the motions of animals. Percussion, by contrast, is properly an impediment to motion. These closing claims widen the analysis of the blow into a general account of what starts and stops motion.
The impetus outlasts the blow
The power of the impetus, creator of the blow, lasts longer than the power of the blow itself, enduring through the whole reflected motion and ending only where natural motion begins. The height of the rebound thus ends with the blow, while its length ends with the impetus. Where percussion is greater, more of the remaining impetus is spent.
Margin sketches of rebound paths and loaded pulleys
Small diagrams in the margins show a fan of rebound trajectories, circles and quadrants for the geometry of the blow, and single pulleys carrying hung weights. They restate visually the cases argued in the text. Several carry compass-drawn arcs.