Why a Body Keeps Moving; Dust Motes in a Sunbeam
Impetus consumed by degrees, and air's varied motions revealed by motes in a sunbeam
Continuing the study of air and motion, Leonardo asks why a body keeps moving after its mover has left it, answering that impetus cannot cease at once but is used up by degrees. Citing his 'eighth' proposition — that the air successively surrounding a moving body makes various motions — he offers a vivid proof: in the motes floating in a sunbeam that enters a dark room, throwing a stone makes the motes whirl about the track the body cut through the air. The 'fifth' and 'fourth' propositions restate that nothing insensate moves of itself and that the air filling the void has varying speed, density and motion; a closing note tells the reader to turn the page. A spiral vortex is drawn at the lower right.
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Impetus is consumed by degrees
Leonardo asks why a body continues the motion begun by its mover, and answers that no impetus can end at once but is consumed gradually by degrees of motion. The air first left behind the track cut through it accompanies the body only a little.
Motes in a sunbeam reveal the air's motion
By the eighth proposition, the air surrounding a moving body makes various motions in itself. This is seen in the motes floating in a sunbeam entering a dark room: throw a stone through the beam and the motes are seen whirling about the spot where the air rushed in to fill the track the body made.
Air fills the void with varied motions
The fifth proposition holds that nothing insensate moves of itself but is moved by another. The fourth adds that the air moving to fill the void made in it by the body has in itself various velocities, densities and motions. A note directs the reader to turn the page.