Why an Oblique Beam Does Not Fall in a Straight Line
Air condensing before and rarefying behind a falling body makes its oblique descent zigzag
The page is dense with text; a small marginal diagram shows an oblique beam a-b divided into eight equal parts above a fringe of hatched lines. Leonardo sets out a premise that air rarefies behind a moving body as much as it densifies in front of it, then reports the adversary's claim that a whole beam does not descend like its separated parts. His main column explains why an oblique descent will not keep a straight path: the air pressed by the leading front condenses and checks it, so the opposite front, meeting rarefied air, gains weight and swings the fall from right to left and back, again and again, until the motion ends. He concludes that a beam of uniform figure and weight nonetheless descends along a straight line, since equal parts fall at equal velocity and the whole does what the parts do.
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Air rarefies behind and condenses before a moving body
As a premise ('conception'), Leonardo states that the air becomes as much more rarefied behind the motion of a moving body as it becomes more dense in front of the same body.
The adversary: the whole beam versus its divided parts
The adversary argues that the whole beam does not descend like its separated parts, because the whole gives its entire oblique weight to the lower front while each part gives its own weight to the front of the part. The velocity from front to front is as that from the whole to the part.
The zigzag of oblique descent through condensing air
The leading front presses and condenses the air, which resists and stops it, while the opposite front in rarefied air gains weight and falls faster; so the rightward impetus turns to the left, then right again, until the motion is spent. Yet by the seventh proposition, uniform bodies falling through an equal medium keep equal velocity, so the beam still descends along a straight line.