Bodies Descending Through the Air: Beams, Boards and Rods
Straight versus oblique descent, the eight degrees of weight, and weighing a rod for air resistance
A cluster of diagrams down the right margin shows a horizontal suspended board, an oblique board, an oblique beam with hatched descent lines, a beam on pulleys with vertical counterweights, and vertical and horizontal rods, all captioned by short labels. Leonardo explains that a body of uniform thickness placed in equilibrium falls straight and very slowly, while the same body set obliquely falls faster and along an oblique path. He divides the weight of an oblique beam a b into eight degrees, two at the front b that loses a quarter and six at the opposite front a. He then gives a practical method: weigh a rod first lying down and then upright, and divide the difference by the rod's partitions to find how much weight the air's resistance removes.
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Horizontal beam in equilibrium: a slow straight descent
A body of uniform thickness and weight placed in the site of equality descends straight, all its parts staying at equal height, without deviating from its first equilibrium, provided the air is immobile and of uniform resistance. This motion, he notes, is extremely slow.
Oblique beam: eight degrees of weight, two in b and six in a
A uniform body set obliquely falls faster and along an oblique line. For obliquity a b, the front b loses a quarter of its weight, which discharges onto the opposite front a; so of the 8 degrees of weight a b, 2 remain in b and 6 in a. The more oblique the rod, the less heavy it is.
Weighing a rod upright and lying to measure air resistance
The rod a c e f, divided into as many parts as its front fits into its length d e, gains that much more weight standing upright than lying down. To find what the air removes, weigh it lying then upright, divide the difference by the rod's divisions, and subtract one; the remainder is the weight taken by air resistance, the subtracted one being the rod's front.