Of Birds: Falling Bodies, Obliquity and Pulley-Ropes
The heaviest part leads a falling body; a bird spread wide weighs less; the science of ropes in tackle-blocks
Headed 'Of birds', the page opens with the principle that the heaviest part of a freely descending body guides its motion, then develops a series of propositions on the ropes of a tackle-block: their varying velocities, the mover always being stronger than the moved, how ropes load a weight, and why the first rope breaks at its highest point rather than at the pulley. Marginal figures apply these ideas to flight, arguing that the front a b falls heavier obliquely than the whole body does perpendicularly, and that a bird never rests upon the air at any degree of obliquity. A closing note observes that a bird which spreads itself more weighs less, while one occupying less space becomes heavier.
On this page
The heaviest part guides descent; ropes and their loads
Every heavy body descending freely makes its heaviest part the guide of the motion. Leonardo then reasons about the ropes of a tackle: the mover is always more powerful than the moved, ropes joined by a perpendicular line to one weight load equally, and immovable ropes carry double the weight sustained — from the weight on one side and the resistance on the other.
Velocities of ropes in a tackle-block and where a rope breaks
The ropes of one tackle-block have as many speeds as their number, and descending ropes that lift the weight are swifter than those that rise. The first rope, in breaking, always breaks at the top of its straightness where it leaves the pulley, not at the pulley's contact, which is proved to be denser and more resistant.
The oblique front falls heavier than the perpendicular body
The front a b, falling obliquely, is heavier than the whole body falling perpendicularly, because that front makes itself the guide of the motion with all the power the weight has through its obliquity. A top-margin figure with letters illustrates the leading edge.
A bird never rests upon the air
One must calculate the degrees of obliquity, for at no degree of obliquity does the bird stop upon the air; it becomes more or less swift as the obliquity is less or more oblique. Leonardo notes that water, outside the site of equality, demonstrates the same behaviour.
Spreading wide reduces a bird's effective weight
That bird which spreads itself more weighs less, and conversely the one that occupies less space becomes heavier. A group of small figures in the lower right corner accompanies the note.