Why a twisted rope's outer strands bear the most load
Straight threads share weight equally, but in a twisted cord the surface fibres stretch and break first
This leaf studies the strength of ropes and wires under load. Leonardo argues that the parallel, untwisted threads of a cord share a hanging weight equally, but once twisted into a rope the outer fibres run in longer helical lines and so feel more weight, and break first, while the central line neither lengthens nor shortens under any twist. He confirms the reasoning with iron: a single straight wire breaks cleanly across its full thickness, whereas in a twisted cord the strain, and the fracture, concentrate at the surface.
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Twisted strands bear more than straight ones
The untwisted threads of a cord, equally loaded by one continuous weight, are equally burdened. But once twisted into a cord, though loaded perpendicularly, the outside feels more weight than the inside, because the inner threads touch and run almost straight while the outer ones form longer lines around the mass.
A bent wire: only the central line keeps its length
An iron wire hanging perpendicular has all its lines equal, but when bent no line keeps its first length save those crossing the middle of the bend. As one bent line grows, its opposite shortens equally; the line made longest by the pulling breaks first.
Proof by iron wire: the cord breaks at its surface
A doubled cord feels more strain from the centre inward, where o p l is the central line. A single straight iron wire breaks across its full thickness, but in a cord of three or four twisted wires only the central line keeps its length, the surface lines are most stretched, and there the break occurs.