Why Lightning Bends: Motion from Dense to Rare Air
The flexuous path of the thunderbolt and the rising of air through water
Two passages — one written in another hand, one in Leonardo's — explain why the thunderbolt travels a bending, flexuous path. Meeting ever-denser air as it descends, the bolt condenses the air ahead of it, which resists and forces it to swerve toward the least-resistant passage; this, Leonardo notes, is why more lightnings run upward than down. He draws an analogy with air rising through water, which likewise wriggles sideways because the water above it descends where it gains weight. The Ambrosiana catalogue also tags the sheet with square-root values (1 to 5, noted as erroneous), faint jottings that are not part of the two transcribed passages.
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Lightning swerves from dense toward rarer air
The motion of the lightning bends from the dense toward the rare because it cannot find a passage of lesser resistance. As it descends into naturally thicker air, the fury of its motion condenses the air ahead, which resists and forces the bolt to swerve toward a quicker passage; for this reason more bolts run upward than downward.
The thunderbolt's flexuous path renewed at each impediment
In Leonardo's own hand, the bolt born in the cloud is called flexuous because it bends from dense to rare, the density itself generated by the fury of its motion. Unable to continue along the straight line it began, it turns toward the readier passage and repeats this until it meets a second impediment, proceeding by the same rule to its end.
Air rising through water moves in flexuous curves
By the same principle of yielding air, Leonardo describes air that rises from the bottom of water to its surface in winding, flexuous motions. Only the water lying on a perpendicular line above the air has weight to press it, so as the water descends the air is forced aside and twists its straight ascent until it reaches the open air.
Catalogue-noted square roots (1 to 5)
The Ambrosiana catalogue records a set of square-root values, from 1 to 5 and marked as erroneous, on this sheet. Faint numeric lines are visible near the foot of the page, but they are not part of the transcribed passages on lightning.