The zig-zag motion of the celestial thunderbolt
Why lightning bends within one and the same air: resistance of the air versus exhaling sparks
A fourth proposition on force, weight, space and time opens the page, stating that half the power moves half the body half the distance in the same time. Leonardo then asks why the celestial thunderbolt moves in bending, sinuous lines through one and the same air, and offers two causes. The first is that the air condenses in resistance before the bolt's furious impetus, bending its motion like reflected motion, just as air rises in bubbles from the bottoms of swamps to the water's surface in a curved, sinuous path. The second is that the bolt's own matter exhales now to the right, now to the left, like a spark bursting from burning coals; he prefers the first, since otherwise one bolt would spawn many as a spark does.
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Fourth proposition: half the power, half the body, half the space
If a power moves a body a certain space in a certain time, the half force will move, in the same time, half of that body half of that space. This continues the numbered series of rules on force, weight, distance and time.
Air resistance bends the thunderbolt into reflected motion
Leonardo proposes that the air condensing before the bolt's furious impetus offers resistance, so the motion comes to bend and is of the nature of reflected motion, though not rectilinear. This is his preferred explanation for the sinuous path.
Analogy of air rising as bubbles from swamp beds
He likens the bending of the bolt to air that issues from the bottoms of swamps in the form of bubbles, coming to the surface of the water with a sinuous, curvilinear motion. A small vessel-like sketch stands in the right margin.