The blow: bombard recoil and shot striking walls and towers
Why a struck wall bulges, a round tower splits, and a bombard kicks back
Leonardo studies the percussion of artillery: a sketch shows a wall in which a ball has lodged, the bulge on the far side centered on the ball, with lines radiating from its center. An overhead diagram shows a round tower being struck and bursting open to the sides. The remaining notes reason about why a bombard recoils, arguing by analogy with a man trying to move a boat or a sack from within: force does nothing until it pushes against something outside, just as the flame moves the gun only when it drives against the air blocking its exit.
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The bulge raised in a bombarded wall
The hump raised on the far side of a wall struck by a bombard has as its center the middle of the ball that struck it. In the sketch, lines radiate from the ball's center out into the swelling of the wall.
A round tower burst open by the blow
Seen from above, a round tower is struck by shot. The blow, together with the air immediately resisting it on the opposite side, is the cause that the tower opens out to the sides.
Why a bombard recoils
That bombard will recoil the more whose fire, multiplying within it, is of readier multiplication and of more immediate assault and percussion into the air situated before it.
The boat-and-sack analogy for recoil
Force exerted wholly inside a boat or a closed sack cannot move it; only a force pressed against an outside obstacle drives it backward. So the flame moves the bombard only once it pushes against the air blocking its exit while braced against the gun's bottom.