The fish's tail as propeller; contact of two currents
A leaping fish over a diagonal boundary between two water flows labelled a, b, c and f, a, b, c
Beneath a sketch of a leaping fish, Leonardo asks in what manner the fish's tail behaves in driving the fish forward, and likewise the eel, the snake, and the leech. A larger drawing shows two currents of water divided by a long straight diagonal, points on the upper flow marked a, b, c and on the lower f, a, b, c. A note headed 'Conception' argues that where parts of two surfaces touch, the part of the one that is touched equals the part of the other that touches it.
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How the fish's tail propels it
Below a leaping fish, Leonardo asks in what manner the fish's tail behaves in propelling the fish forward, and likewise the eel, the snake, and the leech.
Boundary between two currents
A long straight diagonal divides two flows of water; points on the upper flow are marked a, b, c and on the lower flow f, a, b, c, with an eddy curling at the meeting.
Conception: reciprocal contact of surfaces
A 'Conception' states that if parts of two surfaces touch, the part of the one that is touched by the other equals the part of the other touched by the one—as much of the first is touched by the second as of the second touches the first.