Balances on pivots and the key mechanism
How a balance beam behaves upon its pivot, with the eye and teeth of a key
Folio 98v analyses how a balance beam (bilancia) behaves when carried on a pivot (polo), arguing that a balance a b acts the same as a balance c d along any line, and that a pivot must sustain both the beam's circular motion and the pull of its loaded end. Lettered figures of beams, brackets and a boxed lever-and-spring assembly accompany the text. A lower group of drawings treats a key mechanism, noting that a n is the eye of the key moving at f, with g the key, and that two such instruments are of the same nature.
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A balance behaves alike on any pivot line
Leonardo states that the balance a b will always be like the balance c d, even though the upper one is used along any line whatever. The paired figures set out the equivalence of the two beams.
The two burdens a pivot carries under a balance
If the pivot f receives the balance a b, it is as though the beam were straight and hinged at n. The pivot e, by contrast, bears two different labours with its balance c d: sustaining the circular motion the beam makes upon it, and sustaining the motion c wants to make, which would lie beneath r.
The eye and teeth of a key; two instruments of one nature
At the foot Leonardo labels the parts of a key mechanism: a n is the eye of the key, and it moves at f, while g is the key. He adds that these two instruments are of the same nature.