Rule of weight: added length unloads a support
The rule of three shows two added braccia of rod remove the load once carried at support a
Under the heading 'Rule of weight,' folio 167v works a beam problem: a rod a c of two braccia is borne one braccio each by supports a and c, and Leonardo asks how much load is removed from a when two further braccia of rod are added between c and e. Using the half-length of the added rod and the rule of three, he finds that the addition removes the whole braccio formerly carried at a. A second calculation on the half-rod f n divides 54 by 24, reduces to 1/4, and leaves 1 and 3/4 as the remaining load. Several tick-marked beam diagrams at the right, some drawn in red chalk, accompany the arithmetic.
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How added length shifts load off a support
A rod a c of two braccia is borne one braccio by support a and one by c. Adding two more braccia of rod between c and e is shown to remove the whole of the weight formerly carried at a, so that nothing remains there.
Rule of three applied to the half-rod f n
In the half simple rod, three times eight is 24 set under n, and three times three is 9 set under a; then six times nine is 54, which divided by 24 gives 2 and 6/24, reduced to 1/4. Subtracting 2 and 1/4 from the weight a f, which was 4, leaves 1 and 3/4.
