Why water rises only 12 times the lead piston's height
A lead piston of columnar form raises water twelve times its own height, a rule Leonardo says nature never breaks
Folio 150v works out a rule for lifting water with a lead piston (piombo) that presses a columnar cistern. Leonardo states that a lead cylinder of equal thickness to the cistern always raises the water twelve times its own height, no more and no less; a thinner lead raises it proportionally less, a thicker one more. The tall crane-like figures labelled 'lead, air, water' and 'counterweight, lead, air, water' illustrate why simply lengthening the lead fails: a longer piston becomes proportionally thinner, works with less of its weight, and the lever triangle f n m ceases to be equilateral, so the water still climbs only to twelve braccia.
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The rule: water rises twelve times the lead's height
A lead cylinder pressing a columnar cistern of equal thickness always raises the water in its pipe exactly twelve times the lead's height, whatever the pipe's bore, provided it is no wider than the lead. A lead thinner than the cistern raises the water proportionally less than twelve times; a wider lead raises it proportionally more. Leonardo calls this a rule of nature that human ingenuity cannot evade.
Why lengthening the lead does not raise water higher
It seems that drawing a one-braccio lead up to two braccia should carry water 24 braccia. But a longer lead becomes proportionally thinner: at two braccia it is half as thick and works with half its weight, so the water rises only six of its heights, again 12 braccia. At four braccia it is a quarter as thick and lifts water three times its height, still 12 braccia, without remedy.
Lever and counter-lever triangles in the lifting device
If the rod f n is made longer than twelve of lever against one of counter-lever, the cistern n will descend but still not raise the lead to its first height. Because the rod f n is longer than its descent, the triangle f n m is not equilateral, and the same holds for the counter-lever triangle, so the desired motion has no effect.
