A mercury siphon clock; drawing a circle through concurrent lines
The 'cicognola' as a slow timekeeper, and the geometry of sectors and radii
The folio opens 'On the siphon' (cicognola): prepared mercury drawn through hair-thin copper tubes descends so slowly that scarcely a grain passes between vessels in an hour, making a clock in the manner of sand and, Leonardo claims, silently igniting a fire after a year or more. The rest of the sheet is geometry: a problem of finding lines that converge on a point and giving them the appropriate circular periphery, worked through an arc-and-chord figure bound by seven lines (a c b f e t n; d o g p v m) where proportional divisions build a sector of a many-sided circle. A final figure of three intersecting circles proves that the equal arms b a and b c, arising at a circle's centre, are its radii and together a diameter.
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Mercury siphon (cicognola) as a silent slow clock
Prepared mercury drawn through the finest copper tubes of imperceptible bore descends so slowly that in an hour not a grain need pass between vessels, making a clock in the manner of sand. Leonardo claims the same vessel could silently generate fire after a year or more, and refers to a drawing in the margin of the fourth page from the bottom.
Circle through lines converging at a point
The lines b o and e p adjust the periphery and make the angles c b f and f e t. The converging lines c d n m bound the parallel a n d m, whose divisions are proportional, so that d g is to c f as d m is to c n. Raising perpendiculars such as d c on a b builds a sector of a many-sided circle whose angles are the circumference.
Proof that the arms b a and b c are radii
It is proved that the straight arms b a and b c are equal, because they arise at the centre of a circle and are its radii; being one and the same straightness, they form the diameter of that circle.