Funding a navigable canal by selling its surplus water
Canal economics: customs revenue, mills, and drawing water at the tail of the Naviglio
Leonardo answers the objection that maintaining a navigable canal bed is costly by arguing that its surplus water is so abundant that selling it would pay four times the cost, and that where cash is lacking the upkeep can come from customs revenue without burdening the third share. He explains how outfalls taken slowly at the surface and returned quickly below it leave the canal unharmed, and insists that water drawn at the tail — where navigation already ends — does no damage, unlike water taken higher up at San Cristoforo. He notes that the Naviglio della Martesana supplies water in such abundance that the castle's eight millstones run continuously, and remarks that the engineers argue in their own interest to remain masters. The blue-tinted fragment carries only mirror-script text, with no diagrams.
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Surplus water pays to maintain the navigable bed
Leonardo answers the objection about the expense of making and keeping the canal bed navigable: the water is left over in such abundance that its sale would pay four times the cost. Where ready money is lacking, the upkeep can be met from the revenue of its customs dues, provided the third part is not harmed.
Drawing water at the canal's tail spares navigation
He confesses that water taken from the bottom (tail) of the canal does not diminish navigation as it would elsewhere, because it is drawn only where navigation ends. Taking it higher, at San Cristoforo, would starve the two mouths of Santo Vittorio and the Maggi mill, but the Martesana canal supplies ten times as much, keeping the castle's eight millstones in continual water.