Canals and locks beneath the city's palaces
Lighting rooms, unloading boats indoors, and locks holding a steady water level
Leonardo plans navigable canals running beneath the palaces of his ideal city, dimensioning the passage so the rooms under the porticoes stay lit and boats can be unloaded directly into the houses. To hold a constant water height and guard the cellars against flooding he insists on a site beside a clear, non-silting river such as the Ticino or Adda, controlled by a lock (conca) at the town's entrance. A second sketch shows boats rising within a lock when gate M is shut, and a third proposes three locks in series so floodwater meets only a single threshold and cannot swamp the city.
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Canals under the palaces held at a constant level
Navigable canals are planned beneath the palaces, dimensioned a e 6 braccia, a b 8 and b c 30, so the rooms under the porticoes stay lit and boats unload into the houses. To avoid flooding the cellars, Leonardo chooses a site by a clear, non-silting river such as the Ticino or Adda, its level fixed by a lock (conca) at the town's entrance.
Passage sized to light the rooms and unload boats indoors
The face a M is set to give light to the rooms, and the passage c d f is where boats are unloaded directly into the houses. The canal widths are chosen so the rooms under the porticoes remain well lit.
Lock (conca) raising boats to the city level (gate M)
A boat is drawn within a lock in the middle of the sheet. When gate M is shut, the water fills the lock and the low boats rise back to the universal level of the city.
Three locks in series to control flooding
A third figure proposes three locks set level together, so the river's water meets only one threshold on entering the city. This is done for greater speed and so the rise from floods does not soak the town.