The Da Vinci Notebooks
Canals in the Milanese (1009-1013).The canal of San Cristofano at Milan made May 3rd 1509. 539
230:539 : This observation is written above a washed pen and ink drawing which has been published as Tav. VI in the "Saggio." The editors of that work explain the drawing as "uno Studio di bocche per estrazione d'acqua."