Secret flooded conduit within a fortress ditch
A concealed water passage the castellan alone can flood or drain, with hidden brick inlets and a controlling lever K.
The notes and cutaway drawing describe a concealed passage or gallery ('cava') running under water within a fortress, four braccia high and two wide, its wall built up in three layers of brick, beaten potter's clay and brick again. A hidden conduit q p is kept permanently full, fed through almost invisible joints in the brickwork at S t and drained at n f, so that a besieger can neither detect nor block it. Only the castellan, by turning the lever above at K, can admit water to the passage by way of r q, and a single pump can empty it again. A further note specifies that the bricks be joined with fine fresh lime mixed with very little river sand.
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Concealed passage controlled by the castellan
A passage lying under water within the fortress is to be four braccia high and two wide, its wall built in three layers. The water enters and leaves through minute, almost unseen joints so the enemy has no evidence of them and cannot close them. Only the castellan, turning the lever above at K, can flood the passage by way of r q.
Always-full conduit q p and its draining
The first channel or conduit of water, q p, always stands full. It is filled through thin joints of the bricks at the site S t, which all lead into the conduit toward the mouth p, and it discharges within at n f. With a single pump the passage can be emptied again.
Brickwork bedded in fine lime
The bricks laid in such a conduit must be joined with fine, fresh lime mixed with very little river sand, so the joints stay tight and inconspicuous. The wall is raised in three layers, its middle course of well-beaten potter's clay softened like soft dough.
Lettered cutaway of the vaulted passage
The vaulted passage is drawn in cutaway and keyed with letters read from top to bottom, K q r r x n m and S t d a p b c, tying the control lever K and the water channels q, p, n and f to their places in the structure.