Corner Turrets, the Sally-Port, and Testing Foundations
The last common parts of the fortress, and how to prove firm ground with a vessel of water
A prose page (folio 95r) completing the enumeration of the fortress's common parts and passing to the science of foundations. Leonardo sets turrets at the wall angles so each tower can see and relieve the next, and gives the keep an easy sally-port; the last common part is that foundations rest on stable ground. He describes a practical test, a vessel of water set on the site and the ground struck beside it, so that leaping water reveals a hollow or weak stratum beneath a thin vein of tufa. He then prescribes building on solid rock, tufa or piles filled with gravel and lime, and charring and pitching the corner tie-timbers against the corrosive damp of lime. The sheet is continuous text with no diagram.
On this page
Angle turrets and an easy sally-port for the keep
Turrets set at the angles of the walls let one tower see and relieve another and see who strikes it. The keep should have an easy exit so the enemy cannot pen those within, who might otherwise be struck as they sally through a narrow place.
Testing the stability of ground with a vessel of water
To know a foundation's stability, set on the site a vessel full of water and, once it is still, strike the ground beside it. If the place is unstable the impact reverberates upward, shakes the water and makes it leap from the vessel; if the water does not spill, the ground is stable and without cavity. A thin vein of tufa over feeble earth is what breaks under the weight of walls.
Building on rock, tufa, or piles filled with gravel and lime
Build upon solid rock, tufa or firm tenacious ground, taking care with stratified rock that the layer-lines are not oblique, nor cut nearby by a quarry or breach of the mountain. Where firm ground is lacking, drive piles, fill between them with gravel and lime, then raise the wall, first binding the piles well at their ends so the load does not spread them apart.
Charring and pitching the corner tie-timbers
The timbers set as tie-bindings at the corners of the walls should be somewhat charred on the surface, then coated with black pitch. This protects them from the corrosive damp that issues from the lime, which would otherwise consume them.