Mining fortress walls: undermining and sap tunnels
How to excavate beneath walls so they collapse outward; a division sum
A military engineering sheet on sapping and mining fortress walls, worked out in pen over red-chalk studies. Leonardo explains boring through a wall, placing the excavation within its thickness (adjusting for earth-filled or hollow backing) so it breaks outward rather than inward, and above all attacking the corners of the fortifications. A second note lays out an underground sap one and a half braccia wide and three high, turning at right angles from c to d and to the powder 'ovens' set in the middle of the wall. A short division (30000 by 5 = 6000) and many red-chalk sections of walls, vaults and mouldings fill the sheet.
On this page
Undermining a wall so it breaks outward
First bore through the wall at a b and measure its thickness, making the excavation in the middle. If the wall is backed with earth, cut beyond the middle; if hollow behind, cut nearer, so it breaks outward — inward it would hold by leaning on cross walls. A wall pushes and supports more readily than it pulls, so above all make the cut break the corners of the fortress walls.
Layout of the underground sap and powder ovens
Underground excavations to collapse a fortress side must be driven through the lower thickness of the walls. Make a b a gallery one and a half braccia wide and 3 braccia high up to b, then turn at the right angle a b c, bend rectangularly from c to d and from d to the oven f g, continuing always with the ovens set in the middle of the wall.
Division sum
A short worked calculation on the sheet: 30000 by 5, giving 6000.
Red-chalk studies of vaults and wall profiles
The upper left carries red-chalk studies of a curved cornice or vault, and the sheet is crowded with stepped section profiles of battered walls and mouldings, together with a small wheel-and-spiral plan at the lower left. These drawings accompany but extend beyond the transcribed notes.