Ciclo CCXIII

Illustrissimo Signor mio,

I write from Amboise, where the Loire is wide and slow and the king’s court does not ask me to build walls. But my mind is still in the Romagna, where the walls are not yet built as they must be, and I cannot rest from the matter.

You will recall the survey I made of the rampart at Cesena — the outer face, the one that looks toward the hills. I measured it again in memory, pacing the proportions as I paced the ground, and I tell you now what I should have told you then but the campaign moved too fast for honest counsel: the slope is wrong. The scarp rises at an angle that a ladder will hold. Forty-five degrees from the horizontal is what I found there. It should be no more than thirty, and the base should be thickened by two braccia to bear the weight of the earth behind it. A wall that is too steep is a wall that a siege tower will mount. I have drawn the correction — the batter, the added thickness at the foundation, the cordone at the top to shed rain from the face. I enclose the diagram. The cost in stone is perhaps twelve ducats per braccio of frontage, and the work can be done in six weeks if you press the masons from Forlì.

The fossato before that same wall — it is dry now, and a dry ditch is only a trench, not a moat. If you dam the small stream that runs behind the eastern quarter and divert it along the line I marked on the map I left with your engineer, you will have four feet of standing water before the gate within a month. The earth from the digging itself can be thrown against the base of the wall to form the glacis. One labor serves two purposes. This is the principle I have tried to teach: that every excavation should build something, and every wall should drain something, and nothing should be moved twice.

At Fano, the canal. I have not forgotten it. The harbor silts because the current from the Metauro slows where the channel widens, and the sediment falls. The remedy is to narrow the channel at the point of entry — build two stone jetties, converging, so that the water accelerates as it passes between them and carries the silt further out. I have seen this work at the mouth of the Arno near Pisa, where the same problem was solved by the same means. The jetties should extend perhaps forty braccia into the sea, curved slightly inward, built of the local stone laid in courses with the face angled to break the wave. The cost I cannot state precisely without measuring the depth of the water at the point of construction, but I estimate between eighty and one hundred ducates for both arms, including the labor of the divers who must set the foundations below the waterline.

I would do this work myself if my hand were steady and my legs would carry me. But I am in France, and the king keeps me for paintings and for the amusement of his engineers, who are clever men but who have never built a fortification under the threat of artillery. They theorize. I have counted cannonballs in the rubble of breached walls, and I tell you: theory that has not been tested by fire is only a hypothesis, and a hypothesis will not stop a ball of iron weighing sixty pounds.

If you wish the work at Cesena corrected, send word and I will return. The passport you gave me is still valid — I have kept it, though I have not needed it in this country where no one asks a man his business if he speaks Italian and carries drawings. I will come. The rampart must be right before the next campaign, and the next campaign will come whether the wall is ready or not.

I commend myself to your grace, and I remain, as I have been,

Your servant and engineer,

Lionardo da Vinci

Written at Amboise, this day.