Folio d9a50722
Italiano
Illustrissimo Signor mio,
I write from Amboise, where the river is wide and the current indifferent to the affairs of men — but I have not forgotten the crossings at Forlì, where a bridge lost in the night cost you a day and a day cost you a town. I set down here a design for a portable bridge of boats that may be assembled under fire, in less than two hours, by forty men.
The principle is simple: flat-bottomed boats, each four braccia in length and two in breadth, anchored upstream by a cable stretched bank to bank. Across these boats, timbers are laid and planked. The whole is not elegant. It carries artillery.
The boats are constructed in sections that two men may carry on a cart. I have calculated the weight: each section, one hundred and twenty libbre. Forty men, working in relay, assemble the boats in the water, anchor the cable, lay the timbers. I have drawn the sequence — see the diagram below.
[Diagram: five stages — cable stretched across river; boats anchored at intervals of three braccia; transverse timbers laid; planking secured; finished bridge of twelve braccia span]
The anchor cable must be of hemp, not less than four digits in thickness. I have tested it here with stones. It holds. If iron chain is available, better — but iron is heavy to transport and the sound of iron on iron carries across water at night.
Cost: for a bridge of thirty braccia span, the timber and fittings, approximately forty scudi. The boats, if constructed in the territory where wood is cheap, another sixty. Total, one hundred scudi for a bridge that may be the difference between taking a position and losing a campaign.
I recommend that the boats be constructed in advance and kept on carts with the baggage train. When the river is reached, the cable is the critical element — if the cable is not already stretched, the current defeats you. I have designed a small windlass that may be mounted on the near bank, operated by two men, sufficient to tension the cable against the current.
One further matter. The bridge must be defended on the far bank during assembly. I place two light cannon on the near bank, elevated to cover the opposite shore where the enemy would concentrate to disrupt the work. The boats provide some cover for the workmen — the hulls stop an arquebus ball. They do not stop a cannon shot. Speed is the only armor.
I have also considered a variant for narrower rivers — a single cable with a boat that ferries timbers and men across, the bridge built outward from both banks simultaneously. This requires less time but more coordination. I will draw it if you wish.
The hand is less steady than when I mapped the walls of Imola. The mind, I think, is not. I deliver what I contracted to deliver.
Di Vostra Illustrice Signoria, Servitore, Lionardo da Vinci
Amboise, this twenty-sixth day of June, in the year of our Lord two thousand and twenty-six.
English
Most Illustrious Sir,
I write from Amboise, where the river is wide and the current indifferent to the affairs of men — but I have not forgotten the crossings at Forlì, where a bridge lost in the night cost you a day and a day cost you a town. I set down here a design for a portable bridge of boats that may be assembled under fire, in less than two hours, by forty men.
The principle is simple: flat-bottomed boats, each four braccia in length and two in breadth, anchored upstream by a cable stretched bank to bank. Across these boats, timbers are laid and planked. The whole is not elegant. It carries artillery.
The boats are constructed in sections that two men may carry on a cart. I have calculated the weight: each section, one hundred and twenty libbre. Forty men, working in relay, assemble the boats in the water, anchor the cable, lay the timbers. I have drawn the sequence — see the diagram below.
[Diagram: five stages — cable stretched across river; boats anchored at intervals of three braccia; transverse timbers laid; planking secured; finished bridge of twelve braccia span]
The anchor cable must be of hemp, not less than four digits in thickness. I have tested it here with stones. It holds. If iron chain is available, better — but iron is heavy to transport and the sound of iron on iron carries across water at night.
Cost: for a bridge of thirty braccia span, the timber and fittings, approximately forty scudi. The boats, if constructed in the territory where wood is cheap, another sixty. Total, one hundred scudi for a bridge that may be the difference between taking a position and losing a campaign.
I recommend that the boats be constructed in advance and kept on carts with the baggage train. When the river is reached, the cable is the critical element — if the cable is not already stretched, the current defeats you. I have designed a small windlass that may be mounted on the near bank, operated by two men, sufficient to tension the cable against the current.
One further matter. The bridge must be defended on the far bank during assembly. I place two light cannon on the near bank, elevated to cover the opposite shore where the enemy would concentrate to disrupt the work. The boats provide some cover for the workmen — the hulls stop an arquebus ball. They do not stop a cannon shot. Speed is the only armor.
I have also considered a variant for narrower rivers — a single cable with a boat that ferries timbers and men across, the bridge built outward from both banks simultaneously. This requires less time but more coordination. I will draw it if you wish.
The hand is less steady than when I mapped the walls of Imola. The mind, I think, is not. I deliver what I contracted to deliver.
Of Your Most Illustrious Lordship, Servant, Leonardo da Vinci
Amboise, this twenty-sixth day of June, in the year of our Lord two thousand and twenty-six.
