Da Vinci's machines are built

Milan, 1482. Leonardo da Vinci, thirty years old and underemployed in Florence, writes Duke Ludovico Sforza a job application. In ten numbered paragraphs he offers portable bridges, siege engines, mortars, mining machines, unsinkable ships, and covered carts that can carry men safely among the enemy. Painting appears in the last paragraph, almost as a postscript. The Duke hires him, mostly for pageants and portraits. The machines stay in the notebooks: thousands of pages of gears, wings, screws and springs, written in mirror script, centuries ahead of the metallurgy and the engines that could have made them real. Our timeline's physics is firm about this. No bridge of causes leads from those pages to working machines.

The atlas keeps a far shelf for the timelines no bridge of causes reaches, and records them too, honestly labeled. This is one. In the world recorded here, the letter is taken literally, and the universe, for once, cooperates with the drawings. The Duke reads the ten paragraphs and asks for a demonstration. In a courtyard of the Castello, the folio opens, and what Leonardo builds from it works: the springs hold their temper, the gears cut true, the wings answer the arm that drives them. By candlelight the Duke turns the pages of machines the way other princes count soldiers. The postscript about painting is, in this world too, almost forgotten.