A coloured plan of a weir in the Arno two miles east of Florence, showing damage to the embankment caused by the flow of water through the weir, with notes in conventional (not mirror) writing. Melzi's number 168.

This exquisite map dates from a period during which Leonardo was employed by the government of the Florentine republic on a variety of tasks, many of an engineering, cartographic or architectural nature. During the summer of 1504 he was apparently instructed to survey stretches of the river Arno east and west of Florence. The Arno is a mountain torrent, low in the summer (which allowed Leonardo to survey its bed during July and August) but prone to flooding during the autumn rains and spring thaw, and the banks of the river required continual maintenance. In the compilation of Leonardo's papers known as the Codex Arundel (London, British Library) are several pages of sketches and measurements done on the spot, together with records of living expenses incurred while on site. From those measurements Leonardo constructed schematic maps of the two areas, transferred the dimensions to ‘fair copies’, delicately coloured the maps with green and blue washes, and added notes identifying mills, the sizes of sandbanks and so on (RCIN 912678-9). 

The present map is an enlarged detail of the lower part of Leonardo's survey map of the river to the east of Florence (RCIN 912679). South is to the top; at the far left (east) of the sheet is a schematic depiction of a chain ferry, at the point where the main railway line south from Florence now crosses the Arno. The focus of the sheet is the damage caused by the waters passing through a two-stage weir and striking the embankment. Two sections of damage are recorded, the first a result of the immediate impact of the current, and the second – as Leonardo implies in the drawing – caused by the subsequent refraction of the energy of this current, curving back into the riverbank a hundred metres away further downstream. A weir is still to be found at that point in the Arno, though in a different orientation, and the river has been embanked to create a much more regular channel. 

The blue wash, the careful finish, and the notes in conventional left-to-right script indicate that the drawing was done for someone else to see, presumably a government official. Leonardo's annotations read: 'This breakage is 60 braccia (35m) long and 6 braccia (3.5m) wide' and 'here the breakage is 100 braccia (60m) long'.