Defending the Isonzo: river barriers against the Turks
Draft memorials on flood-proof timber barriers, the force of water, and a sketch of the Gorizia crossing
The large sheet is filled with drafts of a memorial to the 'Most Illustrious Lords', arguing that the Turks cannot reach Italy without crossing the river Isonzo and proposing toothed timber supports and 'wild' weirs to obstruct the passage. Leonardo reasons about how floods carry timber above the banks, how turbid water grows heavier and swifter, and why a few defenders aided by the river outmatch many. A hydrographic sketch names the Bridge of Gorizia and Vilpago, while the small upper fragments add numeric tables and short geometrical notes on crossed lines and a suspended beam.
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The Turks must cross the Isonzo
In three drafts of a memorial to the 'Most Illustrious Lords', Leonardo observes that the Turks cannot reach the Italian territories by any part of the mainland without arriving at the river Isonzo. Having examined the river's nature and questioned the country-folk about the routes an enemy might take, he offers to turn the river itself into the frontier's defence.
Toothed timber supports and weirs
For the barrier itself he specifies stakes driven in at the foot of the weirs, which check and slow the water and silt up the gaps between the branches to make 'wild and durable' weirs. He adds that men-at-arms alone are useless against raiders unless massed in one place, and that where the current is slack the gaps can be filled quickly with faggots.
Floods carrying timber over the banks
Leonardo argues that his supports should stand only as high as the lowest part of the banks, so ordinary turbid water runs past without entering the wooded margins or picking up timber. When the river overtops the banks, as it did this year by some 4 braccia, the great trees it sweeps along lodge against standing timber and are held back by their branches. Only branchless trunks enter the channel, and these float over his toothed support without touching it.
Turbid water grows heavier and swifter
Reasoning about the force of the stream, Leonardo states that the more turbid the water the heavier it is, and the heavier it is the swifter its descent and the harder it strikes its object. He adds that water destroys only when it moves, and that a submerged object not fixed to the bed lags behind the current in proportion to its weight.
Hydrographic sketch of the Gorizia crossing
A hydrographic sketch in the lower panel maps the river reach, labelled with the Bridge of Gorizia and the place-name Vilpago. It carries fractional gradient figures 3/4 and 2/3.