River Fall and Whether the Sea Sits Higher Than Mountains
One braccio of fall per mile; the Sea of Tana reckoned 3500 miles from Gibraltar by the nautical chart
Continuing an argument begun on the facing page ('as has been said'), Leonardo reasons that if a river's exit equals its entrance in width, the whole river crossing a lake must still fall one braccio per mile, so the lake's surface lies at a varying distance from the centre of the world. The slowest water is that farthest from the shortest straight line joining entrance and exit. He then concludes that the Sea of Tana, bordering the Tanais, is the highest part of the Mediterranean, which the nautical chart puts 3500 miles from the Strait of Gibraltar, giving a fall of 3500 braccia (about one mile and a sixth), so that this sea stands higher than any mountain of the west. The page carries no diagram.
On this page
A lake's surface tilts: one braccio of fall per mile
Since the width of the exit equals the width of the entrance, the whole river passing through the lake must also have one braccio of fall per mile, so the lake's surface stands at a varying distance from the centre of the world.
Slowest water lies farthest from the entrance-to-exit line
That part of the lake's water is of slower motion which is found farther from the shortest line joining the entrance with the exit of the river passing through the lake.
The Sea of Tana higher than any western mountain
It follows that the Sea of Tana, which borders on the Tanais, is the highest part of the Mediterranean; with a fall of 3500 braccia over that distance, this sea is higher than any mountain the west may have.
3500 miles from Gibraltar, as the nautical chart shows
The Mediterranean is 3500 miles distant from the Strait of Gibraltar, as the nautical chart ('carta da navicare') shows, which gives a fall of 3500 braccia, that is one mile and one sixth.