How by a certain machine many may stay some time under water. And how and wherefore I do not describe my method of remaining under water and how long I can remain without eating. And I do not publish nor divulge these, by reason of the evil nature of men, who would use them for assassinations at the bottom of the sea by destroying ships, and sinking them, together with the men in them. Nevertheless I will impart others, which are not dangerous because the mouth of the tube through which you breathe is above the water, supported on air sacks or cork.
[Footnote: The leaf on which this passage is written, is headed with the words Casi 39, and most of these cases begin with the word ‘Come’, like the two here given, which are the 26th and 27th. 7. Sughero. In the Codex Antlanticus 377a; 1170a there is a sketch, drawn with the pen, representing a man with a tube in his mouth, and at the farther end of the tube a disk. By the tube the word ‘Channa’ is written, and by the disk the word ‘sughero’.]
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.