The Da Vinci Notebooks at sacred-texts.com
Remember that your flying machine must imitate no other than the bat, because the web is what by its union gives the armour, or strength to the wings.
If you imitate the wings of feathered birds, you will find a much stronger structure, because they are pervious; that is, their feathers are separate and the air passes through them. But the bat is aided by the web that connects the whole and is not pervious.