This is one of Da Vinci´s excellent drawings. He made several thousand of them throughout his entire life. Some of them can be viewed in different collections held by museums but some of it have private owners who keep them out of the public eyes. Which I think is a shame because they reveal and show how Da Vinci worked and thought about a project. These drawings was never intended to be viewed by an audience, but I really enjoy browsing through the few I can find online.