If the Lord — who is the light of all things — vouchsafe to enlighten me, I will treat of Light; wherefore I will divide the present work into 3 Part… - Leonardo da Vinci

Swimming upon water teaches men how birds do upon the air.

When you have drawn the same thing so many times that it seems that you know it by heart try to do it without the model; but having a tracing made of the model upon a thin piece of smooth glass and lay this upon the drawing you have made without the model. ...where you find that you have erred bear it in mind in order not to make the mistake again. ...if you cannot procure smooth glass to make a tracing... take a piece of very fine parchment well oiled and then dried, and when you have used it for for one drawing you can wipe this out with a sponge and do a second.

That part of the air which is nearest to the wing which presses on it, will have the greatest density.