inſtead of the planes upon which thoſe bodies roll or ſslide, and which are therefore tangent plane; to the bodies, I ſhall uſe planes parallel to them, in which the centres of the bodies move, and by that motion deſcribe orbits. And by the ſame method I afterwardſ determine the motions of bodies performed in curve ſuperficies.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/9d/Principia_-_1729_-_Book_1%2C_Section_9_-_End_decoration.png/240px-Principia_-_1729_-_Book_1%2C_Section_9_-_End_decoration.png)