books.google.com - I consider philosophy rather than arts and write not concerning manual but natural powers, and consider chiefly those things which relate to gravity, levity, elastic force, the resistance of fluids, and the like forces, whether attractive or impulsive; and therefore I offer this work as the mathematical...https://books.google.com/books/about/Sir_Isaac_Newton_s_Mathematical_Principl.html?id=gB2-Hqdx_LUC&q=observations&utm_source=gb-gplus-shareSir Isaac Newton's Mathematical Principles of Natural Philosophy and His System of the World: The system of the world