A Treatise of the System of the World
London, 1731
Newton's plain-language account of universal gravitation: why the planets, their moons and the comets move as they do, and how the Moon raises the tides.
Newton first wrote this treatise as a popular version of the third book of his Principia. It was published only after his death, in Latin and in English, in 1728. This is the English translation, in its 1731 edition.
The PDF is hosted on a Johns Hopkins University web page, and the link opens that copy.