Abstract. Books I and III of Newton’s Principia develop Newton’s dynamical theory and show how it explains a number of celestial phenomena. Book II has received little attention from historians or educators because it does not play a major role in Newton’s argument. However, it is in Book II that we see most clearly Newton both as a theoretician and an experimenter. In this part of the Principia Newton dealt with terrestrial rather than with celestial phenomena and described a number of experiments he carried out to establish the success of his theory in explaining the properties of fluid resistance. It demonstrates most clearly the activities of a scientist working at the forefront of knowledge and working with phenomena which he did not f...