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 fully understand. In this paper the first of Newton's set of experiments into fluid resistance are described and the theory which underlies his explanation is outlined. A number of issues arising from this portion of the "Principia" together with implications for teaching about the nature of science are discussed.