The aim of this paper is to give a self-reflective account of the building of Galileo's pendulum in order to discover what were the practical contingencies of building and using the pendulum for demonstrating the law of isochronism. In doing this, the unique Lebenswelt structures of "Galilean physics" are explicated through the ethnomethodological concepts developed by Harold Garfinkel. The presupposition is that the practical logic of "Galilean physics" is embedded in the instruments themselves. In building the pendulum and recovering its original use, "Galilean physics" becomes for ethnomethodologists a first-hand practical discovery. This is not a reconstruction of the mind of the historical Galileo but, rather, an explication of Galileo's practical perspective on the instrument as an intersubjective and interchangeable standpoint available for ethnomethodological analysis. This enables us to study historical facts from the standpoint of the practical logic of the original practice with a pedagogical eye for the instructive reproducibility of science.