Historians and sociologists of science and others have long recognised the difficulty of identifying in detail who made up the category 'public', perhaps especially in historical context. Whether the category 'the public' is understood in terms of affiliated groups (public relations theory) or in terms of the ambiguous 'public sphere', whether we deal with audience numbers or with reviews as indicative of a subject's intended public, whether we deal with 'popular science' or with different models of cultural competence and the public understanding of science, the object in question - the public - seems always to escape fine-grained analysis. This paper will explore these issues with reference to several examples from the geographies of nineteenth-century science, looking at questions of audience, public response and public review in the meetings of the British Association for the Advancement of Science, and at the identification of an eponymous public and the idea of 'public interest' in debates between 1871 and 1884 around a single Prime Meridian for the world. If it is the case that understanding the analytic category 'the public' is contingent upon the object or scale of our enquiries, does this matter?