In 1840 the raja of Travancore, Swathi Thirunal, would offer his government's assistance to the British Association for the Advancement of Science and its plan for a global system of magnetic observations. Over the next thirty years, the two directors of this princely state's observatory, John Caldecott and John Allan Broun, would pursue fundamental terrestrial magnetic research. Their efforts would culminate in the Trivandrum [Trevandrum] Magnetical Observations (1874). In what follows, the history of this publication is used to shed light on how and why a semi-autonomous princely state such as Travancore would engage the scientific community in Europe at this time. The article focuses in particular on the work of turning observation data into a published report and on how that labour would be distributed between the Indian subcontinent and Europe. Because the production of such reports required dozens of hands and decades of labour, its history can reveal much about the concrete working relationship between informal colony and imperial metropole within the British Empire. The Trivandrum Magnetic Observations were produced within a global economy of science in which Travancore sometimes had the upper hand. At the same time, data and scientific productions tended to accumulate in Europe (at least for a time), where ultimately the consumers of scientific products and the arbiters of ‘scientific value’ also largely remained. Within the sprawling economic, political and cultural infrastructures that linked geomagnetic research in Travancore and Europe, the relative strengths and weaknesses of each region would cut in different directions. The history of the production of the Trivandrum Observations brings to light this robustly interconnected geography of scientific production within the British Empire. It also reveals some of the processes by which ‘centres’ and ‘peripheries’ in the sciences were then becoming differentiated.