Abstract In the inaugural issue of Past and Present, Eric Hobsbawm cautioned historians against the assumption that a capitalist economy has an inherent tendency to cost-saving and technological innovation, emphasizing that ‘It has a bias only towards profit’. Inspired by Hobsbawm, this article shows how a history of profit can elucidate the economic and social history of machines. Beginning with miners’ protests at the stoppage of Boulton & Watt steam engines in Cornish copper mines in the late 1780s, it shows that these engines’ implications for the people who invested in, and worked, the mines were conditioned by a complex relationship between profit and power: mechanical power, which was crucial to the mining of copper and the costs of its production; imperial power, given fluctuating demand for copper from different parts of the British empire; and market power, since control over price setting on the British copper market was crucial for Cornish mining profits. Understanding what determined mines’ profits helps to account for capitalists’ decisions to stop their engines as well as copper miners’ protests at their actions. More generally, it suggests the potential of studying the economic and social history of new machines through the lens of profit.