In the inaugural issue of Past and Present, Eric Hobsbawm emphasised the complexity of workers' and capitalists' attitudes to new machines. Moreover, noting that: “only rarely were new machines immediate and obvious paying propositions”, he suggested the potential of a history of profit to understand the motivations for introducing machines and the consequences of their adoption. This article grapples with Hobsbawm's “profit puzzle” to understand the implications of the adoption and use of the Boulton & Watt steam engine for capitalists and workers in Cornish copper mines between 1777 and 1791. It shows that the engine's economic implications for the people who invested in, and worked, the Cornish copper mines were conditioned by a complex relationship between power and profit. Power is assigned three meanings in this analysis: steam 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 had decisive implications for Cornish mining profits. The article shows that the evolving relationship between power and profit conditioned both the enthusiasm for the B&W engine in Cornwall and the subsequent hostility that Cornish miners and mining adventurers displayed towards the partners. More generally, it suggests the potential of studying the economic and social history of new machines through the lens of profit to understand the motivations for introducing machines and the consequences of their adoption.