Can the adoption of labor-saving technology lead to social instability and unrest? We examine a canonical historical case, the so-called 'Captain Swing' riots in 1830s Britain. Variously attributed to the adverse consequences of weather shocks, the shortcomings of the Poor Law, or the after-effects of enclosure, we emphasize the importance of a new technology - the threshing machine. Invented in the 1780s, it spread during and after the Napoleonic Wars. Using farm advertisements from newspapers published in 66 English and Welsh towns, we compile a new measure of the technology's diffusion. Parishes with ads for threshing machines had much higher riot probabilities in 1830 - and the relationship was even stronger for machine-breaking attacks. Threshing machines were mainly useful in wheat-growing areas. To establish a causal role for labor-saving technology, we instrument technology adoption the FAO measure of soil suitability for wheat, and with suitability for water mills, and show that this in turn predicts unrest. The link from technology adoption to unrest was weaker where poor law provision was more generous, and where displaced workers could easily access urban labor markets. Finally, we document that technology adoption and patenting rates declined after 1832 in areas with more unrest.