The U.S. Congress's initiative to solve the problem of steamboat boiler explosions in the mid-nineteenth century resulted in the Steamboat Act of 1852. The Act brought radical changes to the western rivers, including reform of the engineering cadre, introduction of new safety devices and procedures, and the creation of a new bureaucracy (the Steam Boat Inspection Service). One of the new safety devices introduced by the Treasury Department was the controversial Evans Patent Safety Guard. This is the story of the safety guard as a central actor in framing the expertise of scientists, inventors, and practical engineers in attempting to make technology safe. The safety guard helps us to understand where expertise came from, how it was defined and justified by government officials, and why the notion of technological expertise depends on a complex mix of technical, institutional, and socioeconomic factors.