ArgumentThis article examines advertisements for potential solutions to the problem of longitude during the year following the announcement of the maximum £20,000 reward in the summer of 1714. While there have been many studies of the race to determine longitude, advertisements have not received close scrutiny. Little attention has been paid to the commoditization of longitude in the marketplace of public science sold within London's public sphere. Although books and lecture series dominated public science in eighteenth-century England, longitude ads are (...) a unique example of this phenomenon. Unlike lecturers or publishers of natural philosophical tracts who sought audiences for demonstrations or readers for books which contained accounts of nature that were not open to debate, longitude seekers were doing something different. Those who attempted to solve the problem of longitude and advertised their plans to the public sought out witnesses to validate and confirm the effectiveness of their experiments or instruments. In other words, the public sphere became the crucible of judgment for schemes of longitude. (shrink)