The hot-air balloon, invented by the Montgolfier brothers in 1783, launched for the second time just days before the Treaty of Paris would end the American Revolutionary War. The ascent in Paris--a technological marvel witnessed by a diverse crowd that included Benjamin Franklin--highlighted celebrations of French military victory against Britain and ignited a balloon mania that swept across Europe at the end of the Enlightenment. This popular frenzy for balloon experiments, which attracted hundreds of thousands of spectators, fundamentally altered the once elite audience for science by...