In 1804, a kind of madness descended upon Britain. A thirteen-year-old boy, William-Henry West Betty, arrived and, in a seeming instant, took Ireland, Scotland, and England by storm. Crowds were so intent upon securing tickets for Betty's performances that officers were called out to stop rioters in the streets. Like the groupies who would a century and a half later mob Elvis or The Beatles, fans raved and regularly fainted when near "the divine Master...