In its heyday, the American circus was the largest showbiz industry the world had ever seen. From the mid-1800s to mid-1900s, traveling circuses performed for audiences of up to 14,000 per show, employed as many as 1,600 men and women, and crisscrossed the country on 20,000 miles of railroad in one season alone. The spectacle of death-defying daredevils, strapping superheroes and scantily clad starlets,...