Every schoolchild knows about Paul Revere's 20-mile ride to warn that the British were coming. Far fewer know that 16-year-old Sybil Ludington rode twice as far to help her father, Colonel Ludington, muster his scattered troops to fight a marauding enemy. Few know about Martha Bratton, who blew up a supply of gunpowder to keep it from approaching British troops and boldly claimed, "It was I who did it " Susan Casey gives Ludington, Bratton, and 18...